cache.go

     1  // Copyright 2018 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 modfetch
     6  
     7  import (
     8  	"bytes"
     9  	"context"
    10  	"encoding/json"
    11  	"errors"
    12  	"fmt"
    13  	"io"
    14  	"io/fs"
    15  	"math/rand"
    16  	"os"
    17  	"path/filepath"
    18  	"strconv"
    19  	"strings"
    20  	"sync"
    21  
    22  	"cmd/go/internal/base"
    23  	"cmd/go/internal/cfg"
    24  	"cmd/go/internal/gover"
    25  	"cmd/go/internal/lockedfile"
    26  	"cmd/go/internal/modfetch/codehost"
    27  	"cmd/internal/par"
    28  	"cmd/internal/robustio"
    29  	"cmd/internal/telemetry/counter"
    30  
    31  	"golang.org/x/mod/module"
    32  	"golang.org/x/mod/semver"
    33  )
    34  
    35  func cacheDir(ctx context.Context, path string) (string, error) {
    36  	if err := checkCacheDir(ctx); err != nil {
    37  		return "", err
    38  	}
    39  	enc, err := module.EscapePath(path)
    40  	if err != nil {
    41  		return "", err
    42  	}
    43  	return filepath.Join(cfg.GOMODCACHE, "cache/download", enc, "/@v"), nil
    44  }
    45  
    46  func CachePath(ctx context.Context, m module.Version, suffix string) (string, error) {
    47  	if gover.IsToolchain(m.Path) {
    48  		return "", ErrToolchain
    49  	}
    50  	dir, err := cacheDir(ctx, m.Path)
    51  	if err != nil {
    52  		return "", err
    53  	}
    54  	if !gover.ModIsValid(m.Path, m.Version) {
    55  		return "", fmt.Errorf("non-semver module version %q", m.Version)
    56  	}
    57  	if module.CanonicalVersion(m.Version) != m.Version {
    58  		return "", fmt.Errorf("non-canonical module version %q", m.Version)
    59  	}
    60  	encVer, err := module.EscapeVersion(m.Version)
    61  	if err != nil {
    62  		return "", err
    63  	}
    64  	return filepath.Join(dir, encVer+"."+suffix), nil
    65  }
    66  
    67  // DownloadDir returns the directory to which m should have been downloaded.
    68  // An error will be returned if the module path or version cannot be escaped.
    69  // An error satisfying errors.Is(err, fs.ErrNotExist) will be returned
    70  // along with the directory if the directory does not exist or if the directory
    71  // is not completely populated.
    72  func DownloadDir(ctx context.Context, m module.Version) (string, error) {
    73  	if gover.IsToolchain(m.Path) {
    74  		return "", ErrToolchain
    75  	}
    76  	if err := checkCacheDir(ctx); err != nil {
    77  		return "", err
    78  	}
    79  	enc, err := module.EscapePath(m.Path)
    80  	if err != nil {
    81  		return "", err
    82  	}
    83  	if !gover.ModIsValid(m.Path, m.Version) {
    84  		return "", fmt.Errorf("non-semver module version %q", m.Version)
    85  	}
    86  	if module.CanonicalVersion(m.Version) != m.Version {
    87  		return "", fmt.Errorf("non-canonical module version %q", m.Version)
    88  	}
    89  	encVer, err := module.EscapeVersion(m.Version)
    90  	if err != nil {
    91  		return "", err
    92  	}
    93  
    94  	// Check whether the directory itself exists.
    95  	dir := filepath.Join(cfg.GOMODCACHE, enc+"@"+encVer)
    96  	if fi, err := os.Stat(dir); os.IsNotExist(err) {
    97  		return dir, err
    98  	} else if err != nil {
    99  		return dir, &DownloadDirPartialError{dir, err}
   100  	} else if !fi.IsDir() {
   101  		return dir, &DownloadDirPartialError{dir, errors.New("not a directory")}
   102  	}
   103  
   104  	// Check if a .partial file exists. This is created at the beginning of
   105  	// a download and removed after the zip is extracted.
