path/filepath: Rel hangs in an infinite loop on Windows for a UNC volume root with a trailing separator

[drakkan]

Go version

go version go1.26.3 windows/amd64

What did you do?

On Windows, filepath.Rel never returns when the base path is a UNC volume root (\\host\share) and the target path is the same volume root with a trailing separator.

filepath.Clean does not normalize this away: it preserves the trailing separator on a volume root (the UNC share root is treated like C:\), so the two arguments stay different and Rel loops.

package main

import (
      "fmt"
      "path/filepath"
)
 
func main() {
      // filepath.Clean(`\\host\share\`) == `\\host\share\` (trailing separator kept,
      // as for any volume root), so Clean does not avoid the problem.
      fmt.Println(filepath.Rel(`\\host\share`, filepath.Clean(`\\host\share\`)))
}

This is pure string manipulation, no filesystem access, so the share does not need to exist, and GOOS=windows is the only requirement.

What did you see instead?

The call never returns; the goroutine consumes a full CPU core indefinitely.

[gabyhelp]

Related Issues

• path/filepath: Rel() fails for UNC volumes when trailing slash is omitted #41230 (closed)
• path/filepath: calling Rel hangs my program #13258 (closed)
• path/filepath: Rel returns error in Windows if no Separator after drive #13208 (closed)
• path/filepath: Rel() returns invalid path if volume case is different #10802 (closed)
• path/filepath: Rel() fails on Windows when only one parameter has `\?\` prefix #68847
• path/filepath: confusing error message for Rel #13259 (closed)
• path/filepath: mishandling of Windows UNC paths #56336 (closed)
• path/filepath: Glob with UNC path causes stack overflow #15879 (closed)
• filepath.Clean mangles UNC paths on Windows #2201 (closed)

_{(Emoji vote if this was helpful or unhelpful; more detailed feedback welcome in this discussion.)}

[seankhliao]

cc @golang/windows

[qmuntal]

Thanks for reporting. I can also reproduce this issue with Go 1.23, 1.24, and 1.25, so it doesn't look like a new regression.

Will submit a fix soon, but we are in Go 1.27 code freeze, so we will have to wait until the Go 1.28 development cycle to land the fix.

[drakkan]

Thanks for reporting. I can also reproduce this issue with Go 1.23, 1.24, and 1.25, so it doesn't look like a new regression.

Will submit a fix soon, but we are in Go 1.27 code freeze, so we will have to wait until the Go 1.28 development cycle to land the fix.

Thanks for confirming. In the meantime the workaround is to do something like this before using filepath.Rel:

if strings.TrimRight(cleanName, `\/`) == strings.TrimRight(fs.rootDir, `\/`) { return something }
rel, err := filepath.Rel(fs.rootDir, cleanName)

[gopherbot]

Change https://go.dev/cl/786100 mentions this issue: path/filepath: avoid Rel loop on exhausted matching paths