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// Copyright 2017 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package cache
import (
"bytes"
"crypto/sha256"
"fmt"
"hash"
"io"
"os"
"runtime"
"strings"
"sync"
)
var debugHash = false // set when GODEBUG=gocachehash=1
// HashSize is the number of bytes in a hash.
const HashSize = 32
// A Hash provides access to the canonical hash function used to index the cache.
// The current implementation uses salted SHA256, but clients must not assume this.
type Hash struct {
h hash.Hash
name string // for debugging
buf *bytes.Buffer // for verify
}
// hashSalt is a salt string added to the beginning of every hash
// created by NewHash. Using the Go version makes sure that different
// versions of the go command (or even different Git commits during
// work on the development branch) do not address the same cache
// entries, so that a bug in one version does not affect the execution
// of other versions. This salt will result in additional ActionID files
// in the cache, but not additional copies of the large output files,
// which are still addressed by unsalted SHA256.
//
// We strip any GOEXPERIMENTs the go tool was built with from this
// version string on the assumption that they shouldn't affect go tool
// execution. This allows bootstrapping to converge faster: dist builds
// go_bootstrap without any experiments, so by stripping experiments
// go_bootstrap and the final go binary will use the same salt.
var hashSalt = []byte(stripExperiment(runtime.Version()))
// stripExperiment strips any GOEXPERIMENT configuration from the Go
// version string.
func stripExperiment(version string) string {
if i := strings.Index(version, " X:"); i >= 0 {
return version[:i]
}
return version
}
// Subkey returns an action ID corresponding to mixing a parent
// action ID with a string description of the subkey.
func Subkey(parent ActionID, desc string) ActionID {
h := sha256.New()
h.Write([]byte("subkey:"))
h.Write(parent[:])
h.Write([]byte(desc))
var out ActionID
h.Sum(out[:0])
if debugHash {
fmt.Fprintf(os.Stderr, "HASH subkey %x %q = %x\n", parent, desc, out)
}
if verify {
hashDebug.Lock()
hashDebug.m[out] = fmt.Sprintf("subkey %x %q", parent, desc)
hashDebug.Unlock()
}
return out
}
// NewHash returns a new Hash.
// The caller is expected to Write data to it and then call Sum.
func NewHash(name string) *Hash {
h := &Hash{h: sha256.New(), name: name}
if debugHash {
fmt.Fprintf(os.Stderr, "HASH[%s]\n", h.name)
}
h.Write(hashSalt)
if verify {
h.buf = new(bytes.Buffer)
}
return h
}
// Write writes data to the running hash.
func (h *Hash) Write(b []byte) (int, error) {
if debugHash {
fmt.Fprintf(os.Stderr, "HASH[%s]: %q\n", h.name, b)
}
if h.buf != nil {
h.buf.Write(b)
}
return h.h.Write(b)
}
// Sum returns the hash of the data written previously.
func (h *Hash) Sum() [HashSize]byte {
var out [HashSize]byte
h.h.Sum(out[:0])
if debugHash {
fmt.Fprintf(os.Stderr, "HASH[%s]: %x\n", h.name, out)
}
if h.buf != nil {
hashDebug.Lock()
if hashDebug.m == nil {
hashDebug.m = make(map[[HashSize]byte]string)
}
hashDebug.m[out] = h.buf.String()
hashDebug.Unlock()
}
return out
}
// In GODEBUG=gocacheverify=1 mode,
// hashDebug holds the input to every computed hash ID,
// so that we can work backward from the ID involved in a
// cache entry mismatch to a description of what should be there.
var hashDebug struct {
sync.Mutex
m map[[HashSize]byte]string
}
// reverseHash returns the input used to compute the hash id.
func reverseHash(id [HashSize]byte) string {
hashDebug.Lock()
s := hashDebug.m[id]
hashDebug.Unlock()
return s
}
var hashFileCache struct {
sync.Mutex
m map[string][HashSize]byte
}
// FileHash returns the hash of the named file.
// It caches repeated lookups for a given file,
// and the cache entry for a file can be initialized
// using SetFileHash.
// The hash used by FileHash is not the same as
// the hash used by NewHash.
func FileHash(file string) ([HashSize]byte, error) {
hashFileCache.Lock()
out, ok := hashFileCache.m[file]
hashFileCache.Unlock()
if ok {
return out, nil
}
h := sha256.New()
f, err := os.Open(file)
if err != nil {
if debugHash {
fmt.Fprintf(os.Stderr, "HASH %s: %v\n", file, err)
}
return [HashSize]byte{}, err
}
_, err = io.Copy(h, f)
f.Close()
if err != nil {
if debugHash {
fmt.Fprintf(os.Stderr, "HASH %s: %v\n", file, err)
}
return [HashSize]byte{}, err
}
h.Sum(out[:0])
if debugHash {
fmt.Fprintf(os.Stderr, "HASH %s: %x\n", file, out)
}
SetFileHash(file, out)
return out, nil
}
// SetFileHash sets the hash returned by FileHash for file.
func SetFileHash(file string, sum [HashSize]byte) {
hashFileCache.Lock()
if hashFileCache.m == nil {
hashFileCache.m = make(map[string][HashSize]byte)
}
hashFileCache.m[file] = sum
hashFileCache.Unlock()
}