os: Fix MkdirAll("/thisdoesnotexist").

Fixes #1637.

R=rsc, rh, msolo
CC=golang-dev
https://golang.org/cl/4317049

src/pkg/os/path.go

@@ -33,7 +33,7 @@ func MkdirAll(path string, perm uint32) Error {
 		j--
 	}
 
-	if j > 0 {
+	if j > 1 {
 		// Create parent
 		err = MkdirAll(path[0:j-1], perm)
 		if err != nil {

src/pkg/os/path_test.go

@@ -179,3 +179,20 @@ func TestMkdirAllWithSymlink(t *testing.T) {
 		t.Errorf("MkdirAll %q: %s", path, err)
 	}
 }
+
+func TestMkdirAllAtSlash(t *testing.T) {
+	if runtime.GOOS == "windows" {
+		return
+	}
+	RemoveAll("/_go_os_test")
+	err := MkdirAll("/_go_os_test/dir", 0777)
+	if err != nil {
+		pathErr, ok := err.(*PathError)
+		// common for users not to be able to write to /
+		if ok && pathErr.Error == EACCES {
+			return
+		}
+		t.Fatalf(`MkdirAll "/_go_os_test/dir": %v`, err)
+	}
+	RemoveAll("/_go_os_test")
+}