lib/Commands/NinjaBuildCommand.cpp

//===-- NinjaBuildCommand.cpp ---------------------------------------------===//
//
// This source file is part of the Swift.org open source project
//
// Copyright (c) 2014 - 2015 Apple Inc. and the Swift project authors
// Licensed under Apache License v2.0 with Runtime Library Exception
//
// See http://swift.org/LICENSE.txt for license information
// See http://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
//
//===----------------------------------------------------------------------===//

#include "NinjaBuildCommand.h"

#include "llbuild/Basic/Compiler.h"
#include "llbuild/Basic/FileInfo.h"
#include "llbuild/Basic/Hashing.h"
#include "llbuild/Basic/PlatformUtility.h"
#include "llbuild/Basic/SerialQueue.h"
#include "llbuild/Basic/Version.h"
#include "llbuild/Commands/Commands.h"
#include "llbuild/Core/BuildDB.h"
#include "llbuild/Core/BuildEngine.h"
#include "llbuild/Core/MakefileDepsParser.h"
#include "llbuild/Ninja/ManifestLoader.h"

#include "llvm/ADT/SmallString.h"
#include "llvm/Support/TimeValue.h"

#include "CommandLineStatusOutput.h"
#include "CommandUtil.h"

#include <atomic>
#include <cerrno>
#include <chrono>
#include <condition_variable>
#include <cstdarg>
#include <cstdlib>
#include <deque>
#include <mutex>
#include <sstream>
#include <thread>
#include <unordered_set>

#include <fcntl.h>
#include <signal.h>
#include <spawn.h>
#include <unistd.h>
#include <sys/stat.h>
#include <sys/wait.h>

using namespace llbuild;
using namespace llbuild::basic;
using namespace llbuild::commands;

#if !defined(_WIN32)
extern "C" {
  extern char **environ;
}
#endif

static uint64_t getTimeInMicroseconds() {
  llvm::sys::TimeValue now = llvm::sys::TimeValue::now();
  return now.msec();
}

static std::string getFormattedString(const char* fmt, va_list ap1) {
  va_list ap2;
  va_copy(ap2, ap1);
  int count = vsnprintf(NULL, 0, fmt, ap1);
  if (count <= 0) {
    return "unable to format message";
  }

  std::string result = std::string(count, '\0');
  if (vsnprintf(const_cast<char *>(result.c_str()), count + 1, fmt, ap2) < 0) {
    return "unable to format message";
  }

  return result;
}

static std::string getFormattedString(const char* fmt, ...) {
  va_list ap;
  va_start(ap, fmt);
  std::string result = getFormattedString(fmt, ap);
  va_end(ap);
  return result;
}

static void usage(int exitCode=1) {
  int optionWidth = 20;
  fprintf(stderr, "Usage: %s ninja build [options] [<targets>...]\n",
          getProgramName());
  fprintf(stderr, "\nOptions:\n");
  fprintf(stderr, "  %-*s %s\n", optionWidth, "--help",
          "show this help message and exit");
  fprintf(stderr, "  %-*s %s\n", optionWidth, "--version",
          "print the Ninja compatible version number");
  fprintf(stderr, "  %-*s %s\n", optionWidth, "--simulate",
          "simulate the build, assuming commands succeed");
  fprintf(stderr, "  %-*s %s\n", optionWidth, "-C, --chdir <PATH>",
          "change directory to PATH before anything else");
  fprintf(stderr, "  %-*s %s\n", optionWidth, "--no-db",
          "do not persist build results");
  fprintf(stderr, "  %-*s %s\n", optionWidth, "--db <PATH>",
          "persist build results at PATH [default='build.db']");
  fprintf(stderr, "  %-*s %s\n", optionWidth, "-f <PATH>",
          "load the manifest at PATH [default='build.ninja']");
  fprintf(stderr, "  %-*s %s\n", optionWidth, "-k <N>",
          "keep building until N commands fail [default=1]");
  fprintf(stderr, "  %-*s %s\n", optionWidth, "-t, --tool <TOOL>",
          "run a ninja tool. use 'list' to list available tools.");
  fprintf(stderr, "  %-*s %s\n", optionWidth, "-j, --jobs <JOBS>",
          "number of jobs to build in parallel [default=cpu dependent]");
  fprintf(stderr, "  %-*s %s\n", optionWidth, "--no-regenerate",
          "disable manifest auto-regeneration");
  fprintf(stderr, "  %-*s %s\n", optionWidth, "--dump-graph <PATH>",
          "dump build graph to PATH in Graphviz DOT format");
  fprintf(stderr, "  %-*s %s\n", optionWidth, "--profile <PATH>",
          "write a build profile trace event file to PATH");
  fprintf(stderr, "  %-*s %s\n", optionWidth, "--strict",
          "use strict mode (no bug compatibility)");
  fprintf(stderr, "  %-*s %s\n", optionWidth, "--trace <PATH>",
          "trace build engine operation to PATH");
  fprintf(stderr, "  %-*s %s\n", optionWidth, "--quiet",
          "don't show information on executed commands");
  fprintf(stderr, "  %-*s %s\n", optionWidth, "-v, --verbose",
          "show full invocation for executed commands");
  fprintf(stderr, "  %-*s %s\n", optionWidth, "-l <N>",
          "start jobs only when load average is below N [not implemented]");
  fprintf(stderr, "  %-*s %s\n", optionWidth, "-d <TOOL>",
          "enable debugging tool TOOL. 'list' for available [not implemented]");
  ::exit(exitCode);
}

namespace {

/// The build execution queue manages the actual parallel execution of jobs
/// which have been enqueued as a result of command orocessing.
///
/// This queue guarantees serial execution when only configured with a single
/// lane.
struct BuildExecutionQueue {
  /// The number of lanes the queue was configured with.
  unsigned numLanes;

  /// A thread for each lane.
  std::vector<std::unique_ptr<std::thread>> lanes;

  /// The ready queue of jobs to execute.
  std::deque<std::function<void(unsigned)>> readyJobs;
  std::mutex readyJobsMutex;
  std::condition_variable readyJobsCondition;

  /// Use LIFO execution.
  bool useLIFO;

public:
  BuildExecutionQueue(unsigned numLanes, bool useLIFO)
      : numLanes(numLanes), useLIFO(useLIFO)
  {
    for (unsigned i = 0; i != numLanes; ++i) {
      lanes.push_back(std::unique_ptr<std::thread>(
                          new std::thread(
                              &BuildExecutionQueue::executeLane, this, i)));
    }
  }
  ~BuildExecutionQueue() {
    // Shut down the lanes.
    for (unsigned i = 0; i != numLanes; ++i) {
      addJob({});
    }
    for (unsigned i = 0; i != numLanes; ++i) {
      lanes[i]->join();
    }
  }

  void executeLane(unsigned laneNumber) {
    // oust execute items from the queue until shutdown.
    while (true) {
      // Take a job from the ready queue.
      std::function<void(unsigned)> job;
      {
        std::unique_lock<std::mutex> lock(readyJobsMutex);

        // While the queue is empty, wait for an item.
        while (readyJobs.empty()) {
          readyJobsCondition.wait(lock);
        }

        // Take an item according to the chosen policy.
        if (useLIFO) {
          job = readyJobs.back();
          readyJobs.pop_back();
        } else {
          job = readyJobs.front();
          readyJobs.pop_front();
        }
      }

      // If we got an empty job, the queue is shutting down.
      if (!job)
        break;

      // Process the job.
      job(laneNumber);
    }
  }

  void addJob(std::function<void(unsigned)> job) {
    std::lock_guard<std::mutex> guard(readyJobsMutex);
    readyJobs.push_back(job);
    readyJobsCondition.notify_one();
  }
};

/// Result value that is computed by the rules for input and command files.
class BuildValue {
private:
  // Copying and move assignment are disabled.
  BuildValue(const BuildValue&) LLBUILD_DELETED_FUNCTION;
  void operator=(const BuildValue&) LLBUILD_DELETED_FUNCTION;
  BuildValue &operator=(BuildValue&& rhs) LLBUILD_DELETED_FUNCTION;

public:
  static const int currentSchemaVersion = 3;

private:
  enum class BuildValueKind : uint32_t {
    /// A value produced by a existing input file.
    ExistingInput = 0,

    /// A value produced by a missing input file.
    MissingInput,

    /// A value produced by a successful command.
    SuccessfulCommand,

    /// A value produced by a failing command.
    FailedCommand,

    /// A value produced by a command that was not run.
    SkippedCommand,
  };

  /// The kind of value.
  BuildValueKind kind;

  /// The number of attached output infos.
  const uint32_t numOutputInfos = 0;

  union {
    /// The file info for the rule output, for existing inputs and successful
    /// commands with a single output.
    FileInfo asOutputInfo;

    /// The file info for successful commands with multiple outputs.
    FileInfo* asOutputInfos;
  } valueData;