   106  	partialPath, err := CachePath(ctx, m, "partial")
   107  	if err != nil {
   108  		return dir, err
   109  	}
   110  	if _, err := os.Stat(partialPath); err == nil {
   111  		return dir, &DownloadDirPartialError{dir, errors.New("not completely extracted")}
   112  	} else if !os.IsNotExist(err) {
   113  		return dir, err
   114  	}
   115  
   116  	// Special case: ziphash is not required for the golang.org/fips140 module,
   117  	// because it is unpacked from a file in GOROOT, not downloaded.
   118  	// We've already checked that it's not a partial unpacking, so we're happy.
   119  	if m.Path == "golang.org/fips140" {
   120  		return dir, nil
   121  	}
   122  
   123  	// Check if a .ziphash file exists. It should be created before the
   124  	// zip is extracted, but if it was deleted (by another program?), we need
   125  	// to re-calculate it. Note that checkMod will repopulate the ziphash
   126  	// file if it doesn't exist, but if the module is excluded by checks
   127  	// through GONOSUMDB or GOPRIVATE, that check and repopulation won't happen.
   128  	ziphashPath, err := CachePath(ctx, m, "ziphash")
   129  	if err != nil {
   130  		return dir, err
   131  	}
   132  	if _, err := os.Stat(ziphashPath); os.IsNotExist(err) {
   133  		return dir, &DownloadDirPartialError{dir, errors.New("ziphash file is missing")}
   134  	} else if err != nil {
   135  		return dir, err
   136  	}
   137  	return dir, nil
   138  }
   139  
   140  // DownloadDirPartialError is returned by DownloadDir if a module directory
   141  // exists but was not completely populated.
   142  //
   143  // DownloadDirPartialError is equivalent to fs.ErrNotExist.
   144  type DownloadDirPartialError struct {
   145  	Dir string
   146  	Err error
   147  }
   148  
   149  func (e *DownloadDirPartialError) Error() string     { return fmt.Sprintf("%s: %v", e.Dir, e.Err) }
   150  func (e *DownloadDirPartialError) Is(err error) bool { return err == fs.ErrNotExist }
   151  
   152  // lockVersion locks a file within the module cache that guards the downloading
   153  // and extraction of the zipfile for the given module version.
   154  func lockVersion(ctx context.Context, mod module.Version) (unlock func(), err error) {
   155  	path, err := CachePath(ctx, mod, "lock")
   156  	if err != nil {
   157  		return nil, err
   158  	}
   159  	if err := os.MkdirAll(filepath.Dir(path), 0777); err != nil {
   160  		return nil, err
   161  	}
   162  	return lockedfile.MutexAt(path).Lock()
   163  }
   164  
   165  // SideLock locks a file within the module cache that previously guarded
   166  // edits to files outside the cache, such as go.sum and go.mod files in the
   167  // user's working directory.
   168  // If err is nil, the caller MUST eventually call the unlock function.
   169  func SideLock(ctx context.Context) (unlock func(), err error) {
   170  	if err := checkCacheDir(ctx); err != nil {
   171  		return nil, err
   172  	}
   173  
   174  	path := filepath.Join(cfg.GOMODCACHE, "cache", "lock")
   175  	if err := os.MkdirAll(filepath.Dir(path), 0777); err != nil {
   176  		return nil, fmt.Errorf("failed to create cache directory: %w", err)
   177  	}
   178  
   179  	return lockedfile.MutexAt(path).Lock()
   180  }
   181  
   182  // A cachingRepo is a cache around an underlying Repo,
   183  // avoiding redundant calls to ModulePath, Versions, Stat, Latest, and GoMod (but not CheckReuse or Zip).
   184  // It is also safe for simultaneous use by multiple goroutines
   185  // (so that it can be returned from Lookup multiple times).
   186  // It serializes calls to the underlying Repo.
