cache.go

Documentation: cmd/go/internal/cache

     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.
     4  
     5  // Package cache implements a build artifact cache.
     6  package cache
     7  
     8  import (
     9  	"bytes"
    10  	"crypto/sha256"
    11  	"encoding/hex"
    12  	"errors"
    13  	"fmt"
    14  	"internal/godebug"
    15  	"io"
    16  	"io/fs"
    17  	"os"
    18  	"path/filepath"
    19  	"strconv"
    20  	"strings"
    21  	"time"
    22  
    23  	"cmd/go/internal/lockedfile"
    24  	"cmd/go/internal/mmap"
    25  )
    26  
    27  // An ActionID is a cache action key, the hash of a complete description of a
    28  // repeatable computation (command line, environment variables,
    29  // input file contents, executable contents).
    30  type ActionID [HashSize]byte
    31  
    32  // An OutputID is a cache output key, the hash of an output of a computation.
    33  type OutputID [HashSize]byte
    34  
    35  // Cache is the interface as used by the cmd/go.
    36  type Cache interface {
    37  	// Get returns the cache entry for the provided ActionID.
    38  	// On miss, the error type should be of type *entryNotFoundError.
    39  	//
    40  	// After a success call to Get, OutputFile(Entry.OutputID) must
    41  	// exist on disk for until Close is called (at the end of the process).
    42  	Get(ActionID) (Entry, error)
    43  
    44  	// Put adds an item to the cache.
    45  	//
    46  	// The seeker is only used to seek to the beginning. After a call to Put,
    47  	// the seek position is not guaranteed to be in any particular state.
    48  	//
    49  	// As a special case, if the ReadSeeker is of type noVerifyReadSeeker,
    50  	// the verification from GODEBUG=goverifycache=1 is skipped.
    51  	//
    52  	// After a success call to Get, OutputFile(Entry.OutputID) must
    53  	// exist on disk for until Close is called (at the end of the process).
    54  	Put(ActionID, io.ReadSeeker) (_ OutputID, size int64, _ error)
    55  
    56  	// Close is called at the end of the go process. Implementations can do
    57  	// cache cleanup work at this phase, or wait for and report any errors from
    58  	// background cleanup work started earlier. Any cache trimming should in one
    59  	// process should not violate cause the invariants of this interface to be
    60  	// violated in another process. Namely, a cache trim from one process should
    61  	// not delete an ObjectID from disk that was recently Get or Put from
    62  	// another process. As a rule of thumb, don't trim things used in the last
    63  	// day.
    64  	Close() error
    65  
    66  	// OutputFile returns the path on disk where OutputID is stored.
    67  	//
    68  	// It's only called after a successful get or put call so it doesn't need
    69  	// to return an error; it's assumed that if the previous get or put succeeded,
    70  	// it's already on disk.
    71  	OutputFile(OutputID) string
    72  
    73  	// FuzzDir returns where fuzz files are stored.
    74  	FuzzDir() string
    75  }
    76  
    77  // A Cache is a package cache, backed by a file system directory tree.
    78  type DiskCache struct {
    79  	dir string
    80  	now func() time.Time
    81  }
    82  
    83  // Open opens and returns the cache in the given directory.
    84  //
    85  // It is safe for multiple processes on a single machine to use the
    86  // same cache directory in a local file system simultaneously.
    87  // They will coordinate using operating system file locks and may
    88  // duplicate effort but will not corrupt the cache.
    89  //
    90  // However, it is NOT safe for multiple processes on different machines
    91  // to share a cache directory (for example, if the directory were stored
    92  // in a network file system). File locking is notoriously unreliable in
    93  // network file systems and may not suffice to protect the cache.
    94  func Open(dir string) (*DiskCache, error) {
    95  	info, err := os.Stat(dir)
    96  	if err != nil {
    97  		return nil, err
    98  	}
    99  	if !info.IsDir() {
   100  		return nil, &fs.PathError{Op: "open", Path: dir, Err: fmt.Errorf("not a directory")}
   101  	}
   102  	for i := 0; i < 256; i++ {
   103  		name := filepath.Join(dir, fmt.Sprintf("%02x", i))
   104  		if err := os.MkdirAll(name, 0777); err != nil {
   105  			return nil, err
   106  		}
   107  	}
   108  	c := &DiskCache{
   109  		dir: dir,
   110  		now: time.Now,
   111  	}
   112  	return c, nil
   113  }
   114  
   115  // fileName returns the name of the file corresponding to the given id.
