return proxyOnce.list, proxyOnce.err
}

// TryProxies iterates f over each configured proxy (including "noproxy" and
// "direct" if applicable) until f returns no error or until f returns an
// error that is not equivalent to fs.ErrNotExist on a proxy configured
// not to fall back on errors.
//
// TryProxies then returns that final error.
//
// If GOPROXY is set to "off", TryProxies invokes f once with the argument
// "off".

	// Note: modfetch.Lookup and repo.Versions are cached,
	// so there's no need for us to add extra caching here.
	err = modfetch.TryProxies(func(proxy string) error {
		repo, err := lookupRepo(ctx, proxy, path)
		if err != nil {
			return err
		}
		allVersions, err := repo.Versions(ctx, "")
		if err != nil {
			return err
		}

}

// autoToolchains returns the list of toolchain versions available to GOTOOLCHAIN=auto or =min+auto mode.
func autoToolchains(ctx context.Context) ([]string, error) {
	var versions *modfetch.Versions
	err := modfetch.TryProxies(func(proxy string) error {
		v, err := modfetch.Lookup(ctx, proxy, "go").Versions(ctx, "")
		if err != nil {
			return err
		}
		versions = v
		return nil
	})
	if err != nil {
		return nil, err

	}

	if file, info, err := readDiskStat(ctx, path, version); err == nil {
		return info, file, nil
	}

	var info *RevInfo
	var err2info map[error]*RevInfo
	err := TryProxies(func(proxy string) error {
		i, err := Lookup(ctx, proxy, path).Stat(ctx, version)
		if err == nil {
			info = i
		} else {
			if err2info == nil {
				err2info = make(map[error]*RevInfo)
			}

	// connect directly to that database.
	//
	// Any other response is treated as the database being unavailable.
	//
	// See https://golang.org/design/25530-sumdb#proxying-a-checksum-database.
	err := TryProxies(func(proxy string) error {
		switch proxy {
		case "noproxy":
			return errUseProxy
		case "direct", "off":
			return errProxyOff
		default:
			proxyURL, err := url.Parse(proxy)
			if err != nil {

func queryReuse(ctx context.Context, path, query, current string, allowed AllowedFunc, reuse map[module.Version]*modinfo.ModulePublic) (*modfetch.RevInfo, error) {
	var info *modfetch.RevInfo
	err := modfetch.TryProxies(func(proxy string) (err error) {
		info, err = queryProxy(ctx, proxy, path, query, current, allowed, reuse)
		return err
	})
	return info, err
}

// checkReuse checks whether a revision of a given module

	if err != nil {
		return err
	}
	defer func() {
		if err != nil {
			f.Close()
			os.Remove(f.Name())
		}
	}()

	var unrecoverableErr error
	err = TryProxies(func(proxy string) error {
		if unrecoverableErr != nil {
			return unrecoverableErr
		}
		repo := Lookup(ctx, proxy, mod.Path)
		err := repo.Zip(ctx, f, mod.Version)
		if err != nil {