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>

AUTHORS

@@ -216,6 +216,7 @@ Matic Potočnik <hairyfotr@gmail.com>
 Matt Burke (burkemw3) <mburke@amplify.com> <burkemw3@gmail.com>
 Matt Robenolt <matt@ydekproductions.com>
 Matteo Ruina <matteo.ruina@gmail.com>
+mattn <mattn.jp@gmail.com>
 Maurizio Tomasi <ziotom78@gmail.com>
 Max <github@germancoding.com>
 Max Schulze (kralo) <max.schulze@online.de> <kralo@users.noreply.github.com>

gui/default/syncthing/core/aboutModalView.html

@@ -30,7 +30,7 @@
         <h4 class="text-center" translate>The Syncthing Authors</h4>
         <div class="row">
           <div class="col-md-12" id="contributor-list">
-Jakob Borg, Audrius Butkevicius, Simon Frei, Tomasz Wilczyński, Alexander Graf, Alexandre Viau, Anderson Mesquita, André Colomb, Antony Male, Ben Schulz, bt90, Caleb Callaway, Daniel Harte, Emil Lundberg, Eric P, Evgeny Kuznetsov, greatroar, Lars K.W. Gohlke, Lode Hoste, Marcus B Spencer, Michael Ploujnikov, Ross Smith II, Stefan Tatschner, Tommy van der Vorst, Wulf Weich, Adam Piggott, Adel Qalieh, Aleksey Vasenev, Alessandro G., Alex Ionescu, Alex Lindeman, Alex Xu, Alexander Seiler, Alexandre Alves, Aman Gupta, Andreas Sommer, andresvia, Andrew Rabert, Andrey D, andyleap, Anjan Momi, Anthony Goeckner, Antoine Lamielle, Anur, Aranjedeath, ardevd, Arkadiusz Tymiński, Aroun, Arthur Axel fREW Schmidt, Artur Zubilewicz, Ashish Bhate, Aurélien Rainone, BAHADIR YILMAZ, Bart De Vries, Beat Reichenbach, Ben Norcombe, Ben Shepherd, Ben Sidhom, Benedikt Heine, Benno Fünfstück, Benny Ng, boomsquared, Boqin Qin, Boris Rybalkin, Brendan Long, Catfriend1, Cathryne Linenweaver, Cedric Staniewski, Chih-Hsuan Yen, Choongkyu, Chris Howie, Chris Joel, Christian Kujau, Christian Prescott, chucic, cjc7373, Colin Kennedy, Cromefire_, cui, Cyprien Devillez, d-volution, Dan, Daniel Barczyk, Daniel Bergmann, Daniel Martí, Daniel Padrta, Daniil Gentili, Darshil Chanpura, dashangcun, David Rimmer, DeflateAwning, Denis A., Dennis Wilson, derekriemer, DerRockWolf, desbma, Devon G. Redekopp, digital, Dimitri Papadopoulos Orfanos, Dmitry Saveliev, domain, Domenic Horner, Dominik Heidler, Elias Jarlebring, Elliot Huffman, Emil Hessman, Eng Zer Jun, entity0xfe, Epifeny, epifeny, Eric Lesiuta, Erik Meitner, Evan Spensley, Federico Castagnini, Felix, Felix Ableitner, Felix Lampe, Felix Unterpaintner, Francois-Xavier Gsell, Frank Isemann, Gahl Saraf, georgespatton, ghjklw, Gilli Sigurdsson, Gleb Sinyavskiy, Graham Miln, Greg, guangwu, gudvinr, Gusted, Han Boetes, HansK-p, Harrison Jones, Hazem Krimi, Heiko Zuerker, Hireworks, Hugo Locurcio, Iain Barnett, Ian Johnson, ignacy123, Iskander Sharipov, Jaakko Hannikainen, Jack Croft, Jacob, Jake Peterson, James O'Beirne, James Patterson, Jaroslav Lichtblau, Jaroslav Malec, Jaspitta, Jaya Chithra, Jaya Kumar, Jeffery To, jelle van der Waa, Jens Diemer, Jochen Voss, Johan Vromans, John Rinehart, Jonas Thelemann, Jonathan, Jose Manuel Delicado, JRNitre, jtagcat, Julian Lehrhuber, Jörg Thalheim, Jędrzej Kula, Kapil Sareen, Karol Różycki, Kebin Liu, Keith Harrison, Kelong Cong, Ken'ichi Kamada, Kevin Allen, Kevin Bushiri, Kevin White, Jr., klemens, Kurt Fitzner, kylosus, Lars Lehtonen, Laurent Etiemble, Leo Arias, Liu Siyuan, Lord Landon Agahnim, LSmithx2, Luiz Angelo Daros de Luca, Lukas Lihotzki, Luke Hamburg, luzpaz, Majed Abdulaziz, Marc Laporte, Marcel Meyer, Marcin Dziadus, Marcus Legendre, Mario Majila, Mark Pulford, Martchus, Mateusz Naściszewski, Mateusz Ż, mathias4833, Matic Potočnik, Matt