   116  func (c *DiskCache) fileName(id [HashSize]byte, key string) string {
   117  	return filepath.Join(c.dir, fmt.Sprintf("%02x", id[0]), fmt.Sprintf("%x", id)+"-"+key)
   118  }
   119  
   120  // An entryNotFoundError indicates that a cache entry was not found, with an
   121  // optional underlying reason.
   122  type entryNotFoundError struct {
   123  	Err error
   124  }
   125  
   126  func (e *entryNotFoundError) Error() string {
   127  	if e.Err == nil {
   128  		return "cache entry not found"
   129  	}
   130  	return fmt.Sprintf("cache entry not found: %v", e.Err)
   131  }
   132  
   133  func (e *entryNotFoundError) Unwrap() error {
   134  	return e.Err
   135  }
   136  
   137  const (
   138  	// action entry file is "v1 <hex id> <hex out> <decimal size space-padded to 20 bytes> <unixnano space-padded to 20 bytes>\n"
   139  	hexSize   = HashSize * 2
   140  	entrySize = 2 + 1 + hexSize + 1 + hexSize + 1 + 20 + 1 + 20 + 1
   141  )
   142  
   143  // verify controls whether to run the cache in verify mode.
   144  // In verify mode, the cache always returns errMissing from Get
   145  // but then double-checks in Put that the data being written
   146  // exactly matches any existing entry. This provides an easy
   147  // way to detect program behavior that would have been different
   148  // had the cache entry been returned from Get.
   149  //
   150  // verify is enabled by setting the environment variable
   151  // GODEBUG=gocacheverify=1.
   152  var verify = false
   153  
   154  var errVerifyMode = errors.New("gocacheverify=1")
   155  
   156  // DebugTest is set when GODEBUG=gocachetest=1 is in the environment.
   157  var DebugTest = false
   158  
   159  func init() { initEnv() }
   160  
   161  var (
   162  	gocacheverify = godebug.New("gocacheverify")
   163  	gocachehash   = godebug.New("gocachehash")
   164  	gocachetest   = godebug.New("gocachetest")
   165  )
   166  
   167  func initEnv() {
   168  	if gocacheverify.Value() == "1" {
   169  		gocacheverify.IncNonDefault()
   170  		verify = true
   171  	}
   172  	if gocachehash.Value() == "1" {
   173  		gocachehash.IncNonDefault()
   174  		debugHash = true
   175  	}
   176  	if gocachetest.Value() == "1" {
   177  		gocachetest.IncNonDefault()
   178  		DebugTest = true
   179  	}
   180  }
   181  
   182  // Get looks up the action ID in the cache,
   183  // returning the corresponding output ID and file size, if any.
   184  // Note that finding an output ID does not guarantee that the
   185  // saved file for that output ID is still available.
   186  func (c *DiskCache) Get(id ActionID) (Entry, error) {
   187  	if verify {
   188  		return Entry{}, &entryNotFoundError{Err: errVerifyMode}
   189  	}
   190  	return c.get(id)
   191  }
   192  
   193  type Entry struct {
   194  	OutputID OutputID
   195  	Size     int64
   196  	Time     time.Time // when added to cache
   197  }
   198  
   199  // get is Get but does not respect verify mode, so that Put can use it.
