godoc never relies on that functionality so it's possible it's just a bug we never noticed.

I have figured out the reason this bug is happening. The issue is that behavior of vfs.NameSpace is not consistent with expectations of how a normal file system behaves and that causes typical directory traversal algorithms to break on vfs.NameSpace.

The issue is that NameSpace is implemented with the implicit assumption that it will be used with a FileSystem mounted at root mount point with other FileSystems mounted somewhere within that. This is of course the use case of godoc as also explained in the documentation of vfs.NameSpace where GOROOT is being mounted at root.

The use case above mounts several folders under root but nothing at root breaking this implicit assumption. Unless we explicitly bind a FileSystem directory at root mount-point "/" of vfs.NameSpace , running Open() or Stat() on root "/" causes a not found error (or a return value that won't be a directory if we mount a file). This happens even though ReadDir() correctly resolves and lists the contents of the root directory as expected.

Most algorithms that traverse a directory structure either Open() or Stat() the root before reading the contents. The not found error that comes when a directory is not explicitly bound at the root mount causes the traversal routine to fail fast without trying to list directory contents. This happens in http.FileServer and I had a similar failure in another piece of directory traversing code I am writing.

I was able to get the right behavior by If i mount an empty directory at root like below.

package main

import (
    "golang.org/x/tools/godoc/vfs"
    "golang.org/x/tools/godoc/vfs/httpfs"
    "log"
    "net/http"
    "os"
)

func main() {
    ns := vfs.NameSpace{}
    // Folder3 is simply an empty folder to be mounted at the root mount point
    ns.Bind("/", vfs.OS("Folder3"), "/", vfs.BindReplace)
    ns.Bind("/music", vfs.OS("Folder1"), "/", vfs.BindReplace)
    ns.Bind("/videos", vfs.OS("Folder2"), "/", vfs.BindReplace)

    fis, err := ns.ReadDir("/")
    if err != nil {
        log.Println(err)
        os.Exit(1)
    }
    for _, fi := range fis {
        log.Println(fi.Name())
    }
    http.Handle("/", http.FileServer(httpfs.New(ns)))
    http.ListenAndServe(":9090", http.DefaultServeMux)
}

I think the least disruptive resolution to this issue will be to add a NewNamespace() function to create a NameSpace that automatically mounts at root mount point "/" an implementation of FileSystem that simply emulates an empty directory so this surprising behavior doesn't occur. The advantage of this approach is that absolutely nothing would change for any current usage of NameSpace by godoc and any other existing programs while new programs can just use the provided function.

Happy to work on a CL if the approach sounds ok.

I have a working implementation of a fix for this issue at https://github.com/srinathh/emptyvfs for review/discussion of the proposed solution approach.

If the approach looks ok, I can move the implementation into vfs package and initiate the code-review package.

Feel free to send a code review. In addition to being easier to review in Gerrit, it will also guarantee that you've signed a CLA (since our Gerrit doesn't allow uploads before a CLA is signed). Reviewers shouldn't be reviewing code elsewhere (e.g. your personal github repo) without knowing a CLA is on file anyway.

Thanks! Submitted CL https://go-review.googlesource.com/19445

CL https://golang.org/cl/19445 mentions this issue.

Hi Brad, did you get a chance to review the CL?