fs: use syscall.Rename rather than os.Rename

This provides more fidelity for reproducing file system behavior. Case
in point, it lets us rename a directory on top of an existing
directory

Fixes #55.

Change-Id: I82572aae32ee225b1629303c448a09f1066eb4ed

fs/loopback.go

@@ -155,9 +155,9 @@ func (n *loopbackNode) Rename(ctx context.Context, name string, newParent InodeE
 	}
 
 	p1 := filepath.Join(n.path(), name)
-
 	p2 := filepath.Join(newParentLoopback.path(), newName)
-	err := os.Rename(p1, p2)
+
+	err := syscall.Rename(p1, p2)
 	return ToErrno(err)
 }
 

posixtest/test.go

@@ -33,6 +33,7 @@ var All = map[string]func(*testing.T, string){
 	"LinkUnlinkRename":           LinkUnlinkRename,
 	"RenameOverwriteDestNoExist": RenameOverwriteDestNoExist,
 	"RenameOverwriteDestExist":   RenameOverwriteDestExist,
+	"RenameOpenDir":              RenameOpenDir,
 	"ReadDir":                    ReadDir,
 	"ReadDirPicksUpCreate":       ReadDirPicksUpCreate,
 	"DirectIO":                   DirectIO,
@@ -354,6 +355,41 @@ func RenameOverwrite(t *testing.T, mnt string, destExists bool) {
 	}
 }
 
+func RenameOpenDir(t *testing.T, mnt string) {
+	if err := os.Mkdir(mnt+"/dir1", 0755); err != nil {
+		t.Fatalf("Mkdir: %v", err)
+	}
+
+	if err := os.Mkdir(mnt+"/dir2", 0755); err != nil {
+		t.Fatalf("Mkdir: %v", err)
+	}
+
+	var st1 syscall.Stat_t
+	if err := syscall.Stat(mnt+"/dir2", &st1); err != nil {
+		t.Fatalf("Stat: %v", err)
+	}
+
+	fd, err := syscall.Open(mnt+"/dir2", syscall.O_RDONLY, 0)
+	if err != nil {
+		t.Fatalf("Open: %v", err)
+	}
+	defer syscall.Close(fd)
+	if err := syscall.Rename(mnt+"/dir1", mnt+"/dir2"); err != nil {
+		t.Fatalf("Rename: %v", err)
+	}
+
+	var st2 syscall.Stat_t
+	if err := syscall.Fstat(fd, &st2); err != nil {
+		t.Fatalf("Fstat: %v", err)
+	}
+	if st2.Mode&syscall.S_IFMT != syscall.S_IFDIR {
+		t.Errorf("got mode %o, want %o", st2.Mode, syscall.S_IFDIR)
+	}
+	if st2.Ino != st1.Ino {
+		t.Errorf("got ino %d, want %d", st2.Ino, st1.Ino)
+	}
+}
+
 // ReadDir creates 110 files one by one, checking that we get the expected
 // entries after each file creation.
 func ReadDir(t *testing.T, mnt string) {