1 files changed, 174 insertions, 0 deletions
diff --git a/src/cmd/go/internal/cache/hash.go b/src/cmd/go/internal/cache/hash.go
new file mode 100644
index 0000000..e4bb2a3
--- /dev/null
+++ b/src/cmd/go/internal/cache/hash.go
@@ -0,0 +1,174 @@
+// 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"
+	"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.
+var hashSalt = []byte(runtime.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()
+}