  /// The command hash, for successful commands.
  uint64_t commandHash;

private:
  BuildValue() {}
  BuildValue(BuildValueKind kind)
    : kind(kind), numOutputInfos(0), commandHash(0)
  {
  }
  BuildValue(BuildValueKind kind, const FileInfo& outputInfo,
             uint64_t commandHash = 0)
    : kind(kind), numOutputInfos(1), commandHash(commandHash)
  {
    valueData.asOutputInfo = outputInfo;
  }
  BuildValue(BuildValueKind kind, const FileInfo* outputInfos,
             uint32_t numOutputInfos, uint64_t commandHash = 0)
    : kind(kind), numOutputInfos(numOutputInfos), commandHash(commandHash)
  {
    valueData.asOutputInfos = new FileInfo[numOutputInfos];
    for (uint32_t i = 0; i != numOutputInfos; ++i) {
      valueData.asOutputInfos[i] = outputInfos[i];
    }
  }

public:
  // Build values can only be moved via construction, not copied.
  BuildValue(BuildValue&& rhs) {
    memcpy(this, &rhs, sizeof(rhs));
    memset(&rhs, 0, sizeof(rhs));
  }
  ~BuildValue() {
    if (hasMultipleOutputs()) {
      delete[] valueData.asOutputInfos;
    }
  }

  static BuildValue makeExistingInput(const FileInfo& outputInfo) {
    return BuildValue(BuildValueKind::ExistingInput, outputInfo);
  }
  static BuildValue makeMissingInput() {
    return BuildValue(BuildValueKind::MissingInput);
  }
  static BuildValue makeSuccessfulCommand(const FileInfo& outputInfo,
                                          uint64_t commandHash) {
    return BuildValue(BuildValueKind::SuccessfulCommand, outputInfo,
                      commandHash);
  }
  static BuildValue makeSuccessfulCommand(const FileInfo* outputInfos,
                                          uint32_t numOutputInfos,
                                          uint64_t commandHash) {
    // This ctor function should only be used for multiple outputs.
    assert(numOutputInfos > 1);
    return BuildValue(BuildValueKind::SuccessfulCommand, outputInfos,
                      numOutputInfos, commandHash);
  }
  static BuildValue makeFailedCommand() {
    return BuildValue(BuildValueKind::FailedCommand);
  }
  static BuildValue makeSkippedCommand() {
    return BuildValue(BuildValueKind::SkippedCommand);
  }

  bool isExistingInput() const { return kind == BuildValueKind::ExistingInput; }
  bool isMissingInput() const { return kind == BuildValueKind::MissingInput; }
  bool isSuccessfulCommand() const {
    return kind == BuildValueKind::SuccessfulCommand;
  }
  bool isFailedCommand() const { return kind == BuildValueKind::FailedCommand; }
  bool isSkippedCommand() const {
    return kind == BuildValueKind::SkippedCommand;
  }

  bool hasMultipleOutputs() const {
    return numOutputInfos > 1;
  }

  unsigned getNumOutputs() const {
    assert((isExistingInput() || isSuccessfulCommand()) &&
           "invalid call for value kind");
    return numOutputInfos;
  }

  const FileInfo& getOutputInfo() const {
    assert((isExistingInput() || isSuccessfulCommand()) &&
           "invalid call for value kind");
    assert(!hasMultipleOutputs() &&
           "invalid call on result with multiple outputs");
    return valueData.asOutputInfo;
  }

  const FileInfo& getNthOutputInfo(unsigned n) const {
    assert((isExistingInput() || isSuccessfulCommand()) &&
           "invalid call for value kind");
    assert(n < getNumOutputs());
    if (hasMultipleOutputs()) {
      return valueData.asOutputInfos[n];
    } else {
      assert(n == 0);
      return valueData.asOutputInfo;
    }
  }

  uint64_t getCommandHash() const {
    assert(isSuccessfulCommand() && "invalid call for value kind");
    return commandHash;
  }

  static BuildValue fromValue(const core::ValueType& value) {
    BuildValue result;
    assert(value.size() >= sizeof(result));
    memcpy(&result, value.data(), sizeof(result));

    // If this result has multiple output values, deserialize them properly.
    if (result.numOutputInfos > 1) {
      assert(value.size() == (sizeof(result) +
                              result.numOutputInfos * sizeof(FileInfo)));
      result.valueData.asOutputInfos = new FileInfo[result.numOutputInfos];
      memcpy(result.valueData.asOutputInfos,
             value.data() + sizeof(result),
             result.numOutputInfos * sizeof(FileInfo));
    } else {
      assert(value.size() == sizeof(result));
    }

    return result;
  }

  core::ValueType toValue() {
    if (numOutputInfos > 1) {
      // FIXME: This could be packed one entry tighter.
      std::vector<uint8_t> result(sizeof(*this) +
                                  numOutputInfos * sizeof(FileInfo));
      memcpy(result.data(), this, sizeof(*this));
      memcpy(result.data() + sizeof(*this), valueData.asOutputInfos,
             numOutputInfos * sizeof(FileInfo));
      return result;
    } else {
      std::vector<uint8_t> result(sizeof(*this));
      memcpy(result.data(), this, sizeof(*this));
      return result;
    }
  }
};

struct NinjaBuildEngineDelegate : public core::BuildEngineDelegate {
  class BuildContext* context = nullptr;

  virtual core::Rule lookupRule(const core::KeyType& key) override;

  virtual void cycleDetected(const std::vector<core::Rule*>& items) override;

  virtual void error(const Twine& message) override;
};

/// Wrapper for information used during a single build.
class BuildContext {
public:
  /// The build engine delegate.
  NinjaBuildEngineDelegate delegate;

  /// The engine in use.
  core::BuildEngine engine;

  /// The Ninja manifest we are operating on.
  std::unique_ptr<ninja::Manifest> manifest;

  /// Whether commands should print status information.
  bool quiet = false;
  /// Whether the build is being "simulated", in which case commands won't be
  /// run and inputs will be assumed to exist.
  bool simulate = false;
  /// Whether to use strict mode.
  bool strict = false;
  /// Whether output should use verbose mode.
  bool verbose = false;
  /// The number of failed commands to tolerate, or 0 if unlimited
  unsigned numFailedCommandsToTolerate = 1;

  /// The build profile output file.
  FILE *profileFP = nullptr;

  /// Whether the build has been cancelled or not.
  std::atomic<bool> isCancelled{false};

  /// Whether the build was cancelled by SIGINT.
  std::atomic<bool> wasCancelledBySigint{false};

  /// The number of generated errors.
  std::atomic<unsigned> numErrors{0};
  /// The number of commands executed during the build
  unsigned numBuiltCommands{0};
  /// The number of output commands written, for numbering purposes.
  unsigned numOutputDescriptions{0};
  /// The number of failed commands.
  std::atomic<unsigned> numFailedCommands{0};

  /// @name Status Reporting Command Counts
  /// @{

  /// The number of commands being scanned.
  std::atomic<unsigned> numCommandsScanning{0};
  /// The number of commands that were up-to-date.
  std::atomic<unsigned> numCommandsUpToDate{0};
  /// The number of commands that have been completed.
  std::atomic<unsigned> numCommandsCompleted{0};
  /// The number of commands that were updated (started, but didn't actually run
  /// the command).
  std::atomic<unsigned> numCommandsUpdated{0};

  /// @}

  /// The status output object.
  CommandLineStatusOutput statusOutput;

  /// The serial queue we used to order output consistently.
  SerialQueue outputQueue;

  /// The limited queue we use to execute parallel jobs.
  std::unique_ptr<BuildExecutionQueue> jobQueue;

  /// The previous SIGINT handler.
  struct sigaction previousSigintHandler;

  /// The set of spawned processes to cancel when interrupted.
  std::unordered_set<pid_t> spawnedProcesses;
  std::mutex spawnedProcessesMutex;

  /// Low-level flag for when a SIGINT has been received.
  static std::atomic<bool> wasInterrupted;

  /// Pipe used to allow detection of signals.
  static int signalWatchingPipe[2];

  static void sigintHandler(int) {
    // Set the atomic interrupt flag.
    BuildContext::wasInterrupted = true;

    // Write to wake up the signal monitoring thread.
    char byte{};
    sys::write(signalWatchingPipe[1], &byte, 1);
  }

  void sendSignalToProcesses(int signal) {
    std::unique_lock<std::mutex> lock(spawnedProcessesMutex);

    for (pid_t pid: spawnedProcesses) {
      // We are killing the whole process group here, this depends on us
      // spawning each process in its own group earlier.
      ::kill(-pid, signal);
    }
  }

  /// Cancel the build in response to an interrupt event.
  void cancelBuildOnInterrupt() {
    sendSignalToProcesses(SIGINT);

    emitNote("cancelling build.");
    isCancelled = true;
    wasCancelledBySigint = true;

    // FIXME: In our model, we still wait for everything to terminate, which
    // means a process that refuses to respond to SIGINT will cause us to just
    // hang here. We should probably detect and report that and be willing to do
    // a hard kill at some point (for example, on the second interrupt).
  }

  /// Check if an interrupt has occurred.
  void checkForInterrupt() {
    // Save and clear the interrupt flag, atomically.
    bool wasInterrupted = BuildContext::wasInterrupted.exchange(false);

