fix(protocol): handle zero-size requests (fixes #10709) (#10710)

- Allow zero-sized requests since they are sent by all current versions
of Syncthing.
- Stop sending zero-sized requests since that's stupid.

---------

Signed-off-by: Jakob Borg <jakob@kastelo.net>

lib/model/folder_sendrecv.go

@@ -1345,6 +1345,12 @@ func (f *sendReceiveFolder) copierRoutine(ctx context.Context, in <-chan copyBlo
 			default:
 			}
 
+			if block.Size == 0 {
+				// Copying zero bytes is a no-op.
+				state.copyDone(block)
+				continue
+			}
+
 			if !f.DisableSparseFiles && state.reused == 0 && block.IsEmpty() {
 				// The block is a block of all zeroes, and we are not reusing
 				// a temp file, so there is no need to do anything with it.
@@ -1534,6 +1540,13 @@ func (f *sendReceiveFolder) pullerRoutine(ctx context.Context, in <-chan pullBlo
 
 		bytes := state.block.Size
 
+		if bytes == 0 {
+			// Pulling zero bytes is a no-op.
+			state.pullDone(state.block)
+			out <- state.sharedPullerState
+			continue
+		}
+
 		if err := requestLimiter.TakeWithContext(ctx, bytes); err != nil {
 			state.fail(err)
 			out <- state.sharedPullerState

lib/model/folder_sendrecv_test.go

@@ -891,7 +891,7 @@ func TestPullCtxCancel(t *testing.T) {
 	emptyState := func() pullBlockState {
 		return pullBlockState{
 			sharedPullerState: newSharedPullerState(protocol.FileInfo{}, nil, f.folderID, "", nil, nil, false, false, protocol.FileInfo{}, false, false),
-			block:             protocol.BlockInfo{},
+			block:             protocol.BlockInfo{Size: 42},
 		}
 	}
 

lib/protocol/protocol.go

@@ -483,7 +483,7 @@ func (c *rawConnection) dispatcherLoop() (err error) {
 			if err := checkFilename(msg.Name); err != nil {
 				return newProtocolError(err, msgContext)
 			}
-			if msg.Size <= 0 {
+			if msg.Size < 0 {
 				return newProtocolError(fmt.Errorf("request size %d too small", msg.Size), msgContext)
 			}
 			if msg.Size > MaxRequestSize {

lib/protocol/protocol_test.go

@@ -544,7 +544,7 @@ func TestDispatcherToCloseDeadlock(t *testing.T) {
 }
 
 func TestRequestMaxSize(t *testing.T) {
-	invalidSize := []int{-65536, 0, MaxRequestSize + 1}
+	invalidSize := []int{-65536, -1, MaxRequestSize + 1}
 	for _, s := range invalidSize {
 		t.Run(fmt.Sprintf("invalid/%d", s), func(t *testing.T) {
 			m := newTestModel()
@@ -592,6 +592,34 @@ func TestRequestMaxSize(t *testing.T) {
 	}
 }
 
+func TestRequestZeroSize(t *testing.T) {
+	// A zero-sized request should be accepted, since current versions of
+	// Syncthing send these. See https://github.com/syncthing/syncthing/issues/10709.
+	m := newTestModel()
+	rw := testutil.NewBlockingRW()
+	c := getRawConnection(NewConnection(c0ID, rw, &testutil.NoopRW{}, testutil.NoopCloser{}, m, new(mockedConnectionInfo), CompressionAlways, testKeyGen))
+	c.Start()
+	defer closeAndWait(c, rw)
+
+	c.inbox <- &bep.ClusterConfig{}
+	c.inbox <- &bep.Request{
+		Id:   1,
+		Name: "valid",
+		Size: 0,
+	}
+
+	select {
+	case res := <-c.outbox:
+		if msg, ok := res.msg.(*bep.Response); !ok || msg.Id != 1 {
+			t.Errorf("bad response %#v", msg)
+		}
+	case <-c.dispatcherLoopStopped:
+		t.Fatal("dispatcher loop terminated, expected zero-sized request to be accepted")
+	case <-time.After(time.Second):
+		t.Fatal("timed out waiting for response")
+	}
+}
+
 func TestRequestInvalidFilename(t *testing.T) {
 	m := newTestModel()
 	rw := testutil.NewBlockingRW()