cmd/go/internal/cache: implement build artifact cache

The cache is stored in $GOCACHE, which is printed by go env and
defaults to a subdirectory named "go-build" in the standard user cache
directory for the host operating system.

This CL only implements the cache. Future CLs will store data in it.

Change-Id: I0b4965a9e50f852e17e44ec3d6dafe05b58f0d22
Reviewed-on: https://go-review.googlesource.com/68116
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>

src/cmd/dist/deps.go

@@ -85,11 +85,20 @@ var builddeps = map[string][]string{
 	},
 
 	"cmd/go/internal/cache": {
-		"crypto/sha256", // cmd/go/internal/cache
-		"fmt",           // cmd/go/internal/cache
-		"hash",          // cmd/go/internal/cache
-		"io",            // cmd/go/internal/cache
-		"os",            // cmd/go/internal/cache
+		"bytes",                // cmd/go/internal/cache
+		"cmd/go/internal/base", // cmd/go/internal/cache
+		"crypto/sha256",        // cmd/go/internal/cache
+		"encoding/hex",         // cmd/go/internal/cache
+		"errors",               // cmd/go/internal/cache
+		"fmt",                  // cmd/go/internal/cache
+		"hash",                 // cmd/go/internal/cache
+		"io",                   // cmd/go/internal/cache
+		"io/ioutil",            // cmd/go/internal/cache
+		"os",                   // cmd/go/internal/cache
+		"path/filepath",        // cmd/go/internal/cache
+		"runtime",              // cmd/go/internal/cache
+		"strconv",              // cmd/go/internal/cache
+		"sync",                 // cmd/go/internal/cache
 	},
 
 	"cmd/go/internal/cfg": {
@@ -483,6 +492,13 @@ var builddeps = map[string][]string{
 		"reflect", // encoding/binary
 	},
 
+	"encoding/hex": {
+		"bytes",  // encoding/hex
+		"errors", // encoding/hex
+		"fmt",    // encoding/hex
+		"io",     // encoding/hex
+	},
+
 	"encoding/json": {
 		"bytes",           // encoding/json
 		"encoding",        // encoding/json

src/cmd/go/internal/cache/cache.go

@@ -2,12 +2,244 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// Package cache implements a package cache,
-// or more properly a build artifact cache,
-// but our only cached build artifacts are packages.
+// Package cache implements a build artifact cache.
 package cache
 
