1 // Copyright 2017 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.
5 // Package cache implements a build artifact cache.
7 // This package is a slightly modified fork of Go's
8 // cmd/go/internal/cache package.
25 "honnef.co/go/tools/internal/renameio"
28 // An ActionID is a cache action key, the hash of a complete description of a
29 // repeatable computation (command line, environment variables,
30 // input file contents, executable contents).
31 type ActionID [HashSize]byte
33 // An OutputID is a cache output key, the hash of an output of a computation.
34 type OutputID [HashSize]byte
36 // A Cache is a package cache, backed by a file system directory tree.
42 // Open opens and returns the cache in the given directory.
44 // It is safe for multiple processes on a single machine to use the
45 // same cache directory in a local file system simultaneously.
46 // They will coordinate using operating system file locks and may
47 // duplicate effort but will not corrupt the cache.
49 // However, it is NOT safe for multiple processes on different machines
50 // to share a cache directory (for example, if the directory were stored
51 // in a network file system). File locking is notoriously unreliable in
52 // network file systems and may not suffice to protect the cache.
54 func Open(dir string) (*Cache, error) {
55 info, err := os.Stat(dir)
60 return nil, &os.PathError{Op: "open", Path: dir, Err: fmt.Errorf("not a directory")}
62 for i := 0; i < 256; i++ {
63 name := filepath.Join(dir, fmt.Sprintf("%02x", i))
64 if err := os.MkdirAll(name, 0777); err != nil {
75 // fileName returns the name of the file corresponding to the given id.
76 func (c *Cache) fileName(id [HashSize]byte, key string) string {
77 return filepath.Join(c.dir, fmt.Sprintf("%02x", id[0]), fmt.Sprintf("%x", id)+"-"+key)
80 var errMissing = errors.New("cache entry not found")
83 // action entry file is "v1 <hex id> <hex out> <decimal size space-padded to 20 bytes> <unixnano space-padded to 20 bytes>\n"
84 hexSize = HashSize * 2
85 entrySize = 2 + 1 + hexSize + 1 + hexSize + 1 + 20 + 1 + 20 + 1
88 // verify controls whether to run the cache in verify mode.
89 // In verify mode, the cache always returns errMissing from Get
90 // but then double-checks in Put that the data being written
91 // exactly matches any existing entry. This provides an easy
92 // way to detect program behavior that would have been different
93 // had the cache entry been returned from Get.
95 // verify is enabled by setting the environment variable
96 // GODEBUG=gocacheverify=1.
99 // DebugTest is set when GODEBUG=gocachetest=1 is in the environment.
100 var DebugTest = false
102 func init() { initEnv() }
107 debug := strings.Split(os.Getenv("GODEBUG"), ",")
108 for _, f := range debug {
109 if f == "gocacheverify=1" {
112 if f == "gocachehash=1" {
115 if f == "gocachetest=1" {
121 // Get looks up the action ID in the cache,
122 // returning the corresponding output ID and file size, if any.
123 // Note that finding an output ID does not guarantee that the
124 // saved file for that output ID is still available.
125 func (c *Cache) Get(id ActionID) (Entry, error) {
127 return Entry{}, errMissing
138 // get is Get but does not respect verify mode, so that Put can use it.
139 func (c *Cache) get(id ActionID) (Entry, error) {
140 missing := func() (Entry, error) {
141 return Entry{}, errMissing
143 f, err := os.Open(c.fileName(id, "a"))
148 entry := make([]byte, entrySize+1) // +1 to detect whether f is too long
149 if n, err := io.ReadFull(f, entry); n != entrySize || err != io.ErrUnexpectedEOF {
152 if entry[0] != 'v' || entry[1] != '1' || entry[2] != ' ' || entry[3+hexSize] != ' ' || entry[3+hexSize+1+hexSize] != ' ' || entry[3+hexSize+1+hexSize+1+20] != ' ' || entry[entrySize-1] != '\n' {
155 eid, entry := entry[3:3+hexSize], entry[3+hexSize:]
156 eout, entry := entry[1:1+hexSize], entry[1+hexSize:]
157 esize, entry := entry[1:1+20], entry[1+20:]
158 //lint:ignore SA4006 See https://github.com/dominikh/go-tools/issues/465
159 etime, entry := entry[1:1+20], entry[1+20:]
160 var buf [HashSize]byte
161 if _, err := hex.Decode(buf[:], eid); err != nil || buf != id {
164 if _, err := hex.Decode(buf[:], eout); err != nil {
168 for i < len(esize) && esize[i] == ' ' {
171 size, err := strconv.ParseInt(string(esize[i:]), 10, 64)
172 if err != nil || size < 0 {
176 for i < len(etime) && etime[i] == ' ' {
179 tm, err := strconv.ParseInt(string(etime[i:]), 10, 64)
180 if err != nil || tm < 0 {
184 c.used(c.fileName(id, "a"))
186 return Entry{buf, size, time.Unix(0, tm)}, nil
189 // GetFile looks up the action ID in the cache and returns
190 // the name of the corresponding data file.
