w.mu.Unlock()

		w.f(item)
	}
}

// ErrCache is like Cache except that it also stores
// an error value alongside the cached value V.
type ErrCache[K comparable, V any] struct {
	Cache[K, errValue[V]]
}

type errValue[V any] struct {
	v   V
	err error
}

func (c *ErrCache[K, V]) Do(key K, f func() (V, error)) (V, error) {
	v := c.Cache.Do(key, func() errValue[V] {

// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package cfg

import (
	"cmd/go/internal/par"
	"os/exec"
)

var lookPathCache par.ErrCache[string, string]

// LookPath wraps exec.LookPath and caches the result
// which can be called by multiple Goroutines at the same time.
func LookPath(file string) (path string, err error) {
	return lookPathCache.Do(file,

// It serializes calls to the underlying Repo.
type cachingRepo struct {
	path          string
	versionsCache par.ErrCache[string, *Versions]
	statCache     par.ErrCache[string, *RevInfo]
	latestCache   par.ErrCache[struct{}, *RevInfo]
	gomodCache    par.ErrCache[string, []byte]

	once     sync.Once
	initRepo func(context.Context) (Repo, error)
	r        Repo
}

	"cmd/go/internal/robustio"
	"cmd/go/internal/str"
	"cmd/go/internal/trace"

	"golang.org/x/mod/module"
	"golang.org/x/mod/sumdb/dirhash"
	modzip "golang.org/x/mod/zip"
)

var downloadCache par.ErrCache[module.Version, string] // version → directory

var ErrToolchain = errors.New("internal error: invalid operation on toolchain module")

// Download downloads the specific module version to the

		if err != nil {
			fmt.Fprintf(os.Stderr, "go proxy_test: %v\n", err)
			continue
		}
		modList = append(modList, module.Version{Path: path, Version: vers})
	}
}

var zipCache par.ErrCache[*txtar.Archive, []byte]

const (
	testSumDBName        = "localhost.localdev/sumdb"
	testSumDBVerifierKey = "localhost.localdev/sumdb+00000c67+AcTrnkbUA+TU4heY3hkjiSES/DSQniBqIeQ/YppAUtK6"

					VersionInterval: ret.VersionInterval,
					Rationale:       ret.Rationale,
				})
			}
		}

		return summary, nil
	})
}

var rawGoModSummaryCache par.ErrCache[module.Version, *modFileSummary]

// rawGoModData returns the content of the go.mod file for module m, ignoring
// all replacements that may apply to m.
//

	err error
}

func (e notExistError) Error() string   { return e.err.Error() }
func (notExistError) Is(err error) bool { return err == fs.ErrNotExist }

const gitWorkDirType = "git3"

var gitRepoCache par.ErrCache[gitCacheKey, Repo]

type gitCacheKey struct {
	remote  string
	localOK bool
}

func newGitRepoCached(ctx context.Context, remote string, localOK bool) (Repo, error) {

	if str.HasFilePathPrefix(modroot, cfg.GOROOTsrc) || !str.HasFilePathPrefix(modroot, cfg.GOMODCACHE) {
		return nil, errNotFromModuleCache
	}
	return openIndexModule(modroot, true)
}

var mcache par.ErrCache[string, *Module]

// openIndexModule returns the module index for modPath.
// It will return ErrNotIndexed if the module can not be read
// using the index because it contains symlinks.

		repo, err := newVCSRepo(ctx, vcs, remote)
		if err != nil {
			return nil, &VCSError{err}
		}
		return repo, nil
	})
}

var vcsRepoCache par.ErrCache[vcsCacheKey, Repo]

type vcsRepo struct {
	mu lockedfile.Mutex // protects all commands, so we don't have to decide which are safe on a per-VCS basis

	remote string
	cmd    *vcsCmd
	dir    string

	return mpath == path ||
		len(path) > len(mpath) && path[len(mpath)] == '/' && path[:len(mpath)] == mpath
}

var (
	haveGoModCache   par.Cache[string, bool]    // dir → bool
	haveGoFilesCache par.ErrCache[string, bool] // dir → haveGoFiles
)

// dirInModule locates the directory that would hold the package named by the given path,
// if it were in the module with module path mpath and root mdir.