+import (
+	"bytes"
+	"crypto/sha256"
+	"encoding/hex"
+	"errors"
+	"fmt"
+	"io"
+	"io/ioutil"
+	"os"
+	"path/filepath"
+	"strconv"
+)
+
 // An ActionID is a cache action key, the hash of a complete description of a
 // repeatable computation (command line, environment variables,
 // input file contents, executable contents).
 type ActionID [HashSize]byte
+
+// An OutputID is a cache output key, the hash of an output of a computation.
+type OutputID [HashSize]byte
+
+// A Cache is a package cache, backed by a file system directory tree.
+type Cache struct {
+	dir string
+}
+
+// Open opens and returns the cache in the given directory.
+//
+// It is safe for multiple processes on a single machine to use the
+// same cache directory in a local file system simultaneously.
+// They will coordinate using operating system file locks and may
+// duplicate effort but will not corrupt the cache.
+//
+// However, it is NOT safe for multiple processes on different machines
+// to share a cache directory (for example, if the directory were stored
+// in a network file system). File locking is notoriously unreliable in
+// network file systems and may not suffice to protect the cache.
+//
+func Open(dir string) (*Cache, error) {
+	info, err := os.Stat(dir)
+	if err != nil {
+		return nil, err
+	}
+	if !info.IsDir() {
+		return nil, &os.PathError{Op: "open", Path: dir, Err: fmt.Errorf("not a directory")}
+	}
+	for i := 0; i < 256; i++ {
+		name := filepath.Join(dir, fmt.Sprintf("%02x", i))
+		if err := os.MkdirAll(name, 0777); err != nil {
+			return nil, err
+		}
+	}
+	c := &Cache{dir: dir}
+	return c, nil
+}
+
+// fileName returns the name of the file corresponding to the given id.
+func (c *Cache) fileName(id [HashSize]byte, key string) string {
+	return filepath.Join(c.dir, fmt.Sprintf("%02x", id[0]), fmt.Sprintf("%x", id)+"-"+key)
+}
+
+var errMissing = errors.New("cache entry not found")
+
+const (
+	// action entry file is "v1 <hex id> <hex out> <decimal size space-padded to 20 bytes>\n"
+	hexSize   = HashSize * 2
+	entrySize = 2 + 1 + hexSize + 1 + hexSize + 1 + 20 + 1
+)
+
+// Get looks up the action ID in the cache,
+// returning the corresponding output ID and file size, if any.
+// Note that finding an output ID does not guarantee that the
+// saved file for that output ID is still available.
+func (c *Cache) Get(id ActionID) (OutputID, int64, error) {
+	missing := func() (OutputID, int64, error) {
+		// TODO: log miss
+		return OutputID{}, 0, errMissing
+	}
+	f, err := os.Open(c.fileName(id, "a"))
+	if err != nil {
+		return missing()
+	}
+	defer f.Close()
+	entry := make([]byte, entrySize+1) // +1 to detect whether f is too long
+	if n, err := io.ReadFull(f, entry); n != entrySize || err != io.ErrUnexpectedEOF {
+		return missing()
+	}
+	if entry[0] != 'v' || entry[1] != '1' || entry[2] != ' ' || entry[3+hexSize] != ' ' || entry[3+hexSize+1+64] != ' ' || entry[entrySize-1] != '\n' {
+		return missing()
+	}
+	eid, eout, esize := entry[3:3+hexSize], entry[3+hexSize+1:3+hexSize+1+hexSize], entry[3+hexSize+1+hexSize+1:entrySize-1]
+	var buf [HashSize]byte
+	if _, err := hex.Decode(buf[:], eid); err != nil || buf != id {
+		return missing()
+	}
+	if _, err := hex.Decode(buf[:], eout); err != nil {
+		return missing()
+	}
+	i := 0
+	for i < len(esize) && esize[i] == ' ' {
+		i++
+	}
+	size, err := strconv.ParseInt(string(esize[i:]), 10, 64)
+	if err != nil || size < 0 {
+		return missing()
+	}
+
+	// TODO: Update modtime of f to give a signal about recently used?
+	// TODO: log hit
+
+	return buf, size, nil
+}
+
+// OutputFile returns the name of the cache file storing output with the given OutputID.
+func (c *Cache) OutputFile(out OutputID) string {
+	return c.fileName(out, "d")
+}
+
