os: on OpenBSD implement Executable using Args[0]

OpenBSD no longer has procfs.

Based on a patch by Matthieu Sarter.

Fixes #19453.

Change-Id: Ia09d16f8a1cbef2f8cc1c5f49e9c61ec7d026a40
Reviewed-on: https://go-review.googlesource.com/46004
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/os/executable_path.go

+// Copyright 2017 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.
+
+// +build openbsd
+
+package os
+
+// We query the working directory at init, to use it later to search for the
+// executable file
+// errWd will be checked later, if we need to use initWd
+var initWd, errWd = Getwd()
+
+func executable() (string, error) {
+	var exePath string
+	if len(Args) == 0 || Args[0] == "" {
+		return "", ErrNotExist
+	}
+	if IsPathSeparator(Args[0][0]) {
+		// Args[0] is an absolute path, so it is the executable.
+		// Note that we only need to worry about Unix paths here.
+		exePath = Args[0]
+	} else {
+		for i := 1; i < len(Args[0]); i++ {
+			if IsPathSeparator(Args[0][i]) {
+				// Args[0] is a relative path: prepend the
+				// initial working directory.
+				if errWd != nil {
+					return "", errWd
+				}
+				exePath = initWd + string(PathSeparator) + Args[0]
+				break
+			}
+		}
+	}
+	if exePath != "" {
+		if err := isExecutable(exePath); err != nil {
+			return "", err
+		}
+		return exePath, nil
+	}
+	// Search for executable in $PATH.
+	for _, dir := range splitPathList(Getenv("PATH")) {
+		if len(dir) == 0 {
+			dir = "."
+		}
+		if !IsPathSeparator(dir[0]) {
+			if errWd != nil {
+				return "", errWd
+			}
+			dir = initWd + string(PathSeparator) + dir
+		}
+		exePath = dir + string(PathSeparator) + Args[0]
+		switch isExecutable(exePath) {
+		case nil:
+			return exePath, nil
+		case ErrPermission:
+			return "", ErrPermission
+		}
+	}
+	return "", ErrNotExist
+}
+
+// isExecutable returns an error if a given file is not an executable.
+func isExecutable(path string) error {
+	stat, err := Stat(path)
+	if err != nil {
+		return err
+	}
+	mode := stat.Mode()
+	if !mode.IsRegular() {
+		return ErrPermission
+	}
+	if (mode & 0111) == 0 {
+		return ErrPermission
+	}
+	return nil
+}
+
+// splitPathList splits a path list.
+// This is based on genSplit from strings/strings.go
+func splitPathList(pathList string) []string {
+	if pathList == "" {
+		return nil
+	}
+	n := 1
+	for i := 0; i < len(pathList); i++ {
+		if pathList[i] == PathListSeparator {
+			n++
+		}
+	}
+	start := 0
+	a := make([]string, n)
+	na := 0
+	for i := 0; i+1 <= len(pathList) && na+1 < n; i++ {
+		if pathList[i] == PathListSeparator {
+			a[na] = pathList[start:i]
+			na++
+			start = i + 1
+		}
+	}
+	a[na] = pathList[start:]
+	return a[:na+1]
+}

src/os/executable_procfs.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build linux netbsd openbsd dragonfly nacl
+// +build linux netbsd dragonfly nacl
 
 package os
 
@@ -23,8 +23,6 @@ var executablePath, executablePathErr = func() (string, error) {
 		procfn = "/proc/self/exe"
 	case "netbsd":
 		procfn = "/proc/curproc/exe"
-	case "openbsd":
-		procfn = "/proc/curproc/file"
 	case "dragonfly":
 		procfn = "/proc/curproc/file"
 	}

src/os/executable_test.go

@@ -20,10 +20,6 @@ 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"
@@ -32,6 +28,13 @@ func TestExecutable(t *testing.T) {
 	if err != nil {
 		t.Fatalf("filepath.Rel: %v", err)
 	}
+
+	if runtime.GOOS == "openbsd" {
+		// The rest of the test doesn't work on OpenBSD,
+		// which relies on argv[0].
+		t.Skipf("skipping remainder of test on %s", runtime.GOOS)
+	}
+
 	cmd := &osexec.Cmd{}
 	// make child start with a relative program path
 	cmd.Dir = dir