191 func (c *Cache) GetFile(id ActionID) (file string, entry Entry, err error) {
192 entry, err = c.Get(id)
194 return "", Entry{}, err
196 file = c.OutputFile(entry.OutputID)
197 info, err := os.Stat(file)
198 if err != nil || info.Size() != entry.Size {
199 return "", Entry{}, errMissing
201 return file, entry, nil
204 // GetBytes looks up the action ID in the cache and returns
205 // the corresponding output bytes.
206 // GetBytes should only be used for data that can be expected to fit in memory.
207 func (c *Cache) GetBytes(id ActionID) ([]byte, Entry, error) {
208 entry, err := c.Get(id)
210 return nil, entry, err
212 data, _ := ioutil.ReadFile(c.OutputFile(entry.OutputID))
213 if sha256.Sum256(data) != entry.OutputID {
214 return nil, entry, errMissing
216 return data, entry, nil
219 // OutputFile returns the name of the cache file storing output with the given OutputID.
220 func (c *Cache) OutputFile(out OutputID) string {
221 file := c.fileName(out, "d")
226 // Time constants for cache expiration.
228 // We set the mtime on a cache file on each use, but at most one per mtimeInterval (1 hour),
229 // to avoid causing many unnecessary inode updates. The mtimes therefore
230 // roughly reflect "time of last use" but may in fact be older by at most an hour.
232 // We scan the cache for entries to delete at most once per trimInterval (1 day).
234 // When we do scan the cache, we delete entries that have not been used for
235 // at least trimLimit (5 days). Statistics gathered from a month of usage by
236 // Go developers found that essentially all reuse of cached entries happened
237 // within 5 days of the previous reuse. See golang.org/issue/22990.
239 mtimeInterval = 1 * time.Hour
240 trimInterval = 24 * time.Hour
241 trimLimit = 5 * 24 * time.Hour
244 // used makes a best-effort attempt to update mtime on file,
245 // so that mtime reflects cache access time.
247 // Because the reflection only needs to be approximate,
248 // and to reduce the amount of disk activity caused by using
249 // cache entries, used only updates the mtime if the current
250 // mtime is more than an hour old. This heuristic eliminates
251 // nearly all of the mtime updates that would otherwise happen,
252 // while still keeping the mtimes useful for cache trimming.
253 func (c *Cache) used(file string) {
254 info, err := os.Stat(file)
255 if err == nil && c.now().Sub(info.ModTime()) < mtimeInterval {
258 os.Chtimes(file, c.now(), c.now())
261 // Trim removes old cache entries that are likely not to be reused.
262 func (c *Cache) Trim() {
265 // We maintain in dir/trim.txt the time of the last completed cache trim.
266 // If the cache has been trimmed recently enough, do nothing.
267 // This is the common case.
268 data, _ := renameio.ReadFile(filepath.Join(c.dir, "trim.txt"))
269 t, err := strconv.ParseInt(strings.TrimSpace(string(data)), 10, 64)
270 if err == nil && now.Sub(time.Unix(t, 0)) < trimInterval {
274 // Trim each of the 256 subdirectories.
275 // We subtract an additional mtimeInterval
276 // to account for the imprecision of our "last used" mtimes.
277 cutoff := now.Add(-trimLimit - mtimeInterval)
278 for i := 0; i < 256; i++ {
279 subdir := filepath.Join(c.dir, fmt.Sprintf("%02x", i))
280 c.trimSubdir(subdir, cutoff)
283 // Ignore errors from here: if we don't write the complete timestamp, the
284 // cache will appear older than it is, and we'll trim it again next time.
285 renameio.WriteFile(filepath.Join(c.dir, "trim.txt"), []byte(fmt.Sprintf("%d", now.Unix())), 0666)
288 // trimSubdir trims a single cache subdirectory.
289 func (c *Cache) trimSubdir(subdir string, cutoff time.Time) {
290 // Read all directory entries from subdir before removing
291 // any files, in case removing files invalidates the file offset
292 // in the directory scan. Also, ignore error from f.Readdirnames,
293 // because we don't care about reporting the error and we still
294 // want to process any entries found before the error.
295 f, err := os.Open(subdir)
299 names, _ := f.Readdirnames(-1)
302 for _, name := range names {
303 // Remove only cache entries (xxxx-a and xxxx-d).
304 if !strings.HasSuffix(name, "-a") && !strings.HasSuffix(name, "-d") {
307 entry := filepath.Join(subdir, name)
308 info, err := os.Stat(entry)
309 if err == nil && info.ModTime().Before(cutoff) {
315 // putIndexEntry adds an entry to the cache recording that executing the action
316 // with the given id produces an output with the given output id (hash) and size.
317 func (c *Cache) putIndexEntry(id ActionID, out OutputID, size int64, allowVerify bool) error {
318 // Note: We expect that for one reason or another it may happen
319 // that repeating an action produces a different output hash
320 // (for example, if the output contains a time stamp or temp dir name).
321 // While not ideal, this is also not a correctness problem, so we
322 // don't make a big deal about it. In particular, we leave the action
323 // cache entries writable specifically so that they can be overwritten.