+// putIndexEntry adds an entry to the cache recording that executing the action
+// with the given id produces an output with the given output id (hash) and size.
+func (c *Cache) putIndexEntry(id ActionID, out OutputID, size int64) error {
+	// Note: We expect that for one reason or another it may happen
+	// that repeating an action produces a different output hash
+	// (for example, if the output contains a time stamp or temp dir name).
+	// While not ideal, this is also not a correctness problem, so we
+	// don't make a big deal about it. In particular, we leave the action
+	// cache entries writable specifically so that they can be overwritten.
+	entry := []byte(fmt.Sprintf("v1 %x %x %20d\n", id, out, size))
+	return ioutil.WriteFile(c.fileName(id, "a"), entry, 0666)
+}
+
+// Put stores the given output in the cache as the output for the action ID.
+// It may read file twice. The content of file must not change between the two passes.
+func (c *Cache) Put(id ActionID, file io.ReadSeeker) (OutputID, int64, error) {
+	// Compute output ID.
+	h := sha256.New()
+	if _, err := file.Seek(0, 0); err != nil {
+		return OutputID{}, 0, err
+	}
+	size, err := io.Copy(h, file)
+	if err != nil {
+		return OutputID{}, 0, err
+	}
+	var out OutputID
+	h.Sum(out[:0])
+
+	// Copy to cached output file (if not already present).
+	if err := c.copyFile(file, out, size); err != nil {
+		return out, size, err
+	}
+
+	// Add to cache index.
+	// TODO: log put
+	return out, size, c.putIndexEntry(id, out, size)
+}
+
+// copyFile copies file into the cache, expecting it to have the given
+// output ID and size, if that file is not present already.
+func (c *Cache) copyFile(file io.ReadSeeker, out OutputID, size int64) error {
+	name := c.fileName(out, "d")
+	info, err := os.Stat(name)
+	if err == nil && info.Size() == size {
+		// Check hash.
+		if f, err := os.Open(name); err == nil {
+			h := sha256.New()
+			io.Copy(h, f)
+			f.Close()
+			var out2 OutputID
+			h.Sum(out2[:0])
+			if out == out2 {
+				return nil
+			}
+		}
+		// Hash did not match. Fall through and rewrite file.
+	}
+
+	// Copy file to cache directory.
+	mode := os.O_RDWR | os.O_CREATE
+	if err == nil && info.Size() > size { // shouldn't happen but fix in case
+		mode |= os.O_TRUNC
+	}
+	f, err := os.OpenFile(name, mode, 0666)
+	if err != nil {
+		return err
+	}
+	defer f.Close()
+	if size == 0 {
+		// File now exists with correct size.
+		// Only one possible zero-length file, so contents are OK too.
+		// Early return here makes sure there's a "last byte" for code below.
+		return nil
+	}
+
+	// From here on, if any of the I/O writing the file fails,
+	// we make a best-effort attempt to truncate the file f
+	// before returning, to avoid leaving bad bytes in the file.
+
+	// Copy file to f, but also into h to double-check hash.
+	if _, err := file.Seek(0, 0); err != nil {
+		f.Truncate(0)
+		return err
+	}
+	h := sha256.New()
+	w := io.MultiWriter(f, h)
+	if _, err := io.CopyN(w, file, size-1); err != nil {
+		f.Truncate(0)
+		return err
+	}
+	// Check last byte before writing it; writing it will make the size match
+	// what other processes expect to find and might cause them to start
+	// using the file.
+	buf := make([]byte, 1)
+	if _, err := file.Read(buf); err != nil {
+		f.Truncate(0)
+		return err
+	}
+	h.Write(buf)
+	sum := h.Sum(nil)
+	if !bytes.Equal(sum, out[:]) {
+		f.Truncate(0)
+		return fmt.Errorf("file content changed underfoot")
+	}
+
+	// Commit cache file entry.
+	if _, err := f.Write(buf); err != nil {
+		f.Truncate(0)
+		return err
+	}
+	if err := f.Close(); err != nil {
+		// Data might not have been written,
+		// but file may look like it is the right size.
+		// To be extra careful, remove cached file.
+		os.Remove(name)
+		return err
+	}
+
+	return nil
+}