   187  type cachingRepo struct {
   188  	path          string
   189  	versionsCache par.ErrCache[string, *Versions]
   190  	statCache     par.ErrCache[string, *RevInfo]
   191  	latestCache   par.ErrCache[struct{}, *RevInfo]
   192  	gomodCache    par.ErrCache[string, []byte]
   193  
   194  	once     sync.Once
   195  	initRepo func(context.Context) (Repo, error)
   196  	r        Repo
   197  }
   198  
   199  func newCachingRepo(ctx context.Context, path string, initRepo func(context.Context) (Repo, error)) *cachingRepo {
   200  	return &cachingRepo{
   201  		path:     path,
   202  		initRepo: initRepo,
   203  	}
   204  }
   205  
   206  func (r *cachingRepo) repo(ctx context.Context) Repo {
   207  	r.once.Do(func() {
   208  		var err error
   209  		r.r, err = r.initRepo(ctx)
   210  		if err != nil {
   211  			r.r = errRepo{r.path, err}
   212  		}
   213  	})
   214  	return r.r
   215  }
   216  
   217  func (r *cachingRepo) CheckReuse(ctx context.Context, old *codehost.Origin) error {
   218  	return r.repo(ctx).CheckReuse(ctx, old)
   219  }
   220  
   221  func (r *cachingRepo) ModulePath() string {
   222  	return r.path
   223  }
   224  
   225  func (r *cachingRepo) Versions(ctx context.Context, prefix string) (*Versions, error) {
   226  	v, err := r.versionsCache.Do(prefix, func() (*Versions, error) {
   227  		return r.repo(ctx).Versions(ctx, prefix)
   228  	})
   229  
   230  	if err != nil {
   231  		return nil, err
   232  	}
   233  	return &Versions{
   234  		Origin: v.Origin,
   235  		List:   append([]string(nil), v.List...),
   236  	}, nil
   237  }
   238  
   239  type cachedInfo struct {
   240  	info *RevInfo
   241  	err  error
   242  }
   243  
   244  func (r *cachingRepo) Stat(ctx context.Context, rev string) (*RevInfo, error) {
   245  	if gover.IsToolchain(r.path) {
   246  		// Skip disk cache; the underlying golang.org/toolchain repo is cached instead.
   247  		return r.repo(ctx).Stat(ctx, rev)
   248  	}
   249  	info, err := r.statCache.Do(rev, func() (*RevInfo, error) {
   250  		file, info, err := readDiskStat(ctx, r.path, rev)
   251  		if err == nil {
   252  			return info, err
   253  		}
   254  
   255  		info, err = r.repo(ctx).Stat(ctx, rev)
   256  		if err == nil {
   257  			// If we resolved, say, 1234abcde to v0.0.0-20180604122334-1234abcdef78,
   258  			// then save the information under the proper version, for future use.
   259  			if info.Version != rev {
   260  				file, _ = CachePath(ctx, module.Version{Path: r.path, Version: info.Version}, "info")
   261  				r.statCache.Do(info.Version, func() (*RevInfo, error) {
   262  					return info, nil
   263  				})
   264  			}
   265  
   266  			if err := writeDiskStat(ctx, file, info); err != nil {
   267  				fmt.Fprintf(os.Stderr, "go: writing stat cache: %v\n", err)
   268  			}
   269  		}
   270  		return info, err
   271  	})
   272  	if info != nil {
   273  		copy := *info
   274  		info = &copy
   275  	}
   276  	return info, err
   277  }
   278  
   279  func (r *cachingRepo) Latest(ctx context.Context) (*RevInfo, error) {
   280  	if gover.IsToolchain(r.path) {
   281  		// Skip disk cache; the underlying golang.org/toolchain repo is cached instead.
   282  		return r.repo(ctx).Latest(ctx)
   283  	}
   284  	info, err := r.latestCache.Do(struct{}{}, func() (*RevInfo, error) {
   285  		info, err := r.repo(ctx).Latest(ctx)
   286  
   287  		// Save info for likely future Stat call.
   288  		if err == nil {
   289  			r.statCache.Do(info.Version, func() (*RevInfo, error) {
   290  				return info, nil
   291  			})
   292  			if file, _, err := readDiskStat(ctx, r.path, info.Version); err != nil {
   293  				writeDiskStat(ctx, file, info)
   294  			}
   295  		}
   296  
   297  		return info, err
   298  	})
   299  	if info != nil {
   300  		copy := *info
   301  		info = &copy
   302  	}
   303  	return info, err
   304  }
   305  
   306  func (r *cachingRepo) GoMod(ctx context.Context, version string) ([]byte, error) {
   307  	if gover.IsToolchain(r.path) {
   308  		// Skip disk cache; the underlying golang.org/toolchain repo is cached instead.