325 // Setting GODEBUG=gocacheverify=1 does make a big deal:
326 // in verify mode we are double-checking that the cache entries
327 // are entirely reproducible. As just noted, this may be unrealistic
328 // in some cases but the check is also useful for shaking out real bugs.
329 entry := fmt.Sprintf("v1 %x %x %20d %20d\n", id, out, size, time.Now().UnixNano())
331 if verify && allowVerify {
332 old, err := c.get(id)
333 if err == nil && (old.OutputID != out || old.Size != size) {
334 // panic to show stack trace, so we can see what code is generating this cache entry.
335 msg := fmt.Sprintf("go: internal cache error: cache verify failed: id=%x changed:<<<\n%s\n>>>\nold: %x %d\nnew: %x %d", id, reverseHash(id), out, size, old.OutputID, old.Size)
339 file := c.fileName(id, "a")
341 // Copy file to cache directory.
342 mode := os.O_WRONLY | os.O_CREATE
343 f, err := os.OpenFile(file, mode, 0666)
347 _, err = f.WriteString(entry)
349 // Truncate the file only *after* writing it.
350 // (This should be a no-op, but truncate just in case of previous corruption.)
352 // This differs from ioutil.WriteFile, which truncates to 0 *before* writing
353 // via os.O_TRUNC. Truncating only after writing ensures that a second write
354 // of the same content to the same file is idempotent, and does not — even
355 // temporarily! — undo the effect of the first write.
356 err = f.Truncate(int64(len(entry)))
358 if closeErr := f.Close(); err == nil {
362 // TODO(bcmills): This Remove potentially races with another go command writing to file.
363 // Can we eliminate it?
367 os.Chtimes(file, c.now(), c.now()) // mainly for tests
372 // Put stores the given output in the cache as the output for the action ID.
373 // It may read file twice. The content of file must not change between the two passes.
374 func (c *Cache) Put(id ActionID, file io.ReadSeeker) (OutputID, int64, error) {
375 return c.put(id, file, true)
378 // PutNoVerify is like Put but disables the verify check
379 // when GODEBUG=goverifycache=1 is set.
380 // It is meant for data that is OK to cache but that we expect to vary slightly from run to run,
381 // like test output containing times and the like.
382 func (c *Cache) PutNoVerify(id ActionID, file io.ReadSeeker) (OutputID, int64, error) {
383 return c.put(id, file, false)
386 func (c *Cache) put(id ActionID, file io.ReadSeeker, allowVerify bool) (OutputID, int64, error) {
387 // Compute output ID.
389 if _, err := file.Seek(0, 0); err != nil {
390 return OutputID{}, 0, err
392 size, err := io.Copy(h, file)
394 return OutputID{}, 0, err
399 // Copy to cached output file (if not already present).
400 if err := c.copyFile(file, out, size); err != nil {
401 return out, size, err
404 // Add to cache index.
405 return out, size, c.putIndexEntry(id, out, size, allowVerify)
408 // PutBytes stores the given bytes in the cache as the output for the action ID.
409 func (c *Cache) PutBytes(id ActionID, data []byte) error {
410 _, _, err := c.Put(id, bytes.NewReader(data))
414 // copyFile copies file into the cache, expecting it to have the given
415 // output ID and size, if that file is not present already.
416 func (c *Cache) copyFile(file io.ReadSeeker, out OutputID, size int64) error {
417 name := c.fileName(out, "d")
418 info, err := os.Stat(name)
419 if err == nil && info.Size() == size {
421 if f, err := os.Open(name); err == nil {
431 // Hash did not match. Fall through and rewrite file.
434 // Copy file to cache directory.
435 mode := os.O_RDWR | os.O_CREATE
436 if err == nil && info.Size() > size { // shouldn't happen but fix in case
439 f, err := os.OpenFile(name, mode, 0666)
445 // File now exists with correct size.
446 // Only one possible zero-length file, so contents are OK too.
447 // Early return here makes sure there's a "last byte" for code below.
451 // From here on, if any of the I/O writing the file fails,
452 // we make a best-effort attempt to truncate the file f
453 // before returning, to avoid leaving bad bytes in the file.
455 // Copy file to f, but also into h to double-check hash.
456 if _, err := file.Seek(0, 0); err != nil {
461 w := io.MultiWriter(f, h)
462 if _, err := io.CopyN(w, file, size-1); err != nil {
466 // Check last byte before writing it; writing it will make the size match
467 // what other processes expect to find and might cause them to start
469 buf := make([]byte, 1)
470 if _, err := file.Read(buf); err != nil {
476 if !bytes.Equal(sum, out[:]) {
478 return fmt.Errorf("file content changed underfoot")
481 // Commit cache file entry.
482 if _, err := f.Write(buf); err != nil {
486 if err := f.Close(); err != nil {
487 // Data might not have been written,
488 // but file may look like it is the right size.
489 // To be extra careful, remove cached file.
493 os.Chtimes(name, c.now(), c.now()) // mainly for tests