    // Process the interrupt flag, if present.
    if (wasInterrupted) {
      // Otherwise, process the interrupt.
      cancelBuildOnInterrupt();
    }
  }

  /// Thread function to wait for indications that signals have arrived and to
  /// process them.
  void signalWaitThread() {
    // Wait for signal arrival indications.
    while (true) {
      char byte;
      int res = sys::read(signalWatchingPipe[0], &byte, 1);

      // If nothing was read, the pipe has been closed and we should shut down.
      if (res == 0)
        break;

      // Otherwise, check if we were awoke because of an interrupt.
      checkForInterrupt();
    }

    // Shut down the pipe.
    sys::close(signalWatchingPipe[0]);
    signalWatchingPipe[0] = -1;
  }

public:
  BuildContext()
    : engine(delegate),
      isCancelled(false)
  {
    // Open the status output.
    std::string error;
    if (!statusOutput.open(&error)) {
      fprintf(stderr, "error: %s: unable to open output: %s\n",
              getProgramName(), error.c_str());
      exit(1);
    }

    // Register the context with the delegate.
    delegate.context = this;

    // Register an interrupt handler.
    struct sigaction action{};
    action.sa_handler = &BuildContext::sigintHandler;
    sigaction(SIGINT, &action, &previousSigintHandler);

    // Create a pipe and thread to watch for signals.
    assert(BuildContext::signalWatchingPipe[0] == -1 &&
           BuildContext::signalWatchingPipe[1] == -1);
    if (basic::sys::pipe(BuildContext::signalWatchingPipe) < 0) {
      perror("pipe");
    }
    new std::thread(&BuildContext::signalWaitThread, this);
  }

  ~BuildContext() {
    // Ensure the output queue is done.
    outputQueue.sync([] {});

    // Restore any previous SIGINT handler.
    sigaction(SIGINT, &previousSigintHandler, NULL);

    // Close the status output.
    std::string error;
    statusOutput.close(&error);

    // Close the signal watching pipe.
    sys::close(BuildContext::signalWatchingPipe[1]);
    signalWatchingPipe[1] = -1;
  }

  /// @name Diagnostics Output
  /// @{

  /// Emit a status line, which can be updated.
  ///
  /// This method should only be called from the outputQueue.
  void emitStatus(const char* fmt, ...) {
    va_list ap;
    va_start(ap, fmt);
    std::string message = getFormattedString(fmt, ap);
    va_end(ap);

    if (verbose) {
      statusOutput.writeText(message + "\n");
    } else {
      statusOutput.setOrWriteLine(message);
    }
  }

  /// Emit a diagnostic to the error stream.
  void emitDiagnostic(std::string kind, std::string message) {
    outputQueue.async([this, kind=std::move(kind), message=std::move(message)] {
        statusOutput.finishLine();
        fprintf(stderr, "%s: %s: %s\n", getProgramName(), kind.c_str(),
                message.c_str());
      });
  }

  /// Emit a diagnostic followed by a block of text, ensuring the text
  /// immediately follows the diagnostic.
  void emitDiagnosticAndText(std::string kind, std::string message,
                             std::string text) {
    outputQueue.async([this, kind=std::move(kind), message=std::move(message),
                       text=std::move(text)] {
        statusOutput.finishLine();
        fprintf(stderr, "%s: %s: %s\n", getProgramName(), kind.c_str(),
                message.c_str());
        fflush(stderr);
        fwrite(text.data(), text.size(), 1, stdout);
        fflush(stdout);
      });
  }

  /// Emit a block of text to the output.
  void emitText(std::string text) {
    outputQueue.async([this, text=std::move(text)] {
        statusOutput.finishLine();
        fwrite(text.data(), text.size(), 1, stdout);
        fflush(stdout);
      });
  }

  void emitText(const char* fmt, ...) {
    va_list ap;
    va_start(ap, fmt);
    emitText(getFormattedString(fmt, ap));
    va_end(ap);
  }

  void emitError(std::string&& message) {
    emitDiagnostic("error", std::move(message));
    ++numErrors;
  }

  void emitErrorAndText(std::string&& message, std::string&& text) {
    emitDiagnosticAndText("error", std::move(message), text);
    ++numErrors;
  }

  void emitError(const char* fmt, ...) {
    va_list ap;
    va_start(ap, fmt);
    emitError(getFormattedString(fmt, ap));
    va_end(ap);
  }

  void emitNote(std::string&& message) {
    emitDiagnostic("note", std::move(message));
  }

  void emitNote(const char* fmt, ...) {
    va_list ap;
    va_start(ap, fmt);
    emitNote(getFormattedString(fmt, ap));
    va_end(ap);
  }

  /// @}

  void reportMissingInput(const ninja::Node* node) {
    // We simply report the missing input here, the build will be cancelled when
    // a rule sees it missing.
    emitError("missing input '%s' and no rule to build it",
              node->getPath().c_str());
  }

  void incrementFailedCommands() {
    // Update our count of the number of failed commands.
    unsigned numFailedCommands = ++this->numFailedCommands;

    // Cancel the build, if the number of command failures exceeds the
    // number to continue past.
    if (numFailedCommandsToTolerate != 0 &&
        numFailedCommands == numFailedCommandsToTolerate) {
      emitError("stopping build due to command failures");
      isCancelled = true;
    }
  }
};

std::atomic<bool> BuildContext::wasInterrupted{false};
int BuildContext::signalWatchingPipe[2]{-1, -1};

class BuildManifestActions : public ninja::ManifestLoaderActions {
  BuildContext& context;
  ninja::ManifestLoader* loader = 0;
  unsigned numErrors = 0;
  unsigned maxErrors = 20;

private:
  virtual void initialize(ninja::ManifestLoader* loader) override {
    this->loader = loader;
  }

  virtual void error(std::string filename, std::string message,
                     const ninja::Token& at) override {
    if (numErrors++ >= maxErrors)
      return;

    util::emitError(filename, message, at, loader->getCurrentParser());
  }

  virtual bool readFileContents(const std::string& fromFilename,
                                const std::string& filename,
                                const ninja::Token* forToken,
                                std::unique_ptr<char[]>* data_out,
                                uint64_t* length_out) override {
    // Load the file contents and return if successful.
    std::string error;
    if (util::readFileContents(filename, data_out, length_out, &error))
      return true;

    // Otherwise, emit the error.
    ++numErrors;
    if (forToken) {
      util::emitError(fromFilename, error, *forToken,
                      loader->getCurrentParser());
    } else {
      context.emitError(std::move(error));
    }

    return false;
  };

public:
  BuildManifestActions(BuildContext& context) : context(context) {}

  unsigned getNumErrors() const { return numErrors; }
};

static core::Task*
buildCommand(BuildContext& context, ninja::Command* command) {
  struct NinjaCommandTask : core::Task {
    BuildContext& context;
    ninja::Command* command;

    /// If true, the command should be skipped (because of an error in an
    /// input).
    bool shouldSkip = false;

    /// If true, the command had a missing input (this implies ShouldSkip is
    /// true).
    bool hasMissingInput = false;

    /// If true, the command can be updated if the output is newer than all of
    /// the inputs.
    bool canUpdateIfNewer = true;

    /// Information on the prior command result, if present.
    bool hasPriorResult = false;
    uint64_t priorCommandHash;

    /// The timestamp of the most recently rebuilt input.
    FileTimestamp newestModTime{ 0, 0 };

    NinjaCommandTask(BuildContext& context, ninja::Command* command)
        : context(context), command(command) {
      // If this command uses discovered dependencies, we can never skip it (we
      // don't yet have a way to account for the discovered dependencies, or
      // preserve them if skipped).
      //
      // FIXME: We should support update-if-newer for commands with deps.
      if (command->getDepsStyle() != ninja::Command::DepsStyleKind::None)
        canUpdateIfNewer = false;
    }

    virtual void provideValue(core::BuildEngine& engine, uintptr_t inputID,
                              const core::ValueType& valueData) override {
      // Process the input value to see if we should skip this command.
      BuildValue value = BuildValue::fromValue(valueData);

      // All direct inputs to NinjaCommandTask objects should be singleton
      // values.
      assert(!value.hasMultipleOutputs());

      // If the value is not an existing input or a successful command, then we
      // shouldn't run this command.
      if (!value.isExistingInput() && !value.isSuccessfulCommand()) {
        shouldSkip = true;
        if (value.isMissingInput()) {
          hasMissingInput = true;

          context.reportMissingInput(command->getInputs()[inputID]);
        }
      } else {
        // Otherwise, track the information used to determine if we can just
        // update the command instead of running it.
        const FileInfo& outputInfo = value.getOutputInfo();