src/cmd/go/internal/cache/cache_test.go

@@ -0,0 +1,106 @@
+// 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 (
+	"encoding/binary"
+	"io/ioutil"
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func TestBasic(t *testing.T) {
+	dir, err := ioutil.TempDir("", "cachetest-")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(dir)
+	_, err = Open(filepath.Join(dir, "notexist"))
+	if err == nil {
+		t.Fatal(`Open("tmp/notexist") succeeded, want failure`)
+	}
+
+	cdir := filepath.Join(dir, "c1")
+	if err := os.Mkdir(cdir, 0777); err != nil {
+		t.Fatal(err)
+	}
+
+	c1, err := Open(cdir)
+	if err != nil {
+		t.Fatalf("Open(c1) (create): %v", err)
+	}
+	if err := c1.putIndexEntry(dummyID(1), dummyID(12), 13); err != nil {
+		t.Fatalf("addIndexEntry: %v", err)
+	}
+	if err := c1.putIndexEntry(dummyID(1), dummyID(2), 3); err != nil { // overwrite entry
+		t.Fatalf("addIndexEntry: %v", err)
+	}
+	if out, size, err := c1.Get(dummyID(1)); err != nil || out != dummyID(2) || size != 3 {
+		t.Fatalf("c1.Get(1) = %x, %v, %v, want %x, %v, nil", out[:], size, err, dummyID(2), 3)
+	}
+
+	c2, err := Open(cdir)
+	if err != nil {
+		t.Fatalf("Open(c2) (reuse): %v", err)
+	}
+	if out, size, err := c2.Get(dummyID(1)); err != nil || out != dummyID(2) || size != 3 {
+		t.Fatalf("c2.Get(1) = %x, %v, %v, want %x, %v, nil", out[:], size, err, dummyID(2), 3)
+	}
+	if err := c2.putIndexEntry(dummyID(2), dummyID(3), 4); err != nil {
+		t.Fatalf("addIndexEntry: %v", err)
+	}
+	if out, size, err := c1.Get(dummyID(2)); err != nil || out != dummyID(3) || size != 4 {
+		t.Fatalf("c1.Get(2) = %x, %v, %v, want %x, %v, nil", out[:], size, err, dummyID(3), 4)
+	}
+}
+
+func TestGrowth(t *testing.T) {
+	dir, err := ioutil.TempDir("", "cachetest-")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(dir)
+
+	c, err := Open(dir)
+	if err != nil {
+		t.Fatalf("Open: %v", err)
+	}
+
+	n := 10000
+	if testing.Short() {
+		n = 1000
+	}
+
+	for i := 0; i < n; i++ {
+		if err := c.putIndexEntry(dummyID(i), dummyID(i*99), int64(i)*101); err != nil {
+			t.Fatalf("addIndexEntry: %v", err)
+		}
+		id := ActionID(dummyID(i))
+		out, size, err := c.Get(id)
+		if err != nil {
+			t.Fatalf("Get(%x): %v", id, err)
+		}
+		if out != dummyID(i*99) || size != int64(i)*101 {
+			t.Errorf("Get(%x) = %x, %d, want %x, %d", id, out, size, dummyID(i*99), int64(i)*101)
+		}
+	}
+	for i := 0; i < n; i++ {
+		id := ActionID(dummyID(i))
+		out, size, err := c.Get(id)
+		if err != nil {
+			t.Fatalf("Get2(%x): %v", id, err)
+		}
+		if out != dummyID(i*99) || size != int64(i)*101 {
+			t.Errorf("Get2(%x) = %x, %d, want %x, %d", id, out, size, dummyID(i*99), int64(i)*101)
+		}
+	}
+}
+
+func dummyID(x int) [HashSize]byte {
+	var out [HashSize]byte
+	binary.LittleEndian.PutUint64(out[:], uint64(x))
+	return out
+}

src/cmd/go/internal/cache/default.go

@@ -0,0 +1,77 @@
+// 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 (
+	"cmd/go/internal/base"
+	"os"
+	"path/filepath"
+	"runtime"
+	"sync"
+)
+
+// Default returns the default cache to use, or nil if no cache should be used.
+func Default() *Cache {
+	defaultOnce.Do(initDefaultCache)
+	return defaultCache
+}
+
+var (
+	defaultOnce  sync.Once
+	defaultCache *Cache
+)
+
+// initDefaultCache does the work of finding the default cache
+// the first time Default is called.
+func initDefaultCache() {
+	dir := os.Getenv("GOCACHE")
+	if dir == "off" {
+		return
+	}
+	if dir == "" {
+		// Compute default location.
+		// TODO(rsc): This code belongs somewhere else,
+		// like maybe ioutil.CacheDir or os.CacheDir.
+		switch runtime.GOOS {
+		case "windows":
+			dir = os.Getenv("LocalAppData")
+
+		case "darwin":
+			dir = os.Getenv("HOME")
+			if dir == "" {
+				return
+			}
+			dir += "/Library/Caches"
+
+		case "plan9":
+			dir = os.Getenv("home")
+			if dir == "" {
+				return
+			}
+			dir += "/lib/cache"
+
+		default: // Unix
+			// https://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html
+			dir = os.Getenv("XDG_CACHE_HOME")
+			if dir == "" {
+				dir = os.Getenv("HOME")
+				if dir == "" {
+					return
+				}
+				dir += "/.cache"
+			}
+		}
+		dir = filepath.Join(dir, "go-build")
+		if err := os.MkdirAll(dir, 0777); err != nil {
+			return
+		}
+	}
+
+	c, err := Open(dir)
+	if err != nil {
+		base.Fatalf("initializing cache in $GOCACHE: %s", err)
+	}
+	defaultCache = c
+}

