path/filepath: implement documented SkipDir behavior

Currently walk() doesn't check for err == SkipDir when iterating
a directory list, but such promise is made in the docs for WalkFunc.

Fixes #3486.

R=rsc, r
CC=golang-dev
https://golang.org/cl/6257059

Author: bflm

src/pkg/path/filepath/path.go

@@ -320,8 +320,11 @@ func walk(path string, info os.FileInfo, walkFn WalkFunc) error {
 	}
 
 	for _, fileInfo := range list {
-		if err = walk(Join(path, fileInfo.Name()), fileInfo, walkFn); err != nil {
-			return err
+		err = walk(Join(path, fileInfo.Name()), fileInfo, walkFn)
+		if err != nil {
+			if !fileInfo.IsDir() || err != SkipDir {
+				return err
+			}
 		}
 	}
 	return nil

src/pkg/path/filepath/path_test.go

@@ -874,3 +874,26 @@ func TestDriveLetterInEvalSymlinks(t *testing.T) {
 		t.Errorf("Results of EvalSymlinks do not match: %q and %q", flp, fup)
 	}
 }
+
+func TestBug3486(t *testing.T) { // http://code.google.com/p/go/issues/detail?id=3486
+	root := os.Getenv("GOROOT")
+	lib := filepath.Join(root, "lib")
+	src := filepath.Join(root, "src")
+	seenSrc := false
+	filepath.Walk(root, func(pth string, info os.FileInfo, err error) error {
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		switch pth {
+		case lib:
+			return filepath.SkipDir
+		case src:
+			seenSrc = true
+		}
+		return nil
+	})
+	if !seenSrc {
+		t.Fatalf("%q not seen", src)
+	}
+}