   309  		return r.repo(ctx).GoMod(ctx, version)
   310  	}
   311  	text, err := r.gomodCache.Do(version, func() ([]byte, error) {
   312  		file, text, err := readDiskGoMod(ctx, r.path, version)
   313  		if err == nil {
   314  			// Note: readDiskGoMod already called checkGoMod.
   315  			return text, nil
   316  		}
   317  
   318  		text, err = r.repo(ctx).GoMod(ctx, version)
   319  		if err == nil {
   320  			if err := checkGoMod(r.path, version, text); err != nil {
   321  				return text, err
   322  			}
   323  			if err := writeDiskGoMod(ctx, file, text); err != nil {
   324  				fmt.Fprintf(os.Stderr, "go: writing go.mod cache: %v\n", err)
   325  			}
   326  		}
   327  		return text, err
   328  	})
   329  	if err != nil {
   330  		return nil, err
   331  	}
   332  	return append([]byte(nil), text...), nil
   333  }
   334  
   335  func (r *cachingRepo) Zip(ctx context.Context, dst io.Writer, version string) error {
   336  	if gover.IsToolchain(r.path) {
   337  		return ErrToolchain
   338  	}
   339  	return r.repo(ctx).Zip(ctx, dst, version)
   340  }
   341  
   342  // InfoFile is like Lookup(ctx, path).Stat(version) but also returns the name of the file
   343  // containing the cached information.
   344  func InfoFile(ctx context.Context, path, version string) (*RevInfo, string, error) {
   345  	if !gover.ModIsValid(path, version) {
   346  		return nil, "", fmt.Errorf("invalid version %q", version)
   347  	}
   348  
   349  	if file, info, err := readDiskStat(ctx, path, version); err == nil {
   350  		return info, file, nil
   351  	}
   352  
   353  	var info *RevInfo
   354  	var err2info map[error]*RevInfo
   355  	err := TryProxies(func(proxy string) error {
   356  		i, err := Lookup(ctx, proxy, path).Stat(ctx, version)
   357  		if err == nil {
   358  			info = i
   359  		} else {
   360  			if err2info == nil {
   361  				err2info = make(map[error]*RevInfo)
   362  			}
   363  			err2info[err] = info
   364  		}
   365  		return err
   366  	})
   367  	if err != nil {
   368  		return err2info[err], "", err
   369  	}
   370  
   371  	// Stat should have populated the disk cache for us.
   372  	file, err := CachePath(ctx, module.Version{Path: path, Version: version}, "info")
   373  	if err != nil {
   374  		return nil, "", err
   375  	}
   376  	return info, file, nil
   377  }
   378  
   379  // GoMod is like Lookup(ctx, path).GoMod(rev) but avoids the
   380  // repository path resolution in Lookup if the result is
   381  // already cached on local disk.
   382  func GoMod(ctx context.Context, path, rev string) ([]byte, error) {
   383  	// Convert commit hash to pseudo-version
   384  	// to increase cache hit rate.
   385  	if !gover.ModIsValid(path, rev) {
   386  		if _, info, err := readDiskStat(ctx, path, rev); err == nil {
   387  			rev = info.Version
   388  		} else {
   389  			if errors.Is(err, statCacheErr) {
   390  				return nil, err
   391  			}
   392  			err := TryProxies(func(proxy string) error {
   393  				info, err := Lookup(ctx, proxy, path).Stat(ctx, rev)
   394  				if err == nil {
   395  					rev = info.Version
   396  				}
   397  				return err
   398  			})
   399  			if err != nil {
   400  				return nil, err
   401  			}
   402  		}
   403  	}
   404  
   405  	_, data, err := readDiskGoMod(ctx, path, rev)
   406  	if err == nil {
   407  		return data, nil
   408  	}
   409  
   410  	err = TryProxies(func(proxy string) (err error) {
   411  		data, err = Lookup(ctx, proxy, path).GoMod(ctx, rev)
   412  		return err
   413  	})
   414  	return data, err
   415  }
   416  
   417  // GoModFile is like GoMod but returns the name of the file containing
   418  // the cached information.
   419  func GoModFile(ctx context.Context, path, version string) (string, error) {
   420  	if !gover.ModIsValid(path, version) {
   421  		return "", fmt.Errorf("invalid version %q", version)
   422  	}
   423  	if _, err := GoMod(ctx, path, version); err != nil {
   424  		return "", err
   425  	}
   426  	// GoMod should have populated the disk cache for us.
