fs: add Options.{UID,GID} for setting default owners

fs/api.go

@@ -462,4 +462,10 @@ type Options struct {
 	// DefaultPermissions sets null file permissions to 755 (dirs)
 	// or 644 (other files.)
 	DefaultPermissions bool
+
+	// If nonzero, replace default (zero) UID with the given UID
+	UID uint32
+
+	// If nonzero, replace default (zero) GID with the given GID
+	GID uint32
 }

fs/bridge.go

@@ -152,6 +152,12 @@ func (b *rawBridge) setAttr(out *fuse.Attr) {
 			out.Mode |= 0111
 		}
 	}
+	if b.options.UID != 0 && out.Uid == 0 {
+		out.Uid = b.options.UID
+	}
+	if b.options.GID != 0 && out.Gid == 0 {
+		out.Gid = b.options.GID
+	}
 }
 
 func (b *rawBridge) setAttrTimeout(out *fuse.AttrOut) {

fs/mem_test.go

@@ -38,6 +38,35 @@ func testMount(t *testing.T, root InodeEmbedder, opts *Options) (string, *fuse.S
 	}
 }
 
+func TestDefaultOwner(t *testing.T) {
+	want := "hello"
+	root := &Inode{}
+	mntDir, _, clean := testMount(t, root, &Options{
+		FirstAutomaticIno: 1,
+		OnAdd: func(ctx context.Context) {
+			n := root.EmbeddedInode()
+			ch := n.NewPersistentInode(
+				ctx,
+				&MemRegularFile{
+					Data: []byte(want),
+				},
+				StableAttr{})
+			n.AddChild("file", ch, false)
+		},
+		UID: 42,
+		GID: 43,
+	})
+	defer clean()
+
+	var st syscall.Stat_t
+	if err := syscall.Lstat(mntDir+"/file", &st); err != nil {
+		t.Fatalf("Lstat: %v", err)
+	} else if st.Uid != 42 || st.Gid != 43 {
+		t.Fatalf("Got Lstat %d, %d want 42,43", st.Uid, st.Gid)
+	}
+
+}
+
 func TestDataFile(t *testing.T) {
 	want := "hello"
 	root := &Inode{}