os: add Executable() (string, error)

// Executable returns the path name for the executable that started
// the current process. There is no guarantee that the path is still
// pointing to the correct executable. If a symlink was used to start
// the process, depending on the operating system, the result might
// be the symlink or the path it pointed to. If a stable result is
// needed, path/filepath.EvalSymlinks might help.
//
// Executable returns an absolute path unless an error occurred.
//
// The main use case is finding resources located relative to an
// executable.
//
// Executable is not supported on nacl or OpenBSD (unless procfs is
// mounted.)
func Executable() (string, error) {
	return executable()
}

Fixes #12773.

Change-Id: I469738d905b12f0b633ea4d88954f8859227a88c
Reviewed-on: https://go-review.googlesource.com/16551
Run-TryBot: Minux Ma <minux@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/os/executable_test.go

@@ -0,0 +1,87 @@
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package os_test
+
+import (
+	"fmt"
+	"internal/testenv"
+	"os"
+	osexec "os/exec"
+	"path/filepath"
+	"runtime"
+	"testing"
+)
+
+const executable_EnvVar = "OSTEST_OUTPUT_EXECPATH"
+
+func TestExecutable(t *testing.T) {
+	testenv.MustHaveExec(t) // will also execlude nacl, which doesn't support Executable anyway
+	ep, err := os.Executable()
+	if err != nil {
+		switch goos := runtime.GOOS; goos {
+		case "openbsd": // procfs is not mounted by default
+			t.Skipf("Executable failed on %s: %v, expected", goos, err)
+		}
+		t.Fatalf("Executable failed: %v", err)
+	}
+	// we want fn to be of the form "dir/prog"
+	dir := filepath.Dir(filepath.Dir(ep))
+	fn, err := filepath.Rel(dir, ep)
+	if err != nil {
+		t.Fatalf("filepath.Rel: %v", err)
+	}
+	cmd := &osexec.Cmd{}
+	// make child start with a relative program path
+	cmd.Dir = dir
+	cmd.Path = fn
+	// forge argv[0] for child, so that we can verify we could correctly
+	// get real path of the executable without influenced by argv[0].
+	cmd.Args = []string{"-", "-test.run=XXXX"}
+	cmd.Env = append(os.Environ(), fmt.Sprintf("%s=1", executable_EnvVar))
+	out, err := cmd.CombinedOutput()
+	if err != nil {
+		t.Fatalf("exec(self) failed: %v", err)
+	}
+	outs := string(out)
+	if !filepath.IsAbs(outs) {
+		t.Fatalf("Child returned %q, want an absolute path", out)
+	}
+	if !sameFile(outs, ep) {
+		t.Fatalf("Child returned %q, not the same file as %q", out, ep)
+	}
+}
+
+func sameFile(fn1, fn2 string) bool {
+	fi1, err := os.Stat(fn1)
+	if err != nil {
+		return false
+	}
+	fi2, err := os.Stat(fn2)
+	if err != nil {
+		return false
+	}
+	return os.SameFile(fi1, fi2)
+}
+
+func init() {
+	if e := os.Getenv(executable_EnvVar); e != "" {
+		// first chdir to another path
+		dir := "/"
+		if runtime.GOOS == "windows" {
+			cwd, err := os.Getwd()
+			if err != nil {
+				panic(err)
+			}
+			dir = filepath.VolumeName(cwd)
+		}
+		os.Chdir(dir)
+		if ep, err := os.Executable(); err != nil {
+			fmt.Fprint(os.Stderr, "ERROR: ", err)
+		} else {
+			fmt.Fprint(os.Stderr, ep)
+		}
+		os.Exit(0)
+	}
+}