        // If there is a missing input file (from a successful command), we
        // always need to run the command.
        if (outputInfo.isMissing()) {
          canUpdateIfNewer = false;
        } else {
          // Otherwise, keep track of the newest input.
          if (outputInfo.modTime > newestModTime) {
            newestModTime = outputInfo.modTime;
          }
        }
      }
    }

    bool isImmediatelyCyclicInput(const ninja::Node* node) const {
      for (const auto* output: command->getOutputs())
        if (node == output)
          return true;
      return false;
    }

    void completeTask(BuildValue&& result, bool forceChange=false) {
      context.engine.taskIsComplete(this, result.toValue(), forceChange);
    }

    virtual void start(core::BuildEngine& engine) override {
      // If this is a phony rule, ignore any immediately cyclic dependencies in
      // non-strict mode, which are generated frequently by CMake, but can be
      // ignored by Ninja. See https://github.com/martine/ninja/issues/935.
      //
      // FIXME: Find a way to harden this more, or see if we can just get CMake
      // to fix it.
      bool isPhony = command->getRule() == context.manifest->getPhonyRule();

      // Request all of the explicit and implicit inputs (the only difference
      // between them is that implicit inputs do not appear in ${in} during
      // variable expansion, but that has already been performed).
      unsigned id = 0;
      for (auto it = command->explicitInputs_begin(),
             ie = command->explicitInputs_end(); it != ie; ++it, ++id) {
        if (!context.strict && isPhony && isImmediatelyCyclicInput(*it))
          continue;

        engine.taskNeedsInput(this, (*it)->getPath(), id);
      }
      for (auto it = command->implicitInputs_begin(),
             ie = command->implicitInputs_end(); it != ie; ++it, ++id) {
        if (!context.strict && isPhony && isImmediatelyCyclicInput(*it))
          continue;

        engine.taskNeedsInput(this, (*it)->getPath(), id);
      }

      // Request all of the order-only inputs.
      for (auto it = command->orderOnlyInputs_begin(),
             ie = command->orderOnlyInputs_end(); it != ie; ++it) {
        if (!context.strict && isPhony && isImmediatelyCyclicInput(*it))
          continue;

        engine.taskMustFollow(this, (*it)->getPath());
      }
    }

    virtual void providePriorValue(core::BuildEngine& engine,
                                   const core::ValueType& valueData) override {
      BuildValue value = BuildValue::fromValue(valueData);

      if (value.isSuccessfulCommand()) {
        hasPriorResult = true;
        priorCommandHash = value.getCommandHash();
      }
    }

    /// Compute the output result for the command.
    BuildValue computeCommandResult(uint64_t commandHash) const {
      unsigned numOutputs = command->getOutputs().size();
      if (numOutputs == 1) {
        return BuildValue::makeSuccessfulCommand(
            FileInfo::getInfoForPath(
                command->getOutputs()[0]->getPath()),
            commandHash);
      } else {
        std::vector<FileInfo> outputInfos(numOutputs);
        for (unsigned i = 0; i != numOutputs; ++i) {
          outputInfos[i] = FileInfo::getInfoForPath(
              command->getOutputs()[i]->getPath());
        }
        return BuildValue::makeSuccessfulCommand(outputInfos.data(), numOutputs,
                                                 commandHash);
      }
    }

    /// Check if it is legal to only update the result (versus rerunning)
    /// because the outputs are newer than all of the inputs.
    bool canUpdateIfNewerWithResult(const BuildValue& result) {
      assert(result.isSuccessfulCommand());

      // Check each output.
      for (unsigned i = 0, e = result.getNumOutputs(); i != e; ++i) {
        const FileInfo& outputInfo = result.getNthOutputInfo(i);

        // If the output is missing, we need to rebuild.
        if (outputInfo.isMissing())
          return false;

        // Check if the output is actually newer than the most recent input.
        //
        // In strict mode, we use a strict "newer-than" check here, to guarantee
        // correctness in the face of equivalent timestamps. This is
        // particularly important on OS X, which has a low resolution mtime.
        //
        // However, in non-strict mode, we need to be compatible with Ninja
        // here, because there are some very important uses cases where this
        // behavior is relied on. One major example is CMake's initial
        // configuration checks using Ninja -- if this is not in place, those
        // rules will try and rerun the generator of the "TRY_COMPILE" steps,
        // and will enter an infinite reconfiguration loop. See also:
        //
        // See: http://www.cmake.org/Bug/view.php?id=15456
        if (context.strict) {
          if (outputInfo.modTime <= newestModTime)
            return false;
        } else {
          if (outputInfo.modTime < newestModTime)
            return false;
        }
      }

      return true;
    }

    virtual void inputsAvailable(core::BuildEngine& engine) override {
      // If the build is cancelled, skip everything.
      if (context.isCancelled) {
        return completeTask(BuildValue::makeSkippedCommand());
      }

      // Ignore phony commands.
      //
      // FIXME: Is it right to bring this up-to-date when one of the inputs
      // indicated a failure? It probably doesn't matter.
      uint64_t commandHash = basic::hashString(command->getCommandString());
      if (command->getRule() == context.manifest->getPhonyRule()) {
        // Get the result.
        BuildValue result = computeCommandResult(commandHash);

        // If any output is missing, then we always want to force the change to
        // propagate.
        bool forceChange = false;
        for (unsigned i = 0, e = result.getNumOutputs(); i != e; ++i) {
            if (result.getNthOutputInfo(i).isMissing()) {
                forceChange = true;
                break;
            }
        }

        return completeTask(std::move(result), forceChange);
      }

      // If it is legal to simply update the command, then if the command output
      // exists and is newer than all of the inputs, don't actually run the
      // command (just bring it up-to-date).
      if (canUpdateIfNewer) {
        // If this isn't a generator command and its command hash differs, we
        // can't update it.
        if (!command->hasGeneratorFlag() &&
            (!hasPriorResult || priorCommandHash != commandHash))
          canUpdateIfNewer = false;

        if (canUpdateIfNewer) {
          BuildValue result = computeCommandResult(commandHash);

          if (canUpdateIfNewerWithResult(result)) {
            // Update the count of the number of commands which have been
            // updated without being rerun.
            ++context.numCommandsUpdated;

            return completeTask(std::move(result));
          }
        }
      }

      // Otherwise, actually run the command.

      ++context.numBuiltCommands;

      // If we are simulating the build, just print the description and
      // complete.
      if (context.simulate) {
        if (!context.quiet)
          writeDescription(context, command);
        return completeTask(BuildValue::makeSkippedCommand());
      }

      // If not simulating, but this command should be skipped, then do nothing.
      if (shouldSkip) {
        // If this command had a failed input, treat it as having failed.
        if (hasMissingInput) {
          context.emitError("cannot build '%s' due to missing input",
                            command->getOutputs()[0]->getPath().c_str());

          // Update the count of failed commands.
          context.incrementFailedCommands();
        }

        return completeTask(BuildValue::makeSkippedCommand());
      }
      assert(!hasMissingInput);

      // Otherwise, enqueue the job to run later.
      context.jobQueue->addJob([&] (unsigned bucket) {
          // Suppress static analyzer false positive on generalized lambda capture
          // (rdar://problem/22165130).
#ifndef __clang_analyzer__
          // Take care to not rely on the ``this`` object, which may disappear
          // before the queue executes this block.
          BuildContext& localContext(context);
          ninja::Command* localCommand(command);

          if (localContext.profileFP) {
            localContext.outputQueue.sync(
              [&localContext=localContext, localCommand=localCommand, bucket] {
                uint64_t startTime = getTimeInMicroseconds();
                fprintf(localContext.profileFP,
                        ("{ \"name\": \"%s\", \"ph\": \"B\", \"pid\": 0, "
                         "\"tid\": %d, \"ts\": %llu},\n"),
                        localCommand->getEffectiveDescription().c_str(), bucket,
                        static_cast<unsigned long long>(startTime));
              });
          }

          executeCommand();

          if (localContext.profileFP) {
            localContext.outputQueue.sync(
              [&localContext=localContext, localCommand=localCommand, bucket] {
                uint64_t endTime = getTimeInMicroseconds();
                fprintf(localContext.profileFP,
                        ("{ \"name\": \"%s\", \"ph\": \"E\", \"pid\": 0, "
                         "\"tid\": %d, \"ts\": %llu},\n"),
                        localCommand->getEffectiveDescription().c_str(), bucket,
                        static_cast<unsigned long long>(endTime));
              });
          }
#endif
        });
    }

    static unsigned getNumPossibleMaxCommands(BuildContext& context) {
      // Compute the "possible" number of maximum commands that will be
      // run. This is only the "possible" max because we can start running
      // commands before dependency scanning is complete -- we include the
      // number of commands that are being scanned so that this number will
      // always be greater than the number of commands that have been executed
      // until the very last command is run.
      int totalPossibleMaxCommands =
        context.numCommandsCompleted + context.numCommandsScanning;

      // Compute the number of max commands to show, subtracting out all the
      // commands that we avoided running.
      int possibleMaxCommands = totalPossibleMaxCommands -
        (context.numCommandsUpToDate + context.numCommandsUpdated);

      return possibleMaxCommands;
    }

    static void writeDescription(BuildContext& context,
                                 ninja::Command* command) {
      const std::string& description =
        context.verbose ? command->getCommandString() :
        command->getEffectiveDescription();
      context.emitStatus(
          "[%d/%d] %s", ++context.numOutputDescriptions,
          getNumPossibleMaxCommands(context), description.c_str());