   200  func (c *DiskCache) get(id ActionID) (Entry, error) {
   201  	missing := func(reason error) (Entry, error) {
   202  		return Entry{}, &entryNotFoundError{Err: reason}
   203  	}
   204  	f, err := os.Open(c.fileName(id, "a"))
   205  	if err != nil {
   206  		return missing(err)
   207  	}
   208  	defer f.Close()
   209  	entry := make([]byte, entrySize+1) // +1 to detect whether f is too long
   210  	if n, err := io.ReadFull(f, entry); n > entrySize {
   211  		return missing(errors.New("too long"))
   212  	} else if err != io.ErrUnexpectedEOF {
   213  		if err == io.EOF {
   214  			return missing(errors.New("file is empty"))
   215  		}
   216  		return missing(err)
   217  	} else if n < entrySize {
   218  		return missing(errors.New("entry file incomplete"))
   219  	}
   220  	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' {
   221  		return missing(errors.New("invalid header"))
   222  	}
   223  	eid, entry := entry[3:3+hexSize], entry[3+hexSize:]
   224  	eout, entry := entry[1:1+hexSize], entry[1+hexSize:]
   225  	esize, entry := entry[1:1+20], entry[1+20:]
   226  	etime, entry := entry[1:1+20], entry[1+20:]
   227  	var buf [HashSize]byte
   228  	if _, err := hex.Decode(buf[:], eid); err != nil {
   229  		return missing(fmt.Errorf("decoding ID: %v", err))
   230  	} else if buf != id {
   231  		return missing(errors.New("mismatched ID"))
   232  	}
   233  	if _, err := hex.Decode(buf[:], eout); err != nil {
   234  		return missing(fmt.Errorf("decoding output ID: %v", err))
   235  	}
   236  	i := 0
   237  	for i < len(esize) && esize[i] == ' ' {
   238  		i++
   239  	}
   240  	size, err := strconv.ParseInt(string(esize[i:]), 10, 64)
   241  	if err != nil {
   242  		return missing(fmt.Errorf("parsing size: %v", err))
   243  	} else if size < 0 {
   244  		return missing(errors.New("negative size"))
   245  	}
   246  	i = 0
   247  	for i < len(etime) && etime[i] == ' ' {
   248  		i++
   249  	}
   250  	tm, err := strconv.ParseInt(string(etime[i:]), 10, 64)
   251  	if err != nil {
   252  		return missing(fmt.Errorf("parsing timestamp: %v", err))
   253  	} else if tm < 0 {
   254  		return missing(errors.New("negative timestamp"))
   255  	}
   256  
   257  	c.used(c.fileName(id, "a"))
   258  
   259  	return Entry{buf, size, time.Unix(0, tm)}, nil
   260  }
   261  
   262  // GetFile looks up the action ID in the cache and returns
   263  // the name of the corresponding data file.
   264  func GetFile(c Cache, id ActionID) (file string, entry Entry, err error) {
   265  	entry, err = c.Get(id)
   266  	if err != nil {
   267  		return "", Entry{}, err
   268  	}
   269  	file = c.OutputFile(entry.OutputID)
   270  	info, err := os.Stat(file)
   271  	if err != nil {
   272  		return "", Entry{}, &entryNotFoundError{Err: err}
   273  	}
   274  	if info.Size() != entry.Size {
   275  		return "", Entry{}, &entryNotFoundError{Err: errors.New("file incomplete")}
   276  	}
   277  	return file, entry, nil
   278  }
   279  
   280  // GetBytes looks up the action ID in the cache and returns
   281  // the corresponding output bytes.
   282  // GetBytes should only be used for data that can be expected to fit in memory.
   283  func GetBytes(c Cache, id ActionID) ([]byte, Entry, error) {
   284  	entry, err := c.Get(id)
   285  	if err != nil {
   286  		return nil, entry, err
   287  	}
   288  	data, _ := os.ReadFile(c.OutputFile(entry.OutputID))
   289  	if sha256.Sum256(data) != entry.OutputID {
   290  		return nil, entry, &entryNotFoundError{Err: errors.New("bad checksum")}
   291  	}
   292  	return data, entry, nil
   293  }
   294  
   295  // GetMmap looks up the action ID in the cache and returns
   296  // the corresponding output bytes.
   297  // GetMmap should only be used for data that can be expected to fit in memory.
   298  func GetMmap(c Cache, id ActionID) ([]byte, Entry, error) {
   299  	entry, err := c.Get(id)
   300  	if err != nil {
   301  		return nil, entry, err
   302  	}
   303  	md, err := mmap.Mmap(c.OutputFile(entry.OutputID))
   304  	if err != nil {
   305  		return nil, Entry{}, err
   306  	}
   307  	if int64(len(md.Data)) != entry.Size {
   308  		return nil, Entry{}, &entryNotFoundError{Err: errors.New("file incomplete")}
   309  	}
   310  	return md.Data, entry, nil
   311  }
   312  
   313  // OutputFile returns the name of the cache file storing output with the given OutputID.
   314  func (c *DiskCache) OutputFile(out OutputID) string {
   315  	file := c.fileName(out, "d")
   316  	c.used(file)
   317  	return file
   318  }
   319  
   320  // Time constants for cache expiration.
   321  //
   322  // We set the mtime on a cache file on each use, but at most one per mtimeInterval (1 hour),
   323  // to avoid causing many unnecessary inode updates. The mtimes therefore
   324  // roughly reflect "time of last use" but may in fact be older by at most an hour.