   427  	file, err := CachePath(ctx, module.Version{Path: path, Version: version}, "mod")
   428  	if err != nil {
   429  		return "", err
   430  	}
   431  	return file, nil
   432  }
   433  
   434  // GoModSum returns the go.sum entry for the module version's go.mod file.
   435  // (That is, it returns the entry listed in go.sum as "path version/go.mod".)
   436  func GoModSum(ctx context.Context, path, version string) (string, error) {
   437  	if !gover.ModIsValid(path, version) {
   438  		return "", fmt.Errorf("invalid version %q", version)
   439  	}
   440  	data, err := GoMod(ctx, path, version)
   441  	if err != nil {
   442  		return "", err
   443  	}
   444  	sum, err := goModSum(data)
   445  	if err != nil {
   446  		return "", err
   447  	}
   448  	return sum, nil
   449  }
   450  
   451  var errNotCached = fmt.Errorf("not in cache")
   452  
   453  // readDiskStat reads a cached stat result from disk,
   454  // returning the name of the cache file and the result.
   455  // If the read fails, the caller can use
   456  // writeDiskStat(file, info) to write a new cache entry.
   457  func readDiskStat(ctx context.Context, path, rev string) (file string, info *RevInfo, err error) {
   458  	if gover.IsToolchain(path) {
   459  		return "", nil, errNotCached
   460  	}
   461  	file, data, err := readDiskCache(ctx, path, rev, "info")
   462  	if err != nil {
   463  		// If the cache already contains a pseudo-version with the given hash, we
   464  		// would previously return that pseudo-version without checking upstream.
   465  		// However, that produced an unfortunate side-effect: if the author added a
   466  		// tag to the repository, 'go get' would not pick up the effect of that new
   467  		// tag on the existing commits, and 'go' commands that referred to those
   468  		// commits would use the previous name instead of the new one.
   469  		//
   470  		// That's especially problematic if the original pseudo-version starts with
   471  		// v0.0.0-, as was the case for all pseudo-versions during vgo development,
   472  		// since a v0.0.0- pseudo-version has lower precedence than pretty much any
   473  		// tagged version.
   474  		//
   475  		// In practice, we're only looking up by hash during initial conversion of a
   476  		// legacy config and during an explicit 'go get', and a little extra latency
   477  		// for those operations seems worth the benefit of picking up more accurate
   478  		// versions.
   479  		//
   480  		// Fall back to this resolution scheme only if the GOPROXY setting prohibits
   481  		// us from resolving upstream tags.
   482  		if cfg.GOPROXY == "off" {
   483  			if file, info, err := readDiskStatByHash(ctx, path, rev); err == nil {
   484  				return file, info, nil
   485  			}
   486  		}
   487  		return file, nil, err
   488  	}
   489  	info = new(RevInfo)
   490  	if err := json.Unmarshal(data, info); err != nil {
   491  		return file, nil, errNotCached
   492  	}
   493  	// The disk might have stale .info files that have Name and Short fields set.
   494  	// We want to canonicalize to .info files with those fields omitted.
   495  	// Remarshal and update the cache file if needed.
   496  	data2, err := json.Marshal(info)
   497  	if err == nil && !bytes.Equal(data2, data) {
   498  		writeDiskCache(ctx, file, data)
   499  	}
   500  	return file, info, nil
   501  }
   502  
   503  // readDiskStatByHash is a fallback for readDiskStat for the case
   504  // where rev is a commit hash instead of a proper semantic version.
   505  // In that case, we look for a cached pseudo-version that matches
   506  // the commit hash. If we find one, we use it.
   507  // This matters most for converting legacy package management
   508  // configs, when we are often looking up commits by full hash.
   509  // Without this check we'd be doing network I/O to the remote repo
   510  // just to find out about a commit we already know about
   511  // (and have cached under its pseudo-version).
   512  func readDiskStatByHash(ctx context.Context, path, rev string) (file string, info *RevInfo, err error) {
   513  	if gover.IsToolchain(path) {
   514  		return "", nil, errNotCached
   515  	}
   516  	if cfg.GOMODCACHE == "" {
   517  		// Do not download to current directory.
