exec: change exec.PathError to exec.Error

There were two issues:
1) It might not be a path error, it might be 'permission denied'.
2) The concept of $PATH is Unix-specific.

R=alex.brainman, rsc, r, mattn.jp
CC=golang-dev
https://golang.org/cl/4530096

src/pkg/exec/exec.go

@@ -14,14 +14,15 @@ import (
 	"strconv"
 )
 
-// PathError records the name of a binary that was not
-// found on the current $PATH.
-type PathError struct {
+// Error records the name of a binary that failed to be be executed
+// and the reason it failed.
+type Error struct {
 	Name  string
+	Error os.Error
 }
 
-func (e *PathError) String() string {
-	return "command " + strconv.Quote(e.Name) + " not found in $PATH"
+func (e *Error) String() string {
+	return "exec: " + strconv.Quote(e.Name) + ": " + e.Error.String()
 }
 
 // Cmd represents an external command being prepared or run.
@@ -32,8 +33,8 @@ type Cmd struct {
 	// value.
 	Path string
 
-	// Args is the command line arguments, including the command as Args[0].
-	// If Args is empty, Run uses {Path}.
+	// Args holds command line arguments, including the command as Args[0].
+	// If the Args field is empty or nil, Run uses {Path}.
 	// 
 	// In typical use, both Path and Args are set by calling Command.
 	Args []string
@@ -44,7 +45,7 @@ type Cmd struct {
 
 	// Dir specifies the working directory of the command.
 	// If Dir is the empty string, Run runs the command in the
-	// process's current directory.
+	// calling process's current directory.
 	Dir string
 
 	// Stdin specifies the process's standard input.
@@ -81,7 +82,7 @@ type Cmd struct {
 // resolve the path to a complete name if possible. Otherwise it uses
 // name directly.
 //
-// The returned Cmd's Args is constructed from the command name
+// The returned Cmd's Args field is constructed from the command name
 // followed by the elements of arg, so arg should not include the
 // command name itself. For example, Command("echo", "hello")
 func Command(name string, arg ...string) *Cmd {
@@ -97,7 +98,7 @@ func Command(name string, arg ...string) *Cmd {
 }
 
 // interfaceEqual protects against panics from doing equality tests on
-// two interface with non-comparable underlying types
+// two interfaces with non-comparable underlying types
 func interfaceEqual(a, b interface{}) bool {
 	defer func() {
 		recover()

src/pkg/exec/lp_test.go

@@ -22,12 +22,12 @@ func TestLookPathNotFound(t *testing.T) {
 		if path != "" {
 			t.Fatalf("LookPath path == %q when err != nil", path)
 		}
-		perr, ok := err.(*PathError)
+		perr, ok := err.(*Error)
 		if !ok {
-			t.Fatal("LookPath error is not a PathError")
+			t.Fatal("LookPath error is not an exec.Error")
 		}
 		if perr.Name != name {
-			t.Fatalf("want PathError name %q, got %q", name, perr.Name)
+			t.Fatalf("want Error name %q, got %q", name, perr.Name)
 		}
 	}
 }

src/pkg/exec/lp_unix.go

@@ -9,12 +9,18 @@ import (
 	"strings"
 )
 
-func canExec(file string) bool {
+// ErrNotFound is the error resulting if a path search failed to find an executable file.
+var ErrNotFound = os.ErrorString("executable file not found in $PATH")
+
+func findExecutable(file string) os.Error {
 	d, err := os.Stat(file)
 	if err != nil {
-		return false
+		return err
+	}
+	if d.IsRegular() && d.Permission()&0111 != 0 {
+		return nil
 	}
-	return d.IsRegular() && d.Permission()&0111 != 0
+	return os.EPERM
 }
 
 // LookPath searches for an executable binary named file
@@ -26,10 +32,11 @@ func LookPath(file string) (string, os.Error) {
 	// but that would not match all the Unix shells.
 
 	if strings.Contains(file, "/") {
-		if canExec(file) {
+		err := findExecutable(file)
+		if err == nil {
 			return file, nil
 		}
-		return "", &PathError{file}
+		return "", &Error{file, err}
 	}
 	pathenv := os.Getenv("PATH")
 	for _, dir := range strings.Split(pathenv, ":", -1) {
@@ -37,9 +44,9 @@ func LookPath(file string) (string, os.Error) {
 			// Unix shell semantics: path element "" means "."
 			dir = "."
 		}
-		if canExec(dir + "/" + file) {
+		if err := findExecutable(dir + "/" + file); err == nil {
 			return dir + "/" + file, nil
 		}
 	}
-	return "", &PathError{file}
+	return "", &Error{file, ErrNotFound}
 }

src/pkg/exec/lp_windows.go

@@ -9,15 +9,21 @@ import (
 	"strings"
 )
 
-func chkStat(file string) bool {
+// ErrNotFound is the error resulting if a path search failed to find an executable file.
+var ErrNotFound = os.ErrorString("executable file not found in %PATH%")
+
+func chkStat(file string) os.Error {
 	d, err := os.Stat(file)
 	if err != nil {
-		return false
+		return err
+	}
+	if d.IsRegular() {
+		return nil
 	}
-	return d.IsRegular()
+	return os.EPERM
 }
 
-func canExec(file string, exts []string) (string, bool) {
+func findExecutable(file string, exts []string) (string, os.Error) {
 	if len(exts) == 0 {
 		return file, chkStat(file)
 	}
@@ -28,14 +34,14 @@ func canExec(file string, exts []string) (string, bool) {
 		}
 	}
 	for _, e := range exts {
-		if f := file + e; chkStat(f) {
-			return f, true
+		if f := file + e; chkStat(f) == nil {
+			return f, nil
 		}
 	}
-	return ``, false
+	return ``, ErrNotFound
 }
 
-func LookPath(file string) (string, os.Error) {
+func LookPath(file string) (f string, err os.Error) {
 	exts := []string{}
 	if x := os.Getenv(`PATHEXT`); x != `` {
 		exts = strings.Split(strings.ToLower(x), `;`, -1)
@@ -46,21 +52,21 @@ func LookPath(file string) (string, os.Error) {
 		}
 	}
 	if strings.Contains(file, `\`) || strings.Contains(file, `/`) {
-		if f, ok := canExec(file, exts); ok {
-			return f, nil
+		if f, err = findExecutable(file, exts); err == nil {
+			return
 		}
-		return ``, &PathError{file}
+		return ``, &Error{file, err}
 	}
 	if pathenv := os.Getenv(`PATH`); pathenv == `` {
-		if f, ok := canExec(`.\`+file, exts); ok {
-			return f, nil
+		if f, err = findExecutable(`.\`+file, exts); err == nil {
+			return
 		}
 	} else {
 		for _, dir := range strings.Split(pathenv, `;`, -1) {
-			if f, ok := canExec(dir+`\`+file, exts); ok {
-				return f, nil
+			if f, err = findExecutable(dir+`\`+file, exts); err == nil {
+				return
 			}
 		}
 	}
-	return ``, &PathError{file}
+	return ``, &Error{file, ErrNotFound}
 }