   325  //
   326  // We scan the cache for entries to delete at most once per trimInterval (1 day).
   327  //
   328  // When we do scan the cache, we delete entries that have not been used for
   329  // at least trimLimit (5 days). Statistics gathered from a month of usage by
   330  // Go developers found that essentially all reuse of cached entries happened
   331  // within 5 days of the previous reuse. See golang.org/issue/22990.
   332  const (
   333  	mtimeInterval = 1 * time.Hour
   334  	trimInterval  = 24 * time.Hour
   335  	trimLimit     = 5 * 24 * time.Hour
   336  )
   337  
   338  // used makes a best-effort attempt to update mtime on file,
   339  // so that mtime reflects cache access time.
   340  //
   341  // Because the reflection only needs to be approximate,
   342  // and to reduce the amount of disk activity caused by using
   343  // cache entries, used only updates the mtime if the current
   344  // mtime is more than an hour old. This heuristic eliminates
   345  // nearly all of the mtime updates that would otherwise happen,
   346  // while still keeping the mtimes useful for cache trimming.
   347  func (c *DiskCache) used(file string) {
   348  	info, err := os.Stat(file)
   349  	if err == nil && c.now().Sub(info.ModTime()) < mtimeInterval {
   350  		return
   351  	}
   352  	os.Chtimes(file, c.now(), c.now())
   353  }
   354  
   355  func (c *DiskCache) Close() error { return c.Trim() }
   356  
   357  // Trim removes old cache entries that are likely not to be reused.
   358  func (c *DiskCache) Trim() error {
   359  	now := c.now()
   360  
   361  	// We maintain in dir/trim.txt the time of the last completed cache trim.
   362  	// If the cache has been trimmed recently enough, do nothing.
   363  	// This is the common case.
   364  	// If the trim file is corrupt, detected if the file can't be parsed, or the
   365  	// trim time is too far in the future, attempt the trim anyway. It's possible that
   366  	// the cache was full when the corruption happened. Attempting a trim on
   367  	// an empty cache is cheap, so there wouldn't be a big performance hit in that case.
   368  	if data, err := lockedfile.Read(filepath.Join(c.dir, "trim.txt")); err == nil {
   369  		if t, err := strconv.ParseInt(strings.TrimSpace(string(data)), 10, 64); err == nil {
   370  			lastTrim := time.Unix(t, 0)
   371  			if d := now.Sub(lastTrim); d < trimInterval && d > -mtimeInterval {
   372  				return nil
   373  			}
   374  		}
   375  	}
   376  
   377  	// Trim each of the 256 subdirectories.
   378  	// We subtract an additional mtimeInterval
   379  	// to account for the imprecision of our "last used" mtimes.
   380  	cutoff := now.Add(-trimLimit - mtimeInterval)
   381  	for i := 0; i < 256; i++ {
   382  		subdir := filepath.Join(c.dir, fmt.Sprintf("%02x", i))
   383  		c.trimSubdir(subdir, cutoff)
   384  	}
   385  
   386  	// Ignore errors from here: if we don't write the complete timestamp, the
   387  	// cache will appear older than it is, and we'll trim it again next time.
   388  	var b bytes.Buffer
   389  	fmt.Fprintf(&b, "%d", now.Unix())
   390  	if err := lockedfile.Write(filepath.Join(c.dir, "trim.txt"), &b, 0666); err != nil {
   391  		return err
   392  	}
   393  
   394  	return nil
   395  }
   396  
   397  // trimSubdir trims a single cache subdirectory.
   398  func (c *DiskCache) trimSubdir(subdir string, cutoff time.Time) {
   399  	// Read all directory entries from subdir before removing
   400  	// any files, in case removing files invalidates the file offset
   401  	// in the directory scan. Also, ignore error from f.Readdirnames,
   402  	// because we don't care about reporting the error and we still
   403  	// want to process any entries found before the error.
   404  	f, err := os.Open(subdir)
   405  	if err != nil {
   406  		return
   407  	}
   408  	names, _ := f.Readdirnames(-1)
   409  	f.Close()
   410  
   411  	for _, name := range names {
   412  		// Remove only cache entries (xxxx-a and xxxx-d).