      // Whenever we write a description for a console job, make sure to finish
      // the output under the expectation that the console job might write to
      // the output. We don't make any attempt to lock this in case the console
      // job can run concurrently with anything else.
      if (command->getExecutionPool() == context.manifest->getConsolePool())
        context.statusOutput.finishLine();
    }

    void executeCommand() {
      // If the build is cancelled, skip the job.
      if (context.isCancelled) {
        return completeTask(BuildValue::makeSkippedCommand());
      }

      // Write the description on the output queue, taking care to not rely on
      // the ``this`` object, which may disappear before the queue executes this
      // block.
      if (!context.quiet) {
          // Suppress static analyzer false positive on generalized lambda capture
          // (rdar://problem/22165130).
#ifndef __clang_analyzer__
        // If this is a console job, do the write synchronously to ensure it
        // appears before the task might start.
        if (command->getExecutionPool() == context.manifest->getConsolePool()) {
          context.outputQueue.sync([&context=context, command=command] {
              writeDescription(context, command);
            });
        } else {
          context.outputQueue.async([&context=context, command=command] {
              writeDescription(context, command);
            });
        }
#endif
      }

      // Actually run the command.
      if (!spawnAndWaitForCommand()) {
        // If the command failed, complete the task with the failed result and
        // always propagate.
        return completeTask(BuildValue::makeFailedCommand(),
                            /*ForceChange=*/true);
      }

      // Otherwise, the command succeeded so process the dependencies.
      if (!processDiscoveredDependencies()) {
        context.incrementFailedCommands();
        return completeTask(BuildValue::makeFailedCommand(),
                            /*ForceChange=*/true);
      }

      // Complete the task with a successful value.
      //
      // We always restat the output, but we honor Ninja's restat flag by
      // forcing downstream propagation if it isn't set.
      uint64_t commandHash = basic::hashString(command->getCommandString());
      BuildValue result = computeCommandResult(commandHash);
      return completeTask(std::move(result),
                          /*ForceChange=*/!command->hasRestatFlag());
    }

    /// Execute the command process and wait for it to complete.
    ///
    /// \returns True if the command succeeded.
    bool spawnAndWaitForCommand() const {
      bool isConsole = command->getExecutionPool() ==
        context.manifest->getConsolePool();

      // Initialize the spawn attributes.
      posix_spawnattr_t attributes;
      posix_spawnattr_init(&attributes);

      // Unmask all signals
      sigset_t noSignals;
      sigemptyset(&noSignals);
      posix_spawnattr_setsigmask(&attributes, &noSignals);

      // Reset all signals to default behavior.
      //
      // On Linux, this can only be used to reset signals that are legal to
      // modify, so we have to take care about the set we use.
#if defined(__linux__)
      sigset_t mostSignals;
      sigemptyset(&mostSignals);
      for (int i = 1; i < SIGSYS; ++i) {
        if (i == SIGKILL || i == SIGSTOP) continue;
        sigaddset(&mostSignals, i);
      }
      posix_spawnattr_setsigdefault(&attributes, &mostSignals);
#else
      sigset_t mostSignals;
      sigfillset(&mostSignals);
      sigdelset(&mostSignals, SIGKILL);
      sigdelset(&mostSignals, SIGSTOP);
      posix_spawnattr_setsigdefault(&attributes, &mostSignals);
#endif

      // Establish a separate process group.
      posix_spawnattr_setpgroup(&attributes, 0);

      // Set the attribute flags.
      unsigned flags = POSIX_SPAWN_SETSIGMASK | POSIX_SPAWN_SETSIGDEF;
      if (!isConsole)
        flags |= POSIX_SPAWN_SETPGROUP;

      // Close all other files by default.
      //
      // FIXME: This is an Apple-specific extension, and we will have to do
      // something else on other platforms (and unfortunately, there isn't
      // really an easy answer other than using a stub executable).
#ifdef __APPLE__
      flags |= POSIX_SPAWN_CLOEXEC_DEFAULT;
#endif

      posix_spawnattr_setflags(&attributes, flags);

      // Setup the file actions.
      posix_spawn_file_actions_t fileActions;
      posix_spawn_file_actions_init(&fileActions);

      // Open /dev/null as stdin.
      posix_spawn_file_actions_addopen(
        &fileActions, 0, "/dev/null", O_RDONLY, 0);

      // Create a pipe to use to read the command output, if necessary.
      int pipe[2]{ -1, -1 };
      if (!isConsole) {
        if (basic::sys::pipe(pipe) < 0) {
          context.emitError("unable to create command pipe (%s)",
                            strerror(errno));
          return false;
        }

        // Open the write end of the pipe as stdout and stderr.
        posix_spawn_file_actions_adddup2(&fileActions, pipe[1], 1);
        posix_spawn_file_actions_adddup2(&fileActions, pipe[1], 2);

        // Close the read and write ends of the pipe.
        posix_spawn_file_actions_addclose(&fileActions, pipe[0]);
        posix_spawn_file_actions_addclose(&fileActions, pipe[1]);
      } else {
        // Otherwise, propagate the current stdout/stderr.
        posix_spawn_file_actions_adddup2(&fileActions, 1, 1);
        posix_spawn_file_actions_adddup2(&fileActions, 2, 2);
      }

      // Spawn the command.
      const char* args[4];
      args[0] = "/bin/sh";
      args[1] = "-c";
      args[2] = command->getCommandString().c_str();
      args[3] = nullptr;

      // Spawn the command.
      pid_t pid;
      {
        // We need to hold the spawn processes lock when we spawn, to ensure that
        // we don't create a process in between when we are cancelled.
        std::lock_guard<std::mutex> guard(context.spawnedProcessesMutex);

        if (posix_spawn(&pid, args[0], /*file_actions=*/&fileActions,
                        /*attrp=*/&attributes, const_cast<char**>(args),
                        environ) != 0) {
          context.emitError("unable to spawn process (%s)", strerror(errno));
          return false;
        }

        // The console process will get interrupted automatically.
        if (!isConsole)
          context.spawnedProcesses.insert(pid);
      }

      posix_spawn_file_actions_destroy(&fileActions);
      posix_spawnattr_destroy(&attributes);

      // Read the command output, if buffering.
      SmallString<1024> outputData;
      if (!isConsole) {
        // Close the write end of the output pipe.
        ::close(pipe[1]);

        // Read all the data from the output pipe.
        while (true) {
          char buf[4096];
          ssize_t numBytes = read(pipe[0], buf, sizeof(buf));
          if (numBytes < 0) {
            context.emitError("unable to read from output pipe (%s)",
                              strerror(errno));
            break;
          }

          if (numBytes == 0)
            break;

          outputData.insert(outputData.end(), &buf[0], &buf[numBytes]);
        }

        // Close the read end of the pipe.
        ::close(pipe[0]);
      }

      // Wait for the command to complete.
      int status, result = waitpid(pid, &status, 0);
      while (result == -1 && errno == EINTR)
        result = waitpid(pid, &status, 0);

      // Update the set of spawned processes.
      {
        std::lock_guard<std::mutex> guard(context.spawnedProcessesMutex);
        context.spawnedProcesses.erase(pid);
      }

      if (result == -1) {
        context.emitError("unable to wait for process (%s)", strerror(errno));
      }

      // If the build has been interrupted, return without writing any output or
      // command status (since they will also have been interrupted).
      if (context.isCancelled && context.wasCancelledBySigint) {
        // We still return an accurate status just in case the command actually
        // completed successfully.
        return status == 0;
      }

      // If the child failed, show the full command and the output.
      if (status != 0) {
        // If the process was killed by SIGINT, assume it is because we were
        // interrupted.
        bool cancelled = WIFSIGNALED(status) && (WTERMSIG(status) == SIGINT || WTERMSIG(status) == SIGKILL);
        if (cancelled)
          return false;

        // Otherwise, report the failure.
        context.emitErrorAndText(
            getFormattedString(
                "process failed: %s", command->getCommandString().c_str()),
            std::string(outputData.data(), outputData.size()));

        // Update the count of failed commands.
        context.incrementFailedCommands();

        return false;
      } else {
        // Write the output data, if buffered.
        if (!outputData.empty()) {
          context.emitText(std::string(outputData.data(), outputData.size()));
        }
      }

      return true;
    }

    bool processDiscoveredDependencies() {
      // Process the discovered dependencies, if used.
      switch (command->getDepsStyle()) {
      case ninja::Command::DepsStyleKind::None:
        return true;
      case ninja::Command::DepsStyleKind::MSVC: {
        context.emitError("MSVC style dependencies are unsupported");
        return false;
      }
      case ninja::Command::DepsStyleKind::GCC: {
        // Read the dependencies file.
        std::string error;
        std::unique_ptr<char[]> data;
        uint64_t length;
        if (!util::readFileContents(command->getDepsFile(), &data, &length,
                                    &error)) {
          // If the file is missing, just ignore it for consistency with Ninja
          // (when using stored deps) in non-strict mode.
          if (!context.strict)
            return true;

