1 //===- DirectoryWatcher-linux.cpp - Linux-platform directory watching -----===//
2 //
3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4 // See https://llvm.org/LICENSE.txt for license information.
5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6 //
7 //===----------------------------------------------------------------------===//
8 
9 #include "DirectoryScanner.h"
10 #include "clang/DirectoryWatcher/DirectoryWatcher.h"
11 
12 #include "llvm/ADT/STLExtras.h"
13 #include "llvm/ADT/ScopeExit.h"
14 #include "llvm/Support/AlignOf.h"
15 #include "llvm/Support/Errno.h"
16 #include "llvm/Support/Mutex.h"
17 #include "llvm/Support/Path.h"
18 #include <atomic>
19 #include <condition_variable>
20 #include <mutex>
21 #include <queue>
22 #include <string>
23 #include <thread>
24 #include <vector>
25 
26 #include <fcntl.h>
27 #include <sys/epoll.h>
28 #include <sys/inotify.h>
29 #include <unistd.h>
30 
31 namespace {
32 
33 using namespace llvm;
34 using namespace clang;
35 
36 /// Pipe for inter-thread synchronization - for epoll-ing on multiple
37 /// conditions. It is meant for uni-directional 1:1 signalling - specifically:
38 /// no multiple consumers, no data passing. Thread waiting for signal should
39 /// poll the FDRead. Signalling thread should call signal() which writes single
40 /// character to FDRead.
41 struct SemaphorePipe {
42   // Expects two file-descriptors opened as a pipe in the canonical POSIX
43   // order: pipefd[0] refers to the read end of the pipe. pipefd[1] refers to
44   // the write end of the pipe.
45   SemaphorePipe(int pipefd[2])
46       : FDRead(pipefd[0]), FDWrite(pipefd[1]), OwnsFDs(true) {}
47   SemaphorePipe(const SemaphorePipe &) = delete;
48   void operator=(const SemaphorePipe &) = delete;
49   SemaphorePipe(SemaphorePipe &&other)
50       : FDRead(other.FDRead), FDWrite(other.FDWrite),
51         OwnsFDs(other.OwnsFDs) // Someone could have moved from the other
52                                // instance before.
53   {
54     other.OwnsFDs = false;
55   };
56 
57   void signal() {
58 #ifndef NDEBUG
59     ssize_t Result =
60 #endif
61     llvm::sys::RetryAfterSignal(-1, write, FDWrite, "A", 1);
62     assert(Result != -1);
63   }
64   ~SemaphorePipe() {
65     if (OwnsFDs) {
66       close(FDWrite);
67       close(FDRead);
68     }
69   }
70   const int FDRead;
71   const int FDWrite;
72   bool OwnsFDs;
73 
74   static llvm::Optional<SemaphorePipe> create() {
75     int InotifyPollingStopperFDs[2];
76     if (pipe2(InotifyPollingStopperFDs, O_CLOEXEC) == -1)
77       return llvm::None;
78     return SemaphorePipe(InotifyPollingStopperFDs);
79   }
80 };
81 
82 /// Mutex-protected queue of Events.
83 class EventQueue {
84   std::mutex Mtx;
85   std::condition_variable NonEmpty;
86   std::queue<DirectoryWatcher::Event> Events;
87 
88 public:
89   void push_back(const DirectoryWatcher::Event::EventKind K,
90                  StringRef Filename) {
91     {
92       std::unique_lock<std::mutex> L(Mtx);
93       Events.emplace(K, Filename);
94     }
95     NonEmpty.notify_one();
96   }
97 
98   // Blocks on caller thread and uses codition_variable to wait until there's an
99   // event to return.
100   DirectoryWatcher::Event pop_front_blocking() {
101     std::unique_lock<std::mutex> L(Mtx);
102     while (true) {
103       // Since we might have missed all the prior notifications on NonEmpty we
104       // have to check the queue first (under lock).
105       if (!Events.empty()) {
106         DirectoryWatcher::Event Front = Events.front();
107         Events.pop();
108         return Front;
109       }
110       NonEmpty.wait(L, [this]() { return !Events.empty(); });
111     }
112   }
113 };
114 
115 class DirectoryWatcherLinux : public clang::DirectoryWatcher {
116 public:
117   DirectoryWatcherLinux(
118       llvm::StringRef WatchedDirPath,
119       std::function<void(llvm::ArrayRef<Event>, bool)> Receiver,
120       bool WaitForInitialSync, int InotifyFD, int InotifyWD,
121       SemaphorePipe &&InotifyPollingStopSignal);
122 
123   ~DirectoryWatcherLinux() override {
124     StopWork();
125     InotifyPollingThread.join();
126     EventsReceivingThread.join();
127     inotify_rm_watch(InotifyFD, InotifyWD);
128     llvm::sys::RetryAfterSignal(-1, close, InotifyFD);
129   }
130 
131 private:
132   const std::string WatchedDirPath;
133   // inotify file descriptor
134   int InotifyFD = -1;
135   // inotify watch descriptor
136   int InotifyWD = -1;
137 
138   EventQueue Queue;
139 
140   // Make sure lifetime of Receiver fully contains lifetime of
141   // EventsReceivingThread.