   518  		return "", nil, errNotCached
   519  	}
   520  
   521  	if !codehost.AllHex(rev) || len(rev) < 12 {
   522  		return "", nil, errNotCached
   523  	}
   524  	rev = rev[:12]
   525  	cdir, err := cacheDir(ctx, path)
   526  	if err != nil {
   527  		return "", nil, errNotCached
   528  	}
   529  	dir, err := os.Open(cdir)
   530  	if err != nil {
   531  		return "", nil, errNotCached
   532  	}
   533  	names, err := dir.Readdirnames(-1)
   534  	dir.Close()
   535  	if err != nil {
   536  		return "", nil, errNotCached
   537  	}
   538  
   539  	// A given commit hash may map to more than one pseudo-version,
   540  	// depending on which tags are present on the repository.
   541  	// Take the highest such version.
   542  	var maxVersion string
   543  	suffix := "-" + rev + ".info"
   544  	err = errNotCached
   545  	for _, name := range names {
   546  		if strings.HasSuffix(name, suffix) {
   547  			v := strings.TrimSuffix(name, ".info")
   548  			if module.IsPseudoVersion(v) && semver.Compare(v, maxVersion) > 0 {
   549  				maxVersion = v
   550  				file, info, err = readDiskStat(ctx, path, strings.TrimSuffix(name, ".info"))
   551  			}
   552  		}
   553  	}
   554  	return file, info, err
   555  }
   556  
   557  // oldVgoPrefix is the prefix in the old auto-generated cached go.mod files.
   558  // We stopped trying to auto-generate the go.mod files. Now we use a trivial
   559  // go.mod with only a module line, and we've dropped the version prefix
   560  // entirely. If we see a version prefix, that means we're looking at an old copy
   561  // and should ignore it.
   562  var oldVgoPrefix = []byte("//vgo 0.0.")
   563  
   564  // readDiskGoMod reads a cached go.mod file from disk,
   565  // returning the name of the cache file and the result.
   566  // If the read fails, the caller can use
   567  // writeDiskGoMod(file, data) to write a new cache entry.
   568  func readDiskGoMod(ctx context.Context, path, rev string) (file string, data []byte, err error) {
   569  	if gover.IsToolchain(path) {
   570  		return "", nil, errNotCached
   571  	}
   572  	file, data, err = readDiskCache(ctx, path, rev, "mod")
   573  
   574  	// If the file has an old auto-conversion prefix, pretend it's not there.
   575  	if bytes.HasPrefix(data, oldVgoPrefix) {
   576  		err = errNotCached
   577  		data = nil
   578  	}
   579  
   580  	if err == nil {
   581  		if err := checkGoMod(path, rev, data); err != nil {
   582  			return "", nil, err
   583  		}
   584  	}
   585  
   586  	return file, data, err
   587  }
   588  
   589  // readDiskCache is the generic "read from a cache file" implementation.
   590  // It takes the revision and an identifying suffix for the kind of data being cached.
   591  // It returns the name of the cache file and the content of the file.
   592  // If the read fails, the caller can use
   593  // writeDiskCache(file, data) to write a new cache entry.
   594  func readDiskCache(ctx context.Context, path, rev, suffix string) (file string, data []byte, err error) {
   595  	if gover.IsToolchain(path) {
   596  		return "", nil, errNotCached
   597  	}
   598  	file, err = CachePath(ctx, module.Version{Path: path, Version: rev}, suffix)
   599  	if err != nil {
   600  		return "", nil, errNotCached
   601  	}
   602  	data, err = robustio.ReadFile(file)
   603  	if err != nil {
   604  		return file, nil, errNotCached
   605  	}
   606  	return file, data, nil
   607  }
   608  
   609  // writeDiskStat writes a stat result cache entry.
   610  // The file name must have been returned by a previous call to readDiskStat.
   611  func writeDiskStat(ctx context.Context, file string, info *RevInfo) error {
   612  	if file == "" {
   613  		return nil
   614  	}
   615  
   616  	if info.Origin != nil {
   617  		// Clean the origin information, which might have too many
   618  		// validation criteria, for example if we are saving the result of
   619  		// m@master as m@pseudo-version.
   620  		clean := *info
   621  		info = &clean
   622  		o := *info.Origin
   623  		info.Origin = &o
   624  
   625  		// Tags never matter if you are starting with a semver version,
   626  		// as we would be when finding this cache entry.