          // FIXME: Error handling.
          context.emitError("unable to read dependency file: %s (%s)",
                  command->getDepsFile().c_str(), error.c_str());
          return false;
        }

        // Parse the output.
        //
        // We just ignore the rule, and add any dependency that we encounter in
        // the file.
        struct DepsActions : public core::MakefileDepsParser::ParseActions {
          BuildContext& context;
          NinjaCommandTask* task;
          const std::string& path;
          unsigned numErrors{0};

          DepsActions(BuildContext& context, NinjaCommandTask* task,
                      const std::string& path)
            : context(context), task(task), path(path) {}

          virtual void error(const char* message, uint64_t position) override {
            context.emitError(
                "error reading dependency file: %s (%s) at offset %u",
                path.c_str(), message, unsigned(position));
            ++numErrors;
          }

          virtual void actOnRuleDependency(const char* dependency,
                                           uint64_t length,
                                           const StringRef
                                             unescapedWord) override {
            context.engine.taskDiscoveredDependency(task, unescapedWord);
          }

          virtual void actOnRuleStart(const char* name, uint64_t length,
                                      const StringRef unescapedWord) override {}
          virtual void actOnRuleEnd() override {}
        };

        DepsActions actions(context, this, command->getDepsFile());
        core::MakefileDepsParser(data.get(), length, actions).parse();
        return actions.numErrors == 0;
      }
      }

      assert(0 && "unexpected case");
      return false;
    }
  };

  return context.engine.registerTask(new NinjaCommandTask(context, command));
}

static core::Task* buildInput(BuildContext& context, ninja::Node* input) {
  struct NinjaInputTask : core::Task {
    BuildContext& context;
    ninja::Node* node;

    NinjaInputTask(BuildContext& context, ninja::Node* node)
        : context(context), node(node) { }

    virtual void provideValue(core::BuildEngine& engine, uintptr_t inputID,
                              const core::ValueType& value) override { }

    virtual void start(core::BuildEngine& engine) override { }

    virtual void inputsAvailable(core::BuildEngine& engine) override {
      if (context.simulate) {
        engine.taskIsComplete(
          this, BuildValue::makeExistingInput({}).toValue());
        return;
      }

      auto outputInfo = FileInfo::getInfoForPath(node->getPath());
      if (outputInfo.isMissing()) {
        engine.taskIsComplete(this, BuildValue::makeMissingInput().toValue());
        return;
      }

      engine.taskIsComplete(
        this, BuildValue::makeExistingInput(outputInfo).toValue());
    }
  };

  return context.engine.registerTask(new NinjaInputTask(context, input));
}

static core::Task*
buildTargets(BuildContext& context,
             const std::vector<std::string>& targetsToBuild) {
  struct TargetsTask : core::Task {
    BuildContext& context;
    std::vector<std::string> targetsToBuild;

    TargetsTask(BuildContext& context,
                const std::vector<std::string>& targetsToBuild)
        : context(context), targetsToBuild(targetsToBuild) { }

    virtual void provideValue(core::BuildEngine& engine, uintptr_t inputID,
                              const core::ValueType& valueData) override {
      BuildValue value = BuildValue::fromValue(valueData);

      if (value.isMissingInput()) {
        context.emitError("unknown target '%s'",
                          targetsToBuild[inputID].c_str());
      }
    }

    virtual void start(core::BuildEngine& engine) override {
      // Request all of the targets.
      unsigned id = 0;
      for (const auto& target: targetsToBuild) {
        engine.taskNeedsInput(this, target, id++);
      }
    }

    virtual void inputsAvailable(core::BuildEngine& engine) override {
      // Complete the job.
      engine.taskIsComplete(
        this, BuildValue::makeSuccessfulCommand({}, 0).toValue());
      return;
    }
  };

  return context.engine.registerTask(new TargetsTask(context, targetsToBuild));
}

static core::Task*
selectCompositeBuildResult(BuildContext& context, ninja::Command* command,
                           unsigned inputIndex,
                           const core::KeyType& compositeRuleName) {
  struct SelectResultTask : core::Task {
    const BuildContext& context;
    const ninja::Command* command;
    const unsigned inputIndex;
    const core::KeyType compositeRuleName;
    const core::ValueType *compositeValueData = nullptr;

    SelectResultTask(BuildContext& context, ninja::Command* command,
                     unsigned inputIndex,
                     const core::KeyType& compositeRuleName)
        : context(context), command(command),
          inputIndex(inputIndex), compositeRuleName(compositeRuleName) { }

    virtual void start(core::BuildEngine& engine) override {
      // Request the composite input.
      engine.taskNeedsInput(this, compositeRuleName, 0);
    }

    virtual void provideValue(core::BuildEngine& engine, uintptr_t inputID,
                              const core::ValueType& valueData) override {
      compositeValueData = &valueData;
    }

    virtual void inputsAvailable(core::BuildEngine& engine) override {
      // Construct the appropriate build value from the result.
      assert(compositeValueData);
      BuildValue value(BuildValue::fromValue(*compositeValueData));

      // If the input was a failed or skipped command, propagate that result.
      if (value.isFailedCommand() || value.isSkippedCommand()) {
        engine.taskIsComplete(this, value.toValue(), /*ForceChange=*/true);
      } else {
        // FIXME: We don't try and set this in response to the restat flag on
        // the incoming command, because it doesn't generally work -- the output
        // will just honor update-if-newer and still not run. We need to move to
        // a different model for handling restat = 0 to get this to work
        // properly.
        bool forceChange = false;

        // Otherwise, the value should be a successful command with file info
        // for each output.
        assert(value.isSuccessfulCommand() && value.hasMultipleOutputs() &&
               inputIndex < value.getNumOutputs());

        // The result is the InputIndex-th element, and the command hash is
        // propagated.
        engine.taskIsComplete(
          this, BuildValue::makeSuccessfulCommand(
            value.getNthOutputInfo(inputIndex),
            value.getCommandHash()).toValue(),
          forceChange);
      }
    }
  };

  return context.engine.registerTask(
    new SelectResultTask(context, command, inputIndex, compositeRuleName));
}

static bool buildInputIsResultValid(ninja::Node* node,
                                    const core::ValueType& valueData) {
  BuildValue value = BuildValue::fromValue(valueData);

  // If the prior value wasn't for an existing input, recompute.
  if (!value.isExistingInput())
    return false;

  // Otherwise, the result is valid if the path exists and the hash has not
  // changed.
  //
  // FIXME: This is inefficient, we will end up doing the stat twice, once when
  // we check the value for up to dateness, and once when we "build" the output.
  //
  // We can solve this by caching ourselves but I wonder if it is something the
  // engine should support more naturally.
  auto info = FileInfo::getInfoForPath(node->getPath());
  if (info.isMissing())
    return false;

  return value.getOutputInfo() == info;
}

static bool buildCommandIsResultValid(ninja::Command* command,
                                      const core::ValueType& valueData) {
  BuildValue value = BuildValue::fromValue(valueData);

  // If the prior value wasn't for a successful command, recompute.
  if (!value.isSuccessfulCommand())
    return false;

  // For non-generator commands, if the command hash has changed, recompute.
  if (!command->hasGeneratorFlag()) {
    if (value.getCommandHash() != basic::hashString(
          command->getCommandString()))
      return false;
  }

  // Check the timestamps on each of the outputs.
  for (unsigned i = 0, e = command->getOutputs().size(); i != e; ++i) {
    // Always rebuild if the output is missing.
    auto info = FileInfo::getInfoForPath(command->getOutputs()[i]->getPath());
    if (info.isMissing())
      return false;

    // Otherwise, the result is valid if file information has not changed.
    //
    // Note that we may still decide not to actually run the command based on
    // the update-if-newer handling, but it does require running the task.
    if (value.getNthOutputInfo(i) != info)
      return false;
  }

  return true;
}

static bool selectCompositeIsResultValid(ninja::Command* command,
                                         const core::ValueType& valueData) {
  BuildValue value = BuildValue::fromValue(valueData);

  // If the prior value wasn't for a successful command, recompute.
  if (!value.isSuccessfulCommand())
    return false;

  // If the command's signature has changed since it was built, rebuild. This is
  // important for ensuring that we properly reevaluate the select rule when
  // it's incoming composite rule no longer exists.
  if (value.getCommandHash() != basic::hashString(command->getCommandString()))
    return false;

  // Otherwise, this result is always valid.
  return true;
}

static void updateCommandStatus(BuildContext& context,
                                ninja::Command* command,
                                core::Rule::StatusKind status) {
  // Ignore phony rules.
  if (command->getRule() == context.manifest->getPhonyRule())
    return;

  // Track the number of commands which are currently being scanned along with
  // the total number of completed commands.
  if (status == core::Rule::StatusKind::IsScanning) {
    ++context.numCommandsScanning;
  } else if (status == core::Rule::StatusKind::IsUpToDate) {
    --context.numCommandsScanning;
    ++context.numCommandsUpToDate;
    ++context.numCommandsCompleted;
  } else {
    assert(status == core::Rule::StatusKind::IsComplete);
    --context.numCommandsScanning;
    ++context.numCommandsCompleted;
  }
}

core::Rule NinjaBuildEngineDelegate::lookupRule(const core::KeyType& key) {
  // We created rules for all of the commands up front, so if we are asked for a
  // rule here it is because we are looking for an input.