142   std::function<void(llvm::ArrayRef<Event>, bool)> Receiver;
143 
144   // Consumes inotify events and pushes directory watcher events to the Queue.
145   void InotifyPollingLoop();
146   std::thread InotifyPollingThread;
147   // Using pipe so we can epoll two file descriptors at once - inotify and
148   // stopping condition.
149   SemaphorePipe InotifyPollingStopSignal;
150 
151   // Does the initial scan of the directory - directly calling Receiver,
152   // bypassing the Queue. Both InitialScan and EventReceivingLoop use Receiver
153   // which isn't necessarily thread-safe.
154   void InitialScan();
155 
156   // Processing events from the Queue.
157   // In case client doesn't want to do the initial scan synchronously
158   // (WaitForInitialSync=false in ctor) we do the initial scan at the beginning
159   // of this thread.
160   std::thread EventsReceivingThread;
161   // Push event of WatcherGotInvalidated kind to the Queue to stop the loop.
162   // Both InitialScan and EventReceivingLoop use Receiver which isn't
163   // necessarily thread-safe.
164   void EventReceivingLoop();
165 
166   // Stops all the async work. Reentrant.
167   void StopWork() {
168     Queue.push_back(DirectoryWatcher::Event::EventKind::WatcherGotInvalidated,
169                     "");
170     InotifyPollingStopSignal.signal();
171   }
172 };
173 
174 void DirectoryWatcherLinux::InotifyPollingLoop() {
175   // We want to be able to read ~30 events at once even in the worst case
176   // (obscenely long filenames).
177   constexpr size_t EventBufferLength =
178       30 * (sizeof(struct inotify_event) + NAME_MAX + 1);
179   // http://man7.org/linux/man-pages/man7/inotify.7.html
180   // Some systems cannot read integer variables if they are not
181   // properly aligned. On other systems, incorrect alignment may
182   // decrease performance. Hence, the buffer used for reading from
183   // the inotify file descriptor should have the same alignment as
184   // struct inotify_event.
185 
186   auto ManagedBuffer =
187       llvm::make_unique<llvm::AlignedCharArray<alignof(struct inotify_event),
188                                                EventBufferLength>>();
189   char *const Buf = ManagedBuffer->buffer;
190 
191   const int EpollFD = epoll_create1(EPOLL_CLOEXEC);
192   if (EpollFD == -1) {
193     StopWork();
194     return;
195   }
196   auto EpollFDGuard = llvm::make_scope_exit([EpollFD]() { close(EpollFD); });
197 
198   struct epoll_event EventSpec;
199   EventSpec.events = EPOLLIN;
200   EventSpec.data.fd = InotifyFD;
201   if (epoll_ctl(EpollFD, EPOLL_CTL_ADD, InotifyFD, &EventSpec) == -1) {
202     StopWork();
203     return;
204   }
205 
206   EventSpec.data.fd = InotifyPollingStopSignal.FDRead;
207   if (epoll_ctl(EpollFD, EPOLL_CTL_ADD, InotifyPollingStopSignal.FDRead,
208                 &EventSpec) == -1) {
209     StopWork();
210     return;
211   }
212 
213   std::array<struct epoll_event, 2> EpollEventBuffer;
214 
215   while (true) {
216     const int EpollWaitResult = llvm::sys::RetryAfterSignal(
217         -1, epoll_wait, EpollFD, EpollEventBuffer.data(),
218         EpollEventBuffer.size(), /*timeout=*/-1 /*== infinity*/);
219     if (EpollWaitResult == -1) {
220       StopWork();
221       return;
222     }
223 
224     // Multiple epoll_events can be received for a single file descriptor per
225     // epoll_wait call.
226     for (int i = 0; i < EpollWaitResult; ++i) {
227       if (EpollEventBuffer[i].data.fd == InotifyPollingStopSignal.FDRead) {
228         StopWork();
229         return;
230       }
231     }
232 
233     // epoll_wait() always return either error or >0 events. Since there was no
234     // event for stopping, it must be an inotify event ready for reading.
