reexec: only consider basename, not path

[thaJeztah]

The intent of reexec is to provide functionality similar to multi-call binaries (like "busybox"). However, currently, re-exec also includes path information when matching. This does not match behavior like busybox;

date --help 2>&1 | grep Usage
Usage: date [OPTIONS] [+FMT] [[-s] TIME]

mkdir foo
ln -s /bin/busybox ./foo/date
./foo/date --help 2>&1 | grep Usage

This patch changes the behavior to only consider the basename, and disregard path.

With this patch applied:

Basic example:

package main

import (
	"fmt"
	"os"

	"github.com/moby/sys/reexec"
)

func init() {
	reexec.Register("hello-foo", func() {
		fmt.Println("Hello Foo")
		os.Exit(0)
	})
	reexec.Register("hello-bar", func() {
		fmt.Println("Hello Bar")
		os.Exit(0)
	})

	reexec.Init()
}

func main() {
	fmt.Println("Hello Main")
}

executing the binary, including path (./) now selects the correct
function to execute.

go build -o ./example
ln -s ./example hello-foo
ln -s ./example hello-bar
ln -s ./example hello-baz

./example
# Hello Main

./hello-foo
# Hello Foo

./hello-bar
# Hello Bar

./hello-baz
# Hello Main

Before this patch, all of the above would fall through to use main instead;

./example
# Hello Main

./hello-foo
# Hello Main

./hello-bar
# Hello Main

./hello-baz
# Hello Main

[Copilot]

Pull Request Overview

This PR updates the reexec package to consider only the basename when matching registered commands, aligning its behavior with multi-call binaries like busybox.

• Updated reexec.Register to panic if the name includes a path component.
• Modified reexec.Init to use the basename of os.Args[0].
• Enhanced tests in reexec_test.go to cover duplicate registration and basename versus full path behavior.

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
reexec/reexec.go	Enforces basename-only registration and lookup by using filepath.Base.
reexec/reexec_test.go	Adds test cases for duplicate names and verifies behavior for full paths.

[thaJeztah]

/cc @tianon @vvoland ptal

[thaJeztah]

I'll bring this one in; did a quick check, and I didn't find any place where we call Register with a command that includes a path (but will open draft PR's before doing a new release - some other changes still coming)