vfs.NameSpace.ReadDir is documented to:

Logically, ReadDir must return the union of all the directories that are named by path. In order to avoid misinterpreting Go packages, of all the directories that contain Go source code, we only include the files from the first, but we include subdirectories from all.

Consider the following case, where there are no .go files in either directory, yet ReadDir fails to include files from both directories:

package main

import (
	"fmt"
	"log"

	"golang.org/x/tools/godoc/vfs"
	"golang.org/x/tools/godoc/vfs/mapfs"
)

func main() {
	fs := vfs.NameSpace{}
	fs.Bind("/", mapfs.New(map[string]string{
		"doc/a.txt":          "1",
		"doc/b.txt":          "1",
		"doc/dir1/empty.txt": "",
	}), "/", vfs.BindReplace)
	fs.Bind("/", mapfs.New(map[string]string{
		"doc/a.txt":          "22",
		"doc/dir2/empty.txt": "",
	}), "/", vfs.BindBefore)

	fis, err := fs.ReadDir("/doc")
	if err != nil {
		log.Fatalln(err)
	}
	for _, fi := range fis {
		fmt.Println(fi.Name(), fi.Size())
	}
}

It produces output:

a.txt 2
dir1 0
dir2 0

But should produce:

a.txt 2
b.txt 1
dir1 0
dir2 0

This bug came up in the past, and was fixed by CL 5783076. If re-applied today, it fixes the problem again. But it was replaced with a more general fix in CL 5783079, which causes the issue to occur under the above conditions.

/cc @jayconrod