   627  		o.TagSum = ""
   628  		o.TagPrefix = ""
   629  		// Ref doesn't matter if you have a pseudoversion.
   630  		if module.IsPseudoVersion(info.Version) {
   631  			o.Ref = ""
   632  		}
   633  	}
   634  
   635  	js, err := json.Marshal(info)
   636  	if err != nil {
   637  		return err
   638  	}
   639  	return writeDiskCache(ctx, file, js)
   640  }
   641  
   642  // writeDiskGoMod writes a go.mod cache entry.
   643  // The file name must have been returned by a previous call to readDiskGoMod.
   644  func writeDiskGoMod(ctx context.Context, file string, text []byte) error {
   645  	return writeDiskCache(ctx, file, text)
   646  }
   647  
   648  // writeDiskCache is the generic "write to a cache file" implementation.
   649  // The file must have been returned by a previous call to readDiskCache.
   650  func writeDiskCache(ctx context.Context, file string, data []byte) error {
   651  	if file == "" {
   652  		return nil
   653  	}
   654  	// Make sure directory for file exists.
   655  	if err := os.MkdirAll(filepath.Dir(file), 0777); err != nil {
   656  		return err
   657  	}
   658  
   659  	// Write the file to a temporary location, and then rename it to its final
   660  	// path to reduce the likelihood of a corrupt file existing at that final path.
   661  	f, err := tempFile(ctx, filepath.Dir(file), filepath.Base(file), 0666)
   662  	if err != nil {
   663  		return err
   664  	}
   665  	defer func() {
   666  		// Only call os.Remove on f.Name() if we failed to rename it: otherwise,
   667  		// some other process may have created a new file with the same name after
   668  		// the rename completed.
   669  		if err != nil {
   670  			f.Close()
   671  			os.Remove(f.Name())
   672  		}
   673  	}()
   674  
   675  	if _, err := f.Write(data); err != nil {
   676  		return err
   677  	}
   678  	if err := f.Close(); err != nil {
   679  		return err
   680  	}
   681  	if err := robustio.Rename(f.Name(), file); err != nil {
   682  		return err
   683  	}
   684  
   685  	if strings.HasSuffix(file, ".mod") {
   686  		rewriteVersionList(ctx, filepath.Dir(file))
   687  	}
   688  	return nil
   689  }
   690  
   691  // tempFile creates a new temporary file with given permission bits.
   692  func tempFile(ctx context.Context, dir, prefix string, perm fs.FileMode) (f *os.File, err error) {
   693  	for i := 0; i < 10000; i++ {
   694  		name := filepath.Join(dir, prefix+strconv.Itoa(rand.Intn(1000000000))+".tmp")
   695  		f, err = os.OpenFile(name, os.O_RDWR|os.O_CREATE|os.O_EXCL, perm)
   696  		if os.IsExist(err) {
   697  			if ctx.Err() != nil {
   698  				return nil, ctx.Err()
   699  			}
   700  			continue
   701  		}
   702  		break
   703  	}
   704  	return
   705  }
   706  
   707  // rewriteVersionList rewrites the version list in dir
   708  // after a new *.mod file has been written.
   709  func rewriteVersionList(ctx context.Context, dir string) (err error) {
   710  	if filepath.Base(dir) != "@v" {
   711  		base.Fatalf("go: internal error: misuse of rewriteVersionList")
   712  	}
   713  
   714  	listFile := filepath.Join(dir, "list")
   715  
   716  	// Lock listfile when writing to it to try to avoid corruption to the file.
   717  	// Under rare circumstances, for instance, if the system loses power in the
   718  	// middle of a write it is possible for corrupt data to be written. This is
   719  	// not a problem for the go command itself, but may be an issue if the
   720  	// cache is being served by a GOPROXY HTTP server. This will be corrected
   721  	// the next time a new version of the module is fetched and the file is rewritten.
   722  	// TODO(matloob): golang.org/issue/43313 covers adding a go mod verify
   723  	// command that removes module versions that fail checksums. It should also
   724  	// remove list files that are detected to be corrupt.
