os: fix rename on Plan 9

Rename should remove newname if the file already exists
and is not a directory.

Fixes #13844.

Change-Id: I85a5cc28e8d161637a8bc1de33f4a637d9154cd1
Reviewed-on: https://go-review.googlesource.com/18291Reviewed-by: Russ Cox <rsc@golang.org>

src/os/file_plan9.go

@@ -339,7 +339,9 @@ func rename(oldname, newname string) error {
 
 	// If newname still contains slashes after removing the oldname
 	// prefix, the rename is cross-directory and must be rejected.
-	// This case is caught by d.Marshal below.
+	if lastIndex(newname, '/') >= 0 {
+		return &LinkError{"rename", oldname, newname, ErrInvalid}
+	}
 
 	var d syscall.Dir
 
@@ -351,6 +353,13 @@ func rename(oldname, newname string) error {
 	if err != nil {
 		return &LinkError{"rename", oldname, newname, err}
 	}
+
+	// If newname already exists and is not a directory, rename replaces it.
+	f, err := Stat(dirname + newname)
+	if err == nil && !f.IsDir() {
+		Remove(dirname + newname)
+	}
+
 	if err = syscall.Wstat(oldname, buf[:n]); err != nil {
 		return &LinkError{"rename", oldname, newname, err}
 	}

src/os/os_test.go

@@ -773,9 +773,6 @@ func TestRename(t *testing.T) {
 }
 
 func TestRenameOverwriteDest(t *testing.T) {
-	if runtime.GOOS == "plan9" {
-		t.Skip("skipping on plan9")
-	}
 	defer chtmpdir(t)()
 	from, to := "renamefrom", "renameto"
 	// Just in case.