Fix:

diff --git a/src/testing/fstest/mapfs.go b/src/testing/fstest/mapfs.go
index a0b1f65668..b3fc0c8ad7 100644
--- a/src/testing/fstest/mapfs.go
+++ b/src/testing/fstest/mapfs.go
@@ -98,14 +98,14 @@ func (fsys MapFS) Open(name string) (fs.File, error) {
                delete(need, fi.name)
        }
        for name := range need {
-               list = append(list, mapFileInfo{name, &MapFile{Mode: fs.ModeDir}})
+               list = append(list, mapFileInfo{name, &MapFile{Mode: fs.ModeDir | 0555}})
        }
        sort.Slice(list, func(i, j int) bool {
                return list[i].name < list[j].name
        })
 
        if file == nil {
-               file = &MapFile{Mode: fs.ModeDir}
+               file = &MapFile{Mode: fs.ModeDir | 0555}
        }
        return &mapDir{name, mapFileInfo{elem, file}, list, 0}, nil
 }

CC @bcmills, @ianlancetaylor.

Want to send a patch? https://go.dev/doc/contribute.

Change https://go.dev/cl/534075 mentions this issue: testing/fstest: MapFS: set perm 0555 on synthetized dirs

🎉

While I do agree that it makes sense for directories to have 0555 set, this change actually broke my tests that relied on fstest.MapFS synthesizing directories with only mode fs.ModeDir. The exact behaviour of fstest.MapFS wasn't documented and I suppose one might even be able to call the previous behaviour a bug, so I don't think it breaks the Go 1 compatibility promise, and I have no issue with that part.

However, the documentation does still suggest that a directory can be created by setting the file mode to fs.ModeDir. This is true, but following this recommendation now results in a directory that differs from what fs.MapFS would synthesize by default. Would it make sense to update the documentation to recommend setting the mode to fs.ModeDir | 0555 (or perhaps fs.ModeDir | 0o555)?

The map need not include parent directories for files contained in the map; those will be synthesized if needed. But a directory can still be included by setting the [MapFile.Mode]'s fs.ModeDir bit; this may be necessary for detailed control over the directory's fs.FileInfo or to create an empty directory.

https://pkg.go.dev/testing/fstest