1// Copyright 2018 The Go Authors. All rights reserved. 2// Use of this source code is governed by a BSD-style 3// license that can be found in the LICENSE file. 4 5// Package detrand provides deterministically random functionality. 6// 7// The pseudo-randomness of these functions is seeded by the program binary 8// itself and guarantees that the output does not change within a program, 9// while ensuring that the output is unstable across different builds. 10package detrand 11 12import ( 13 "encoding/binary" 14 "hash/fnv" 15 "os" 16) 17 18// Disable disables detrand such that all functions returns the zero value. 19// This function is not concurrent-safe and must be called during program init. 20func Disable() { 21 randSeed = 0 22} 23 24// Bool returns a deterministically random boolean. 25func Bool() bool { 26 return randSeed%2 == 1 27} 28 29// randSeed is a best-effort at an approximate hash of the Go binary. 30var randSeed = binaryHash() 31 32func binaryHash() uint64 { 33 // Open the Go binary. 34 s, err := os.Executable() 35 if err != nil { 36 return 0 37 } 38 f, err := os.Open(s) 39 if err != nil { 40 return 0 41 } 42 defer f.Close() 43 44 // Hash the size and several samples of the Go binary. 45 const numSamples = 8 46 var buf [64]byte 47 h := fnv.New64() 48 fi, err := f.Stat() 49 if err != nil { 50 return 0 51 } 52 binary.LittleEndian.PutUint64(buf[:8], uint64(fi.Size())) 53 h.Write(buf[:8]) 54 for i := int64(0); i < numSamples; i++ { 55 if _, err := f.ReadAt(buf[:], i*fi.Size()/numSamples); err != nil { 56 return 0 57 } 58 h.Write(buf[:]) 59 } 60 return h.Sum64() 61} 62