  // Get the node for this input.
  //
  // FIXME: This is frequently a redundant lookup, given that the caller might
  // well have had the Node* available. This is something that would be nice
  // to avoid when we support generic key types.
  ninja::Node* node = context->manifest->getOrCreateNode(key);

  return core::Rule{
    node->getPath(),
      [&, node] (core::BuildEngine&) {
      return buildInput(*context, node);
    },
    [&, node] (core::BuildEngine&, const core::Rule&,
               const core::ValueType& value) {
      // If simulating, assume cached results are valid.
      if (context->simulate)
        return true;

      return buildInputIsResultValid(node, value);
    } };
}

void NinjaBuildEngineDelegate::cycleDetected(
    const std::vector<core::Rule*>& cycle) {
  // Report the cycle.
  std::stringstream message;
  message << "cycle detected among targets:";
  bool first = true;
  for (const auto* rule: cycle) {
    if (!first)
      message << " ->";
    message << " \"" << rule->key << '"';
    first = false;
  }

  context->emitError(message.str());

  // Cancel the build.
  context->isCancelled = true;
}

void NinjaBuildEngineDelegate::error(const Twine& message) {
  // Report the error.
  context->emitError("error: " + message.str());

  // Cancel the build.
  context->isCancelled = true;
}

}

int commands::executeNinjaBuildCommand(std::vector<std::string> args) {
  std::string chdirPath = "";
  std::string customTool = "";
  std::string dbFilename = "build.db";
  std::string dumpGraphPath, profileFilename, traceFilename;
  std::string manifestFilename = "build.ninja";

  // Create a context for the build.
  bool autoRegenerateManifest = true;
  bool quiet = false;
  bool simulate = false;
  bool strict = false;
  bool verbose = false;
  unsigned numJobsInParallel = 0;
  unsigned numFailedCommandsToTolerate = 1;
  double maximumLoadAverage = 0.0;
  std::vector<std::string> debugTools;

  while (!args.empty() && args[0][0] == '-') {
    const std::string option = args[0];
    args.erase(args.begin());

    if (option == "--")
      break;

    if (option == "--version") {
      // Report a fake version for tools (like CMake) that detect compatibility
      // based on the 'Ninja' version.
      printf("1.5 Ninja Compatible (%s)\n", getLLBuildFullVersion().c_str());
      return 0;
    } else if (option == "--help") {
      usage(/*exitCode=*/0);
    } else if (option == "--simulate") {
      simulate = true;
    } else if (option == "--quiet") {
      quiet = true;
    } else if (option == "-C" || option == "--chdir") {
      if (args.empty()) {
        fprintf(stderr, "%s: error: missing argument to '%s'\n\n",
                getProgramName(), option.c_str());
        usage();
      }
      chdirPath = args[0];
      args.erase(args.begin());
    } else if (option == "--no-db") {
      dbFilename = "";
    } else if (option == "--db") {
      if (args.empty()) {
        fprintf(stderr, "%s: error: missing argument to '%s'\n\n",
                getProgramName(), option.c_str());
        usage();
      }
      dbFilename = args[0];
      args.erase(args.begin());
    } else if (option == "--dump-graph") {
      if (args.empty()) {
        fprintf(stderr, "%s: error: missing argument to '%s'\n\n",
                getProgramName(), option.c_str());
        usage();
      }
      dumpGraphPath = args[0];
      args.erase(args.begin());
    } else if (option == "-f") {
      if (args.empty()) {
        fprintf(stderr, "%s: error: missing argument to '%s'\n\n",
                getProgramName(), option.c_str());
        usage();
      }
      manifestFilename = args[0];
      args.erase(args.begin());
    } else if (option == "-k") {
      if (args.empty()) {
        fprintf(stderr, "%s: error: missing argument to '%s'\n\n",
                getProgramName(), option.c_str());
        usage();
      }
      char *end;
      numFailedCommandsToTolerate = ::strtol(args[0].c_str(), &end, 10);
      if (*end != '\0') {
          fprintf(stderr, "%s: error: invalid argument '%s' to '%s'\n\n",
                  getProgramName(), args[0].c_str(), option.c_str());
          usage();
      }
      args.erase(args.begin());
    } else if (option == "-l") {
      if (args.empty()) {
        fprintf(stderr, "%s: error: missing argument to '%s'\n\n",
                getProgramName(), option.c_str());
        usage();
      }
      char *end;
      maximumLoadAverage = ::strtod(args[0].c_str(), &end);
      if (*end != '\0') {
          fprintf(stderr, "%s: error: invalid argument '%s' to '%s'\n\n",
                  getProgramName(), args[0].c_str(), option.c_str());
          usage();
      }
      args.erase(args.begin());
    } else if (option == "-j" || option == "--jobs") {
      if (args.empty()) {
        fprintf(stderr, "%s: error: missing argument to '%s'\n\n",
                getProgramName(), option.c_str());
        usage();
      }
      char *end;
      numJobsInParallel = ::strtol(args[0].c_str(), &end, 10);
      if (*end != '\0') {
          fprintf(stderr, "%s: error: invalid argument '%s' to '%s'\n\n",
                  getProgramName(), args[0].c_str(), option.c_str());
          usage();
      }
      args.erase(args.begin());
    } else if (StringRef(option).startswith("-j")) {
      char *end;
      numJobsInParallel = ::strtol(&option[2], &end, 10);
      if (*end != '\0') {
          fprintf(stderr, "%s: error: invalid argument '%s' to '-j'\n\n",
                  getProgramName(), &option[2]);
          usage();
      }
    } else if (option == "--no-regenerate") {
      autoRegenerateManifest = false;
    } else if (option == "--profile") {
      if (args.empty()) {
        fprintf(stderr, "%s: error: missing argument to '%s'\n\n",
                getProgramName(), option.c_str());
        usage();
      }
      profileFilename = args[0];
      args.erase(args.begin());
    } else if (option == "--strict") {
      strict = true;
    } else if (option == "-t" || option == "--tool") {
      if (args.empty()) {
        fprintf(stderr, "%s: error: missing argument to '%s'\n\n",
                getProgramName(), option.c_str());
        usage();
      }
      customTool = args[0];
      args.erase(args.begin());
    } else if (option == "-d") {
      if (args.empty()) {
        fprintf(stderr, "%s: error: missing argument to '%s'\n\n",
                getProgramName(), option.c_str());
        usage();
      }
      debugTools.push_back(args[0]);
      args.erase(args.begin());
    } else if (option == "--trace") {
      if (args.empty()) {
        fprintf(stderr, "%s: error: missing argument to '%s'\n\n",
                getProgramName(), option.c_str());
        usage();
      }
      traceFilename = args[0];
      args.erase(args.begin());
    } else if (option == "-v" || option == "--verbose") {
      verbose = true;
    } else {
      fprintf(stderr, "%s: error: invalid option: '%s'\n\n",
              getProgramName(), option.c_str());
      usage();
    }
  }

  if (maximumLoadAverage > 0.0) {
    fprintf(stderr, "%s: warning: maximum load average %.8g not implemented\n",
            getProgramName(), maximumLoadAverage);
  }

  if (!debugTools.empty()) {
    fprintf(stderr, "%s: warning: debug tools not implemented\n",
            getProgramName());
  }

  // Honor the --chdir option, if used.
  if (!chdirPath.empty()) {
    if (!sys::chdir(chdirPath.c_str())) {
      fprintf(stderr, "%s: error: unable to honor --chdir: %s\n",
              getProgramName(), strerror(errno));
      return 1;
    }

    // Print a message about the changed directory. The exact format here is
    // important, it is recognized by other tools (like Emacs).
    fprintf(stdout, "%s: Entering directory `%s'\n", getProgramName(),
            chdirPath.c_str());
    fflush(stdout);
  }

  if (!customTool.empty()) {
    std::vector<std::string> availableTools = {
      "targets",
      "list",
    };

    if (std::find(availableTools.begin(), availableTools.end(), customTool) ==
        availableTools.end()) {
        fprintf(stderr, "error: unknown tool '%s'\n", customTool.c_str());
        return 1;
    } else if (customTool == "list") {
      if (!args.empty()) {
        fprintf(stderr, "error: unsupported arguments to tool '%s'\n",
                customTool.c_str());
        return 1;
      }

      fprintf(stdout, "available ninja tools:\n");
      for (const auto& tool: availableTools) {
        fprintf(stdout, "  %s\n", tool.c_str());
      }

      return 0;
    }
  }

  // Run up to two iterations, the first one loads the manifest and rebuilds it
  // if necessary, the second only runs if the manifest needs to be reloaded.
  //
  // This is somewhat inefficient in the case where the manifest needs to be
  // reloaded (we reopen the database, for example), but we don't expect that to
  // be a common case spot in practice.
  for (int iteration = 0; iteration != 2; ++iteration) {
    BuildContext context;

    context.numFailedCommandsToTolerate = numFailedCommandsToTolerate;
    context.quiet = quiet;
    context.simulate = simulate;
    context.strict = strict;
    context.verbose = verbose;