src/cmd/go/internal/cache/hash.go

@@ -10,6 +10,7 @@ import (
 	"hash"
 	"io"
 	"os"
+	"sync"
 )
 
 var debugHash = os.Getenv("GOCMDDEBUGHASH") == "1"
@@ -52,8 +53,24 @@ func (h *Hash) Sum() [HashSize]byte {
 	return out
 }
 
+var hashFileCache struct {
+	sync.Mutex
+	m map[string][HashSize]byte
+}
+
 // HashFile returns the hash of the named file.
-func HashFile(file string) ([HashSize]byte, error) {
+// It caches repeated lookups for a given file,
+// and the cache entry for a file can be initialized
+// using SetFileHash.
+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 {
@@ -62,12 +79,29 @@ func HashFile(file string) ([HashSize]byte, error) {
 		}
 		return [HashSize]byte{}, err
 	}
-	io.Copy(h, f)
+	_, err = io.Copy(h, f)
 	f.Close()
-	var out [HashSize]byte
+	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()
+}

src/cmd/go/internal/cache/hash_test.go

@@ -34,7 +34,7 @@ func TestHashFile(t *testing.T) {
 	}
 
 	var h ActionID // make sure hash result is assignable to ActionID
-	h, err = HashFile(name)
+	h, err = FileHash(name)
 	if err != nil {
 		t.Fatal(err)
 	}

src/cmd/go/internal/work/action.go

@@ -47,10 +47,9 @@ type Builder struct {
 	readySema chan bool
 	ready     actionQueue
 
-	id            sync.Mutex
-	toolIDCache   map[string]string // tool name -> tool ID
-	buildIDCache  map[string]string // file name -> build ID
-	fileHashCache map[string]string // file name -> content hash
+	id           sync.Mutex
+	toolIDCache  map[string]string // tool name -> tool ID
+	buildIDCache map[string]string // file name -> build ID
 }
 
 // NOTE: Much of Action would not need to be exported if not for test.
@@ -195,7 +194,6 @@ func (b *Builder) Init() {
 	b.mkdirCache = make(map[string]bool)
 	b.toolIDCache = make(map[string]string)
 	b.buildIDCache = make(map[string]string)
-	b.fileHashCache = make(map[string]string)
 
 	if cfg.BuildN {
 		b.WorkDir = "$WORK"

src/cmd/go/internal/work/buildid.go

@@ -216,25 +216,11 @@ func (b *Builder) buildID(file string) string {
 
 // fileHash returns the content hash of the named file.
 func (b *Builder) fileHash(file string) string {
-	b.id.Lock()
-	id := b.fileHashCache[file]
-	b.id.Unlock()
-
-	if id != "" {
-		return id
-	}
-
-	sum, err := cache.HashFile(file)
+	sum, err := cache.FileHash(file)
 	if err != nil {
 		return ""
 	}
-	id = hashToString(sum)
-
-	b.id.Lock()
-	b.fileHashCache[file] = id
-	b.id.Unlock()
-
-	return id
+	return hashToString(sum)
 }
 
 // useCache tries to satisfy the action a, which has action ID actionHash,