exec: use custom error for LookPath

Make the error message and the needed action more obvious
when a command isn't found to obtain the source code
of a project.  Users seem to strugle with the existing
wording in practice.

R=rsc
CC=golang-dev
https://golang.org/cl/4058047

src/cmd/goinstall/main.go

@@ -244,8 +244,7 @@ func quietRun(dir string, stdin []byte, cmd ...string) os.Error {
 func genRun(dir string, stdin []byte, cmd []string, quiet bool) os.Error {
 	bin, err := exec.LookPath(cmd[0])
 	if err != nil {
-		// report binary as well as the error
-		return os.NewError(cmd[0] + ": " + err.String())
+		return err
 	}
 	p, err := exec.Run(bin, cmd, os.Environ(), dir, exec.Pipe, exec.Pipe, exec.MergeWithStdout)
 	if *verbose {

src/pkg/exec/exec.go

@@ -7,6 +7,7 @@ package exec
 
 import (
 	"os"
+	"strconv"
 )
 
 // Arguments to Run.
@@ -29,6 +30,16 @@ type Cmd struct {
 	Pid    int
 }
 
+// PathError records the name of a binary that was not
+// found on the current $PATH.
+type PathError struct {
+	Name string
+}
+
+func (e *PathError) String() string {
+	return "command " + strconv.Quote(e.Name) + " not found in $PATH"
+}
+
 // Given mode (DevNull, etc), return file for child
 // and file to record in Cmd structure.
 func modeToFiles(mode, fd int) (*os.File, *os.File, os.Error) {

src/pkg/exec/lp_test.go

@@ -0,0 +1,33 @@
+// Copyright 2011 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 (
+	"testing"
+)
+
+var nonExistentPaths = []string{
+	"some-non-existent-path",
+	"non-existent-path/slashed",
+}
+
+func TestLookPathNotFound(t *testing.T) {
+	for _, name := range nonExistentPaths {
+		path, err := LookPath(name)
+		if err == nil {
+			t.Fatalf("LookPath found %q in $PATH", name)
+		}
+		if path != "" {
+			t.Fatalf("LookPath path == %q when err != nil", path)
+		}
+		perr, ok := err.(*PathError)
+		if !ok {
+			t.Fatal("LookPath error is not a PathError")
+		}
+		if perr.Name != name {
+			t.Fatal("want PathError name %q, got %q", name, perr.Name)
+		}
+	}
+}

src/pkg/exec/lp_unix.go

@@ -29,7 +29,7 @@ func LookPath(file string) (string, os.Error) {
 		if canExec(file) {
 			return file, nil
 		}
-		return "", &os.PathError{"lookpath", file, os.ENOENT}
+		return "", &PathError{file}
 	}
 	pathenv := os.Getenv("PATH")
 	for _, dir := range strings.Split(pathenv, ":", -1) {
@@ -41,5 +41,5 @@ func LookPath(file string) (string, os.Error) {
 			return dir + "/" + file, nil
 		}
 	}
-	return "", &os.PathError{"lookpath", file, os.ENOENT}
+	return "", &PathError{file}
 }

src/pkg/exec/lp_windows.go

@@ -49,7 +49,7 @@ func LookPath(file string) (string, os.Error) {
 		if f, ok := canExec(file, exts); ok {
 			return f, nil
 		}
-		return ``, &os.PathError{"lookpath", file, os.ENOENT}
+		return ``, &PathError{file}
 	}
 	if pathenv := os.Getenv(`PATH`); pathenv == `` {
 		if f, ok := canExec(`.\`+file, exts); ok {
@@ -62,5 +62,5 @@ func LookPath(file string) (string, os.Error) {
 			}
 		}
 	}
-	return ``, &os.PathError{"lookpath", file, os.ENOENT}
+	return ``, &PathError{file}
 }