    // Create the job queue to use.
    //
    // When running in parallel, we use a LIFO queue to work around the default
    // traversal of the BuildEngine tending to build in BFS order. This is
    // generally at avoiding clustering of links.
    //
    // FIXME: Do a serious analysis of scheduling, including ideally an active
    // scheduler in the execution queue.
    if (numJobsInParallel == 0) {
      unsigned numCPUs = std::thread::hardware_concurrency();
      if (numCPUs == 0) {
        context.emitError("unable to detect number of CPUs (%s)",
                          strerror(errno));
        return 1;
      }

      numJobsInParallel = numCPUs + 2;
    }
    bool useLIFO = (numJobsInParallel > 1);
    context.jobQueue.reset(new BuildExecutionQueue(numJobsInParallel, useLIFO));

    // Load the manifest.
    BuildManifestActions actions(context);
    ninja::ManifestLoader loader(manifestFilename, actions);
    context.manifest = loader.load();

    // If there were errors loading, we are done.
    if (unsigned numErrors = actions.getNumErrors()) {
      context.emitNote("%d errors generated.", numErrors);
      return 1;
    }

    // Run the targets tool, if specified.
    if (!customTool.empty() && customTool == "targets") {
      if (args.size() != 1 || args[0] != "all") {
        if (args.empty()) {
          context.emitError("unsupported arguments to tool '%s'",
                            customTool.c_str());
        } else {
          context.emitError("unsupported argument to tool '%s': '%s'",
                            customTool.c_str(), args[0].c_str());
        }
        return 1;
      }

      for (const auto command: context.manifest->getCommands()) {
        for (const auto& output: command->getOutputs()) {
          fprintf(stdout, "%s: %s\n", output->getPath().c_str(),
                  command->getRule()->getName().c_str());
        }
      }

      return 0;
    }

    // Otherwise, run the build.

    // Parse the positional arguments.
    std::vector<std::string> targetsToBuild(args);

    // Attach the database, if requested.
    if (!dbFilename.empty()) {
      std::string error;
      std::unique_ptr<core::BuildDB> db(
        core::createSQLiteBuildDB(dbFilename,
                                  BuildValue::currentSchemaVersion,
                                  &error));
      if (!db || !context.engine.attachDB(std::move(db), &error)) {
        context.emitError("unable to open build database: %s", error.c_str());
        return 1;
      }
    }

    // Enable tracing, if requested.
    if (!traceFilename.empty()) {
      std::string error;
      if (!context.engine.enableTracing(traceFilename, &error)) {
        context.emitError("unable to enable tracing: %s", error.c_str());
        return 1;
      }
    }

    // Create rules for all of the build commands up front.
    //
    // FIXME: We should probably also move this to be dynamic.
    for (const auto command: context.manifest->getCommands()) {
      // If this command has a single output, create the trivial rule.
      if (command->getOutputs().size() == 1) {
        context.engine.addRule({
            command->getOutputs()[0]->getPath(),
            [=, &context](core::BuildEngine& engine) {
              return buildCommand(context, command);
            },
            [=, &context](core::BuildEngine&, const core::Rule& rule,
                          const core::ValueType value) {
              // If simulating, assume cached results are valid.
              if (context.simulate)
                return true;

              return buildCommandIsResultValid(command, value);
            },
            [=, &context](core::BuildEngine&, core::Rule::StatusKind status) {
              updateCommandStatus(context, command, status);
            } });
        continue;
      }

      // Otherwise, create a composite rule group for the multiple outputs.

      // Create a signature for the composite rule.
      //
      // FIXME: Make efficient.
      std::string compositeRuleName = "";
      for (auto& output: command->getOutputs()) {
        if (!compositeRuleName.empty())
          compositeRuleName += "&&";
        compositeRuleName += output->getPath();
      }

      // Add the composite rule, which will run the command and build all
      // outputs.
      context.engine.addRule({
          compositeRuleName,
          [=, &context](core::BuildEngine& engine) {
            return buildCommand(context, command);
          },
          [=, &context](core::BuildEngine&, const core::Rule& rule,
                        const core::ValueType value) {
            // If simulating, assume cached results are valid.
            if (context.simulate)
              return true;

            return buildCommandIsResultValid(command, value);
          },
          [=, &context](core::BuildEngine&, core::Rule::StatusKind status) {
            updateCommandStatus(context, command, status);
          } });

      // Create the per-output selection rules that select the individual output
      // result from the composite result.
      for (unsigned i = 0, e = command->getOutputs().size(); i != e; ++i) {
        context.engine.addRule({
            command->getOutputs()[i]->getPath(),
            [=, &context] (core::BuildEngine&) {
              return selectCompositeBuildResult(context, command, i,
                                                compositeRuleName);
            },
            [=, &context] (core::BuildEngine&, const core::Rule& rule,
                           const core::ValueType value) {
              // If simulating, assume cached results are valid.
              if (context.simulate)
                return true;

              return selectCompositeIsResultValid(command, value);
            } });
      }
    }

    // If this is the first iteration, build the manifest, unless disabled.
    if (autoRegenerateManifest && iteration == 0) {
      context.engine.build(manifestFilename);

      // If the manifest was rebuilt, then reload it and build again.
      if (context.numBuiltCommands) {
        continue;
      }

      // Otherwise, perform the main build.
      //
      // FIXME: This is somewhat inefficient, as we will end up repeating any
      // dependency scanning that was required for checking the manifest. We can
      // fix this by building the manifest inline with the targets...
    }

    // If using a build profile, open it.
    if (!profileFilename.empty()) {
      context.profileFP = ::fopen(profileFilename.c_str(), "w");
      if (!context.profileFP) {
        context.emitError("unable to open build profile '%s' (%s)\n",
                          profileFilename.c_str(), strerror(errno));
        return 1;
      }

      fprintf(context.profileFP, "[\n");
    }

    // If no explicit targets were named, build the default targets.
    if (targetsToBuild.empty()) {
      for (auto& target: context.manifest->getDefaultTargets())
        targetsToBuild.push_back(target->getPath());

      // If there are no default targets, then build all of the root targets.
      if (targetsToBuild.empty()) {
        std::unordered_set<const ninja::Node*> inputNodes;

        // Collect all of the input nodes.
        for (const auto& command: context.manifest->getCommands()) {
          for (const auto* input: command->getInputs()) {
            inputNodes.emplace(input);
          }
        }

        // Build all of the targets that are not an input.
        for (const auto& command: context.manifest->getCommands()) {
          for (const auto& output: command->getOutputs()) {
            if (!inputNodes.count(output)) {
              targetsToBuild.push_back(output->getPath());
            }
          }
        }
      }
    }

    // Generate an error if there is nothing to build.
    if (targetsToBuild.empty()) {
      context.emitError("no targets to build");
      return 1;
    }

    // If building multiple targets, do so via a dummy rule to allow them to
    // build concurrently (and without duplicates).
    //
    // FIXME: We should sort out eventually whether the engine itself should
    // support this. It seems like an obvious feature, but it is also trivial
    // for the client to implement on top of the existing API.
    if (targetsToBuild.size() > 1) {
      // Create a dummy rule to build all targets.
      context.engine.addRule({
          "<<build>>",
          [&](core::BuildEngine&) {
            return buildTargets(context, targetsToBuild);
          },
          [&](core::BuildEngine&, const core::Rule&, const core::ValueType&) {
            // Always rebuild the dummy rule.
            return false;
          } });

      context.engine.build("<<build>>");
    } else {
      context.engine.build(targetsToBuild[0]);
    }

    if (!dumpGraphPath.empty()) {
      context.engine.dumpGraphToFile(dumpGraphPath);
    }

    // Close the build profile, if used.
    if (context.profileFP) {
      ::fclose(context.profileFP);

      context.emitNote(
          "wrote build profile to '%s', use Chrome's about:tracing to view.",
          profileFilename.c_str());
    }

    // If the build was cancelled by SIGINT, cause ourself to also die by SIGINT
    // to support proper shell behavior.
    if (context.wasCancelledBySigint) {
      // Ensure SIGINT action is default.
      struct sigaction action{};
      action.sa_handler = SIG_DFL;
      sigaction(SIGINT, &action, 0);

      kill(getpid(), SIGINT);
      std::this_thread::sleep_for(std::chrono::microseconds(1000));
      return 2;
    }

    // If there were command failures, report the count.
    if (context.numFailedCommands) {
      context.emitError("build had %d command failures",
                        context.numFailedCommands.load());
    }

    // If the build was stopped because of an error, return an error status.
    if (context.numErrors) {
      return 1;
    }

    // Otherwise, if nothing was done, print a single message to let the user
    // know we completed successfully.
    if (!context.quiet && !context.numBuiltCommands) {
      context.emitNote("no work to do.");
    }

    // If we reached here on the first iteration, then we don't need a second
    // and are done.
    if (iteration == 0)
        break;
  }

  // Return an appropriate exit status.
  return 0;
}