235     ssize_t NumRead = llvm::sys::RetryAfterSignal(-1, read, InotifyFD, Buf,
236                                                   EventBufferLength);
237     for (char *P = Buf; P < Buf + NumRead;) {
238       if (P + sizeof(struct inotify_event) > Buf + NumRead) {
239         StopWork();
240         llvm_unreachable("an incomplete inotify_event was read");
241         return;
242       }
243 
244       struct inotify_event *Event = reinterpret_cast<struct inotify_event *>(P);
245       P += sizeof(struct inotify_event) + Event->len;
246 
247       if (Event->mask & (IN_CREATE | IN_MODIFY | IN_MOVED_TO | IN_DELETE) &&
248           Event->len <= 0) {
249         StopWork();
250         llvm_unreachable("expected a filename from inotify");
251         return;
252       }
253 
254       if (Event->mask & (IN_CREATE | IN_MOVED_TO | IN_MODIFY)) {
255         Queue.push_back(DirectoryWatcher::Event::EventKind::Modified,
256                         Event->name);
257       } else if (Event->mask & (IN_DELETE | IN_MOVED_FROM)) {
258         Queue.push_back(DirectoryWatcher::Event::EventKind::Removed,
259                         Event->name);
260       } else if (Event->mask & (IN_DELETE_SELF | IN_MOVE_SELF)) {
261         Queue.push_back(DirectoryWatcher::Event::EventKind::WatchedDirRemoved,
262                         "");
263         StopWork();
264         return;
265       } else if (Event->mask & IN_IGNORED) {
266         StopWork();
267         return;
268       } else {
269         StopWork();
270         llvm_unreachable("Unknown event type.");
271         return;
272       }
273     }
274   }
275 }
276 
277 void DirectoryWatcherLinux::InitialScan() {
278   this->Receiver(getAsFileEvents(scanDirectory(WatchedDirPath)),
279                  /*IsInitial=*/true);
280 }
281 
282 void DirectoryWatcherLinux::EventReceivingLoop() {
283   while (true) {
284     DirectoryWatcher::Event Event = this->Queue.pop_front_blocking();
285     this->Receiver(Event, false);
286     if (Event.Kind ==
287         DirectoryWatcher::Event::EventKind::WatcherGotInvalidated) {
288       StopWork();
289       return;
290     }
291   }
292 }
293 
294 DirectoryWatcherLinux::DirectoryWatcherLinux(
295     StringRef WatchedDirPath,
296     std::function<void(llvm::ArrayRef<Event>, bool)> Receiver,
297     bool WaitForInitialSync, int InotifyFD, int InotifyWD,
298     SemaphorePipe &&InotifyPollingStopSignal)
299     : WatchedDirPath(WatchedDirPath), InotifyFD(InotifyFD),
300       InotifyWD(InotifyWD), Receiver(Receiver),
301       InotifyPollingStopSignal(std::move(InotifyPollingStopSignal)) {
302 
303   InotifyPollingThread = std::thread([this]() { InotifyPollingLoop(); });
304   // We have no guarantees about thread safety of the Receiver which is being
305   // used in both InitialScan and EventReceivingLoop. We shouldn't run these
306   // only synchronously.
307   if (WaitForInitialSync) {
308     InitialScan();
309     EventsReceivingThread = std::thread([this]() { EventReceivingLoop(); });
310   } else {
311     EventsReceivingThread = std::thread([this]() {
312       // FIXME: We might want to terminate an async initial scan early in case
313       // of a failure in EventsReceivingThread.
314       InitialScan();
315       EventReceivingLoop();
316     });
317   }
318 }
319 
320 } // namespace
321 
322 std::unique_ptr<DirectoryWatcher> clang::DirectoryWatcher::create(
323     StringRef Path,
324     std::function<void(llvm::ArrayRef<DirectoryWatcher::Event>, bool)> Receiver,
325     bool WaitForInitialSync) {
326   if (Path.empty())
327     return nullptr;
328 
329   const int InotifyFD = inotify_init1(IN_CLOEXEC);
330   if (InotifyFD == -1)
331     return nullptr;
332 
333   const int InotifyWD = inotify_add_watch(
334       InotifyFD, Path.str().c_str(),
335       IN_CREATE | IN_DELETE | IN_DELETE_SELF | IN_MODIFY |
336       IN_MOVED_FROM | IN_MOVE_SELF | IN_MOVED_TO | IN_ONLYDIR | IN_IGNORED
337 #ifdef IN_EXCL_UNLINK
338       | IN_EXCL_UNLINK
339 #endif
340       );
341   if (InotifyWD == -1)
342     return nullptr;
343 
344   auto InotifyPollingStopper = SemaphorePipe::create();
345 
346   if (!InotifyPollingStopper)
347     return nullptr;
348 
349   return llvm::make_unique<DirectoryWatcherLinux>(
350       Path, Receiver, WaitForInitialSync, InotifyFD, InotifyWD,
351       std::move(*InotifyPollingStopper));
352 }