exec.LookPath() unix/windows separation

R=brainman, rsc, vcc, rsc1
CC=golang-dev
https://golang.org/cl/2068041

src/pkg/exec/lp_unix.go

@@ -0,0 +1,45 @@
+// Copyright 2010 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 exec
+
+import (
+	"os"
+	"strings"
+)
+
+func canExec(file string) bool {
+	d, err := os.Stat(file)
+	if err != nil {
+		return false
+	}
+	return d.IsRegular() && d.Permission()&0111 != 0
+}
+
+// LookPath searches for an executable binary named file
+// in the directories named by the PATH environment variable.
+// If file contains a slash, it is tried directly and the PATH is not consulted.
+func LookPath(file string) (string, os.Error) {
+	// NOTE(rsc): I wish we could use the Plan 9 behavior here
+	// (only bypass the path if file begins with / or ./ or ../)
+	// but that would not match all the Unix shells.
+
+	if strings.Index(file, "/") >= 0 {
+		if canExec(file) {
+			return file, nil
+		}
+		return "", os.ENOENT
+	}
+	pathenv := os.Getenv("PATH")
+	for _, dir := range strings.Split(pathenv, ":", -1) {
+		if dir == "" {
+			// Unix shell semantics: path element "" means "."
+			dir = "."
+		}
+		if canExec(dir + "/" + file) {
+			return dir + "/" + file, nil
+		}
+	}
+	return "", os.ENOENT
+}