os/exec: calling Cmd.Start after setting Cmd.Path manually to absolute path without ".exe" no longer implicitly adds ".exe" in Go 1.22

[dmitshur]

https://go.dev/doc/go1.22#os/exec includes:

On Windows, Command and Cmd.Start no longer call LookPath if the path to the executable is already absolute and has an executable file extension.

I don't quite understand what criteria is used for determining whether a path has an executable file extension. Which extensions are included? Is the PATHEXT environment variable involved?

---

That said, I've narrowed down a following behavior change between Go 1.21 and 1.22 that I'm not sure if it's working as intended, so reporting it for investigation.

Consider the output of the following Go program on a Windows machine that has an executable file at the path "C:\Program Files\Go\bin\gofmt.exe", in a roughly default environment (i.e., PATHEXT is not modified):

package main

import (
	"fmt"
	"os"
	"os/exec"
	"strings"

	"github.com/google/go-cmp/cmp"
	"github.com/google/go-cmp/cmp/cmpopts"
)

func main() {
	if _, err := os.Stat(`C:\Program Files\Go\bin\gofmt.exe`); err != nil {
		fmt.Println("returning early; if gofmt.exe doesn't exist the rest of the output will be misleading")
		return
	} else if !strings.Contains(os.Getenv("PATHEXT"), ".EXE") {
		fmt.Println("returning early; if .exe isn't included in PATHEXT the rest of the output will be misleading")
		return
	}

	cmdViaCommand := exec.Command(`C:\Program Files\Go\bin\gofmt`)
	cmdManualPath := &exec.Cmd{
		Path: `C:\Program Files\Go\bin\gofmt`,
		Args: []string{`C:\Program Files\Go\bin\gofmt`},
	}
	diff := cmp.Diff(cmdViaCommand, cmdManualPath, cmpopts.IgnoreUnexported(exec.Cmd{}))
	if diff == "" {
		diff = "(no diff)\n"
	}
	fmt.Printf("diff (-cmdViaCommand +cmdManualPath):\n%s\n", diff)

	err := cmdManualPath.Run()
	fmt.Println("err:", err)
}

When running it using Go 1.21.8, the output is:

$ go run .
diff (-cmdViaCommand +cmdManualPath):
(no diff)

err: <nil>

But when running it with Go 1.22.1:

$ go run .
diff (-cmdViaCommand +cmdManualPath):
  &exec.Cmd{
-       Path: `C:\Program Files\Go\bin\gofmt.exe`,
+       Path: `C:\Program Files\Go\bin\gofmt`,
        Args: {`C:\Program Files\Go\bin\gofmt`},
        Env:  nil,
        ... // 8 ignored and 11 identical fields
  }

err: fork/exec C:\Program Files\Go\bin\gofmt: The system cannot find the file specified.

Not having to manually add ".exe" to the path and instead relying on the PATHEXT mechanism is very convenient when writing multi-platform Go programs, since it permits there not to be special cases for one of the GOOS values.

In that context, it seems there's no change in behavior when using exec.Command to create a *exec.Cmd and then calling Cmd.Start on it. But when creating it manually, Go 1.21 would use PATHEXT compensate for Cmd.Path missing a ".exe" suffix, whereas Go 1.22 doesn't. I can't quite tell from os/exec documentation (or the seemingly relevant release note) if this is a bug fix or a bug.

CC @golang/windows.

[rsc]

@gopherbot please backport go1.22

[gopherbot]

Backport issue(s) opened: #66598 (for 1.22).

Remember to create the cherry-pick CL(s) as soon as the patch is submitted to master, according to https://go.dev/wiki/MinorReleases.

[gopherbot]

Change https://go.dev/cl/575255 mentions this issue: os/exec: revert "avoid calling LookPath in cmd.Start for resolved paths"

[gopherbot]

Change https://go.dev/cl/575275 mentions this issue: os/exec: resolve extension in Cmd.Start if Command wasn't used

[cagedmantis]

This issue was reviewed in the release meeting. Is there any status update to report on this issue?

[joedian]

@dmitshur I see the CL was abandoned, is this issue also obsolete?

[dmitshur]

@joedian It was a draft CL associated with a review comment. Russ has go.dev/cl/575255 which is still open but needs attention.

[gopherbot]

Change https://go.dev/cl/581695 mentions this issue: os/exec: not add a suffix to Cmd.Path

[cagedmantis]

@rsc Is there is status update on this issue? There is a CL that is ready for review. This came up in the release meeting.

[mdempsky]

Ping? Is this still a 1.23 release blocker? It seems like it's an issue present in 1.22, and so it merits backporting either way.

[rsc]

I'll take a look later today.

[gopherbot]

Change https://go.dev/cl/589016 mentions this issue: os/exec: add new test

[gopherbot]

Change https://go.dev/cl/594495 mentions this issue: [release-branch.go1.22] os/exec: on Windows look for extensions in Run if not already done

[gopherbot]

Change https://go.dev/cl/596875 mentions this issue: os/exec: only use cachedLookExtensions if Cmd.Path is unmodified

[gopherbot]

Change https://go.dev/cl/596976 mentions this issue: [release-branch.go1.22] os/exec: only use cachedLookExtensions if Cmd.Path is unmodified