testing/fstest: TestFS should return an unwrappable error

[dolmen]

testing/fstest.TestFS reports a single error which is the concatenation of all errors. This doesn't allow to analyze the errors by unwraping using errors.Is or errors.As or calling Unwrap() []error.

See TestFS code: https://github.com/golang/go/blob/master/src/testing/fstest/testfs.go#L107

Instead TestFS should use errors.Join or a custom error that implements the Unwrap() []error method like errors.Join.

What version of Go are you using (go version)?

$ go version
go version go1.21.3 darwin/amd64

Does this issue reproduce with the latest release?

Yes, on master branch.

Check code of the fsTester.errorf method:
https://github.com/golang/go/blob/master/src/testing/fstest/testfs.go#L107

What did you do?

I'm testing a filesystem that enforces permissions: https://github.com/dolmen-go/sqlar

I applied TestFS to a filesystem that returns fs.ErrPermission for ReadDir on multiple directories.

What did you expect to see?

I would like to analyze the errors using errors.Is to ignore expected fs.ErrPermission errors.

What did you see instead?

I got a big string containing the error messages concatenated.

[dolmen]

I'm working on a patch.

[gopherbot]

Change https://go.dev/cl/537015 mentions this issue: testing/fstest: return structured errors in TestFS

[dolmen]

Reviews of CL #537015 would be welcome.

[dolmen]

Here is the workaround I had to implement in my sqlarfs testsuite when testing for fs.ErrPermission: string.Contains applied to err.Error.
https://github.com/dolmen-go/sqlar/blob/main/sqlarfs/sqlarfs_test.go#L178

		var errs interface{ Unwrap() []error }
		switch {
		case err == nil: // ignore
		case errors.As(err, &errs):
			for _, err := range errs.Unwrap() {
				if errors.Is(err, fs.ErrPermission) {
					t.Logf("%T: %[1]q", err)
				} else {
					t.Fatal(err)
				}
			}
		case errors.Is(err, fs.ErrPermission):
			t.Logf("%T: %[1]q", err)
		default:
			t.Logf("%T: %[1]q", err)
			if strings.Contains(err.Error(), fs.ErrPermission.Error()) {
				t.Logf("%T: %[1]q", err)
				t.Log("Fallback to error message check")
				t.Skip("Skip. See https://github.com/golang/go/issues/63675")
			}
			t.Fatalf("%T: %[1]q", err)
		}

My CL 537015 is still waiting.