   413  		if !strings.HasSuffix(name, "-a") && !strings.HasSuffix(name, "-d") {
   414  			continue
   415  		}
   416  		entry := filepath.Join(subdir, name)
   417  		info, err := os.Stat(entry)
   418  		if err == nil && info.ModTime().Before(cutoff) {
   419  			os.Remove(entry)
   420  		}
   421  	}
   422  }
   423  
   424  // putIndexEntry adds an entry to the cache recording that executing the action
   425  // with the given id produces an output with the given output id (hash) and size.
   426  func (c *DiskCache) putIndexEntry(id ActionID, out OutputID, size int64, allowVerify bool) error {
   427  	// Note: We expect that for one reason or another it may happen
   428  	// that repeating an action produces a different output hash
   429  	// (for example, if the output contains a time stamp or temp dir name).
   430  	// While not ideal, this is also not a correctness problem, so we
   431  	// don't make a big deal about it. In particular, we leave the action
   432  	// cache entries writable specifically so that they can be overwritten.
   433  	//
   434  	// Setting GODEBUG=gocacheverify=1 does make a big deal:
   435  	// in verify mode we are double-checking that the cache entries
   436  	// are entirely reproducible. As just noted, this may be unrealistic
   437  	// in some cases but the check is also useful for shaking out real bugs.
   438  	entry := fmt.Sprintf("v1 %x %x %20d %20d\n", id, out, size, time.Now().UnixNano())
   439  	if verify && allowVerify {
   440  		old, err := c.get(id)
   441  		if err == nil && (old.OutputID != out || old.Size != size) {
   442  			// panic to show stack trace, so we can see what code is generating this cache entry.
   443  			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)
   444  			panic(msg)
   445  		}
   446  	}
   447  	file := c.fileName(id, "a")
   448  
   449  	// Copy file to cache directory.
   450  	mode := os.O_WRONLY | os.O_CREATE
   451  	f, err := os.OpenFile(file, mode, 0666)
   452  	if err != nil {
   453  		return err
   454  	}
   455  	_, err = f.WriteString(entry)
   456  	if err == nil {
   457  		// Truncate the file only *after* writing it.
   458  		// (This should be a no-op, but truncate just in case of previous corruption.)
   459  		//
   460  		// This differs from os.WriteFile, which truncates to 0 *before* writing
   461  		// via os.O_TRUNC. Truncating only after writing ensures that a second write
   462  		// of the same content to the same file is idempotent, and does not — even
   463  		// temporarily! — undo the effect of the first write.
   464  		err = f.Truncate(int64(len(entry)))
   465  	}
   466  	if closeErr := f.Close(); err == nil {
   467  		err = closeErr
   468  	}
   469  	if err != nil {
   470  		// TODO(bcmills): This Remove potentially races with another go command writing to file.
   471  		// Can we eliminate it?
   472  		os.Remove(file)
   473  		return err
   474  	}
   475  	os.Chtimes(file, c.now(), c.now()) // mainly for tests
   476  
   477  	return nil
   478  }
   479  
   480  // noVerifyReadSeeker is an io.ReadSeeker wrapper sentinel type
   481  // that says that Cache.Put should skip the verify check
   482  // (from GODEBUG=goverifycache=1).
   483  type noVerifyReadSeeker struct {
   484  	io.ReadSeeker
   485  }
   486  
   487  // Put stores the given output in the cache as the output for the action ID.
   488  // It may read file twice. The content of file must not change between the two passes.
   489  func (c *DiskCache) Put(id ActionID, file io.ReadSeeker) (OutputID, int64, error) {
   490  	wrapper, isNoVerify := file.(noVerifyReadSeeker)
   491  	if isNoVerify {
   492  		file = wrapper.ReadSeeker
   493  	}
   494  	return c.put(id, file, !isNoVerify)
   495  }
   496  
   497  // PutNoVerify is like Put but disables the verify check
   498  // when GODEBUG=goverifycache=1 is set.
   499  // It is meant for data that is OK to cache but that we expect to vary slightly from run to run,
   500  // like test output containing times and the like.
   501  func PutNoVerify(c Cache, id ActionID, file io.ReadSeeker) (OutputID, int64, error) {
   502  	return c.Put(id, noVerifyReadSeeker{file})
   503  }
   504  
   505  func (c *DiskCache) put(id ActionID, file io.ReadSeeker, allowVerify bool) (OutputID, int64, error) {
   506  	// Compute output ID.
   507  	h := sha256.New()
   508  	if _, err := file.Seek(0, 0); err != nil {
   509  		return OutputID{}, 0, err
   510  	}
   511  	size, err := io.Copy(h, file)
   512  	if err != nil {
   513  		return OutputID{}, 0, err
   514  	}
   515  	var out OutputID
   516  	h.Sum(out[:0])
   517  
   518  	// Copy to cached output file (if not already present).