   725  	f, err := lockedfile.Edit(listFile)
   726  	if err != nil {
   727  		return err
   728  	}
   729  	defer func() {
   730  		if cerr := f.Close(); cerr != nil && err == nil {
   731  			err = cerr
   732  		}
   733  	}()
   734  	infos, err := os.ReadDir(dir)
   735  	if err != nil {
   736  		return err
   737  	}
   738  	var list []string
   739  	for _, info := range infos {
   740  		// We look for *.mod files on the theory that if we can't supply
   741  		// the .mod file then there's no point in listing that version,
   742  		// since it's unusable. (We can have *.info without *.mod.)
   743  		// We don't require *.zip files on the theory that for code only
   744  		// involved in module graph construction, many *.zip files
   745  		// will never be requested.
   746  		name := info.Name()
   747  		if v, found := strings.CutSuffix(name, ".mod"); found {
   748  			if v != "" && module.CanonicalVersion(v) == v {
   749  				list = append(list, v)
   750  			}
   751  		}
   752  	}
   753  	semver.Sort(list)
   754  
   755  	var buf bytes.Buffer
   756  	for _, v := range list {
   757  		buf.WriteString(v)
   758  		buf.WriteString("\n")
   759  	}
   760  	if fi, err := f.Stat(); err == nil && int(fi.Size()) == buf.Len() {
   761  		old := make([]byte, buf.Len()+1)
   762  		if n, err := f.ReadAt(old, 0); err == io.EOF && n == buf.Len() && bytes.Equal(buf.Bytes(), old) {
   763  			return nil // No edit needed.
   764  		}
   765  	}
   766  	// Remove existing contents, so that when we truncate to the actual size it will zero-fill,
   767  	// and we will be able to detect (some) incomplete writes as files containing trailing NUL bytes.
   768  	if err := f.Truncate(0); err != nil {
   769  		return err
   770  	}
   771  	// Reserve the final size and zero-fill.
   772  	if err := f.Truncate(int64(buf.Len())); err != nil {
   773  		return err
   774  	}
   775  	// Write the actual contents. If this fails partway through,
   776  	// the remainder of the file should remain as zeroes.
   777  	if _, err := f.Write(buf.Bytes()); err != nil {
   778  		f.Truncate(0)
   779  		return err
   780  	}
   781  
   782  	return nil
   783  }
   784  
   785  var (
   786  	statCacheOnce sync.Once
   787  	statCacheErr  error
   788  
   789  	counterErrorsGOMODCACHEEntryRelative = counter.New("go/errors:gomodcache-entry-relative")
   790  )
   791  
   792  // checkCacheDir checks if the directory specified by GOMODCACHE exists. An
   793  // error is returned if it does not.
   794  func checkCacheDir(ctx context.Context) error {
   795  	if cfg.GOMODCACHE == "" {
   796  		// modload.Init exits if GOPATH[0] is empty, and cfg.GOMODCACHE
   797  		// is set to GOPATH[0]/pkg/mod if GOMODCACHE is empty, so this should never happen.
   798  		return fmt.Errorf("module cache not found: neither GOMODCACHE nor GOPATH is set")
   799  	}
   800  	if !filepath.IsAbs(cfg.GOMODCACHE) {
   801  		counterErrorsGOMODCACHEEntryRelative.Inc()
   802  		return fmt.Errorf("GOMODCACHE entry is relative; must be absolute path: %q.\n", cfg.GOMODCACHE)
   803  	}
   804  
   805  	// os.Stat is slow on Windows, so we only call it once to prevent unnecessary
   806  	// I/O every time this function is called.
   807  	statCacheOnce.Do(func() {
   808  		fi, err := os.Stat(cfg.GOMODCACHE)
   809  		if err != nil {
   810  			if !os.IsNotExist(err) {
   811  				statCacheErr = fmt.Errorf("could not create module cache: %w", err)
   812  				return
   813  			}
   814  			if err := os.MkdirAll(cfg.GOMODCACHE, 0777); err != nil {
   815  				statCacheErr = fmt.Errorf("could not create module cache: %w", err)
   816  				return
   817  			}
   818  			return
   819  		}
   820  		if !fi.IsDir() {
   821  			statCacheErr = fmt.Errorf("could not create module cache: %q is not a directory", cfg.GOMODCACHE)
   822  			return
   823  		}
   824  	})
   825  	return statCacheErr
   826  }
   827
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