os: CopyFS fails with ErrClosed

[gopherbot]

#!watchflakes
default <- pkg == "golang.org/x/tools/internal/refactor/inline" && test == "TestData/err-unexported.txtar"

Issue created automatically to collect these failures.

Example (log):

=== RUN   TestData/err-unexported.txtar
=== PAUSE TestData/err-unexported.txtar
=== CONT  TestData/err-unexported.txtar
    testfiles.go:49: close /home/swarming/.swarming/w/ir/x/t/TestDataerr-unexported.txtar735483480/001/go.mod: file already closed
--- FAIL: TestData/err-unexported.txtar (0.21s)

— watchflakes

[gopherbot]

Found new dashboard test flakes for:

#!watchflakes
default <- pkg == "golang.org/x/tools/internal/refactor/inline" && test == "TestData/err-unexported.txtar"

2025-05-13 17:01 x_tools-gotip-linux-amd64-longtest-race tools@ade411c2 go@76f63ee8 x/tools/internal/refactor/inline.TestData/err-unexported.txtar (log)

=== RUN   TestData/err-unexported.txtar
=== PAUSE TestData/err-unexported.txtar
=== CONT  TestData/err-unexported.txtar
    testfiles.go:49: close /home/swarming/.swarming/w/ir/x/t/TestDataerr-unexported.txtar735483480/001/go.mod: file already closed
--- FAIL: TestData/err-unexported.txtar (0.21s)

— watchflakes

[adonovan]

The symptom is a sporadically failing test (on linux/amd64):

--- FAIL: TestData (0.02s)
--- FAIL: TestData/err-unexported.txtar (0.21s)
    testfiles.go:49: close /home/swarming/.swarming/w/ir/x/t/TestDataerr-unexported.txtar735483480/001/go.mod: file already closed

The implicated line (at the indicated commit) is:

func CopyToTmp(t testing.TB, src fs.FS, rename ...string) string {
	dstdir := t.TempDir()

	if err := os.CopyFS(dstdir, src); err != nil {
		t.Fatal(err) <-- here
	}

This means os.CopyFS returned ErrClosed. I don't see how that's possible, as none of the return statements below can possibly return ErrClosed. (fsys refers to an x/tools/txtar.filesystem.) The source, also at the indicated commit, is:

func CopyFS(dir string, fsys fs.FS) error {
	return fs.WalkDir(fsys, ".", func(path string, d fs.DirEntry, err error) error {
		if err != nil {
			return err
		}

		fpath, err := filepathlite.Localize(path)
		if err != nil {
			return err
		}
		newPath := joinPath(dir, fpath)

		switch d.Type() {
		case ModeDir:
			return MkdirAll(newPath, 0777)
		case ModeSymlink:
			target, err := fs.ReadLink(fsys, path) 
			if err != nil {
				return err
			}
			return Symlink(target, newPath)
		case 0:
			r, err := fsys.Open(path)
			if err != nil {
				return err
			}
			defer r.Close()
			info, err := r.Stat()
			if err != nil {
				return err
			}
			w, err := OpenFile(newPath, O_CREATE|O_EXCL|O_WRONLY, 0666|info.Mode()&0777)
			if err != nil {
				return err
			}

			if _, err := io.Copy(w, r); err != nil {
				w.Close()
				return &PathError{Op: "Copy", Path: newPath, Err: err}
			}
			return w.Close()
		default:
			return &PathError{Op: "CopyFS", Path: path, Err: ErrInvalid}
		}
	})
}

@neild any ideas?

[neild]

This is very confusing. I can't see where an ErrClosed can possibly come from.

I've had no luck replicating the failure, either.