   519  	if err := c.copyFile(file, out, size); err != nil {
   520  		return out, size, err
   521  	}
   522  
   523  	// Add to cache index.
   524  	return out, size, c.putIndexEntry(id, out, size, allowVerify)
   525  }
   526  
   527  // PutBytes stores the given bytes in the cache as the output for the action ID.
   528  func PutBytes(c Cache, id ActionID, data []byte) error {
   529  	_, _, err := c.Put(id, bytes.NewReader(data))
   530  	return err
   531  }
   532  
   533  // copyFile copies file into the cache, expecting it to have the given
   534  // output ID and size, if that file is not present already.
   535  func (c *DiskCache) copyFile(file io.ReadSeeker, out OutputID, size int64) error {
   536  	name := c.fileName(out, "d")
   537  	info, err := os.Stat(name)
   538  	if err == nil && info.Size() == size {
   539  		// Check hash.
   540  		if f, err := os.Open(name); err == nil {
   541  			h := sha256.New()
   542  			io.Copy(h, f)
   543  			f.Close()
   544  			var out2 OutputID
   545  			h.Sum(out2[:0])
   546  			if out == out2 {
   547  				return nil
   548  			}
   549  		}
   550  		// Hash did not match. Fall through and rewrite file.
   551  	}
   552  
   553  	// Copy file to cache directory.
   554  	mode := os.O_RDWR | os.O_CREATE
   555  	if err == nil && info.Size() > size { // shouldn't happen but fix in case
   556  		mode |= os.O_TRUNC
   557  	}
   558  	f, err := os.OpenFile(name, mode, 0666)
   559  	if err != nil {
   560  		return err
   561  	}
   562  	defer f.Close()
   563  	if size == 0 {
   564  		// File now exists with correct size.
   565  		// Only one possible zero-length file, so contents are OK too.
   566  		// Early return here makes sure there's a "last byte" for code below.
   567  		return nil
   568  	}
   569  
   570  	// From here on, if any of the I/O writing the file fails,
   571  	// we make a best-effort attempt to truncate the file f
   572  	// before returning, to avoid leaving bad bytes in the file.
   573  
   574  	// Copy file to f, but also into h to double-check hash.
   575  	if _, err := file.Seek(0, 0); err != nil {
   576  		f.Truncate(0)
   577  		return err
   578  	}
   579  	h := sha256.New()
   580  	w := io.MultiWriter(f, h)
   581  	if _, err := io.CopyN(w, file, size-1); err != nil {
   582  		f.Truncate(0)
   583  		return err
   584  	}
   585  	// Check last byte before writing it; writing it will make the size match
   586  	// what other processes expect to find and might cause them to start
   587  	// using the file.
   588  	buf := make([]byte, 1)
   589  	if _, err := file.Read(buf); err != nil {
   590  		f.Truncate(0)
   591  		return err
   592  	}
   593  	h.Write(buf)
   594  	sum := h.Sum(nil)
   595  	if !bytes.Equal(sum, out[:]) {
   596  		f.Truncate(0)
   597  		return fmt.Errorf("file content changed underfoot")
   598  	}
   599  
   600  	// Commit cache file entry.
   601  	if _, err := f.Write(buf); err != nil {
   602  		f.Truncate(0)
   603  		return err
   604  	}
   605  	if err := f.Close(); err != nil {
   606  		// Data might not have been written,
   607  		// but file may look like it is the right size.
   608  		// To be extra careful, remove cached file.
   609  		os.Remove(name)
   610  		return err
   611  	}
   612  	os.Chtimes(name, c.now(), c.now()) // mainly for tests
   613  
   614  	return nil
   615  }
   616  
   617  // FuzzDir returns a subdirectory within the cache for storing fuzzing data.
   618  // The subdirectory may not exist.
   619  //
   620  // This directory is managed by the internal/fuzz package. Files in this
   621  // directory aren't removed by the 'go clean -cache' command or by Trim.
   622  // They may be removed with 'go clean -fuzzcache'.
   623  //
   624  // TODO(#48526): make Trim remove unused files from this directory.
   625  func (c *DiskCache) FuzzDir() string {
   626  	return filepath.Join(c.dir, "fuzz")
   627  }
   628
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