testing/fstest: `TestFS` falsely fails for certain directory trees on macOS

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

$ go version
go version go1.19 darwin/arm64

Does this issue reproduce with the latest release?

Yep! I've also tested it with 1.17.13 and 1.18.5.

What operating system and processor architecture are you using (go env)?

go env Output

$ go env
...
GOHOSTARCH="arm64"
GOHOSTOS="darwin"
...

What did you do?

On a macOS host, create this testdata/ tree. The file contents don't matter, only the hierarchy and file names are important:

$ mkdir -p testdata/bug/sub/{new,paths}
$ touch testdata/bug/{hello,sub/bar,sub/new/file,sub/paths/foo}
$ tree testdata
testdata
└── bug
    ├── hello
    └── sub
        ├── bar
        ├── new
        │   └── file
        └── paths
            └── foo

Then run the following test

package testfsbug

import (
    "io/fs"
    "os"
    "testing"
    "testing/fstest"
)

func TestTestFS(t *testing.T) {
    tests := map[string]struct {
        src           fs.FS
        expectedFiles []string
    }{
        // Fails on macOS hosts because of ordering of dirents returned
        // by `readDir`.
        "dirfs - bug": {
            src: os.DirFS("testdata/bug"),
            expectedFiles: []string{
                "hello",
                "sub/bar",
                "sub/paths/foo",
                "sub/new/file",
            },
        },
        // This spells out the same file tree as the previous test, but this
        // time as a fstest.MapFS. This one passes because of the ordering in
        // reading dirents in `readDir`.
        "mapfs": {
            src: fstest.MapFS{
                "hello":         &fstest.MapFile{Data: []byte("hello world!"), Mode: 0644},
                "sub/bar":       &fstest.MapFile{Data: []byte("this is bar"), Mode: 0644},
                "sub/new/file":  &fstest.MapFile{Data: []byte("cool file"), Mode: 0644},
                "sub/paths/foo": &fstest.MapFile{Data: []byte("foo"), Mode: 0644},
            },
            expectedFiles: []string{
                "hello",
                "sub/bar",
                "sub/paths/foo",
                "sub/new/file",
            },
        },
    }
    for name, tt := range tests {
        t.Run(name, func(t *testing.T) {
            err := fstest.TestFS(tt.src, tt.expectedFiles...)
            if err != nil {
                t.Fatal(err)
            }
        })
    }
}

What did you expect to see?

The dirfs - bug test case should pass, because all expected files exist under testdata/bug. The mapfs test case at least proves that fstest.TestFS can handle a file tree like this, because it always passes.

What did you see instead?

The dirfs - bug test case fails on macOS, with an error case I like to call "Schrödinger's bar":

$ go version
go version go1.19 darwin/arm64

$ go test -v ./...
=== RUN   TestTestFS
=== RUN   TestTestFS/dirfs_-_bug
    fstest_test.go:93: testing fs.Sub(fsys, sub): TestFS found errors:
        .: Glob(`*a*`): wrong output:
        extra: bar
        missing: bar
=== RUN   TestTestFS/mapfs
--- FAIL: TestTestFS (0.00s)
    --- FAIL: TestTestFS/dirfs_-_bug (0.00s)
    --- PASS: TestTestFS/mapfs (0.00s)
FAIL
FAIL    testfsbug       0.186s
FAIL

But always passes on linux:

$ docker run \
    -v $(pwd):/test \
    -w /test \
    golang:1.19 \
    go test -v ./...
=== RUN   TestTestFS
=== RUN   TestTestFS/dirfs_-_bug
=== RUN   TestTestFS/mapfs
--- PASS: TestTestFS (0.09s)
    --- PASS: TestTestFS/dirfs_-_bug (0.09s)
    --- PASS: TestTestFS/mapfs (0.00s)
PASS
ok      testfsbug       0.093s

Analysis

This one fails (on darwin), but we'd expect it not to. All expected files exist in that test directory. The checks at the end of fstester.checkGlob rely on both names and want being sorted, but it only checks that names is sorted. The ordering of want is dependent on what ReadDirFile.ReadDir returns here and, for os.DirFS, that ordering is platform dependent. The docs for ReadDirFile.ReadDir say to the files being returned in "directory order":

ReadDir reads the contents of the directory and returns
a slice of up to n DirEntry values in directory order.

There are consistency checks in fstester.checkDir for reading the dir entries before checking the glob, but these fail to prevent the ordering issue and list ultimately ends up as want. The closest this come to checking list's order is checking that the elements match with what fs.ReadDir returns (which is list2) and then checking that list2 is sorted. Passing list2 into t.checkGlob would likely fix the issue because it is guaranteed to be sorted.

This only happens when there are at least two paths in the same directory that match the internally computed glob. In this test, both sub/bar and sub/paths match glob *a* in sub/. Obviously if there is only one path that matches the glob, the ordering doesn't matter.

In this test case (on darwin), you end up with the following state at the want/names check here:

• want = ["paths", "bar"]
• names = ["bar", "paths"]

That code doesn't properly compare two unsorted slices, and so you end with the error:

    fstest_test.go:76: testing fs.Sub(fsys, sub): TestFS found errors:
        .: Glob(`*a*`): wrong output:
        extra: bar
        missing: bar

[seankhliao]

I believe this has since been fixed, the test no longer fails on my mac.
Note that the docs for ReadDir nowsay the files are returned in sorted order.