Burke, Matt Robenolt, Matteo Ruina, Maurizio Tomasi, Max, Max Schulze, MaximAL, Maximilian, Maxwell G, Michael Jephcote, Michael Rienstra, Michael Wang 汪東陽, MichaIng, Migelo, Mike Boone, MikeLund, MikolajTwarog, Mingxuan Lin, mv1005, Nate Morrison, nf, Nicholas Rishel, Nick Busey, Nico Stapelbroek, Nicolas Braud-Santoni, Nicolas Perraut, Niels Peter Roest, Nils Jakobi, NinoM4ster, Nitroretro, NoLooseEnds, Oliver Freyermuth, orangekame3, otbutz, overkill, Oyebanji Jacob Mayowa, Pablo, Pascal Jungblut, Paul Brit, Paul Donald, Pawel Palenica, perewa, Peter Badida, Peter Dave Hello, Peter Hoeg, Peter Marquardt, Phani Rithvij, Phil Davis, Philippe Schommers, Phill Luby, Piotr Bejda, polyfloyd, Prathik P Kulkarni, pullmerge, Quentin Hibon, Rahmi Pruitt, RealCharlesChia, red_led, Robert Carosi, Roberto Santalla, Robin Schoonover, Roman Zaynetdinov, rubenbe, Ruslan Yevdokymov, Ryan Qian, Ryan Sullivan, Sacheendra Talluri, Scott Klupfel, sec65, Sergey Mishin, Sertonix, Severin von Wnuck-Lipinski, Shaarad Dalvi, Shivam Kumar, Simon Mwepu, Simon Pickup, Sly_tom_cat, Sonu Kumar Saw, Stefan Kuntz, Steven Eckhoff, Suhas Gundimeda, Sven Bachmann, Sébastien WENSKE, Tao, Taylor Khan, Terrance, TheCreeper, Thomas, Thomas Hipp, Tim Abell, Tim Howes, Tobias Frölich, Tobias Klauser, Tobias Nygren, Tobias Tom, Tom Jakubowski, Tully Robinson, Tyler Brazier, Tyler Kropp, Umer-Azaz, Unrud, Val Markovic, vapatel2, Veeti Paananen, Victor Buinsky, Vik, Vil Brekin, villekalliomaki, Vladimir Rusinov, vvaswani, wangguoliang, WangXi, Will Rouesnel, William A. Kennington III, wouter bolsterlee, xarx00, Xavier O., xjtdy888, Yannic A., yparitcher, 佛跳墙, 落心
+Jakob Borg, Audrius Butkevicius, Simon Frei, Tomasz Wilczyński, Alexander Graf, Alexandre Viau, Anderson Mesquita, André Colomb, Antony Male, Ben Schulz, bt90, Caleb Callaway, Daniel Harte, Emil Lundberg, Eric P, Evgeny Kuznetsov, greatroar, Lars K.W. Gohlke, Lode Hoste, Marcus B Spencer, Michael Ploujnikov, Ross Smith II, Stefan Tatschner, Tommy van der Vorst, Wulf Weich, Adam Piggott, Adel Qalieh, Aleksey Vasenev, Alessandro G., Alex Ionescu, Alex Lindeman, Alex Xu, Alexander Seiler, Alexandre Alves, Aman Gupta, Andreas Sommer, andresvia, Andrew Rabert, Andrey D, andyleap, Anjan Momi, Anthony Goeckner, Antoine Lamielle, Anur, Aranjedeath, ardevd, Arkadiusz Tymiński, Aroun, Arthur Axel fREW Schmidt, Artur Zubilewicz, Ashish Bhate, Aurélien Rainone, BAHADIR YILMAZ, Bart De Vries, Beat Reichenbach, Ben Norcombe, Ben Shepherd, Ben Sidhom, Benedikt Heine, Benno Fünfstück, Benny Ng, boomsquared, Boqin Qin, Boris Rybalkin, Brendan Long, Catfriend1, Cathryne Linenweaver, Cedric Staniewski, Chih-Hsuan Yen, Choongkyu, Chris Howie, Chris Joel, Christian Kujau, Christian Prescott, chucic, cjc7373, Colin Kennedy, Cromefire_, cui, Cyprien Devillez, d-volution, Dan, Daniel Barczyk, Daniel Bergmann, Daniel Martí, Daniel Padrta, Daniil Gentili, Darshil Chanpura, dashangcun, David Rimmer, DeflateAwning, Denis A., Dennis Wilson, derekriemer, DerRockWolf, desbma, Devon G. Redekopp, digital, Dimitri Papadopoulos Orfanos, Dmitry Saveliev, domain, Domenic Horner, Dominik Heidler, Elias Jarlebring, Elliot Huffman, Emil Hessman, Eng Zer Jun, entity0xfe, Epifeny, epifeny, Eric Lesiuta, Erik Meitner, Evan Spensley, Federico Castagnini, Felix, Felix Ableitner, Felix Lampe, Felix Unterpaintner, Francois-Xavier Gsell, Frank Isemann, Gahl Saraf, georgespatton, ghjklw, Gilli Sigurdsson, Gleb Sinyavskiy, Graham Miln, Greg, guangwu, gudvinr, Gusted, Han Boetes, HansK-p, Harrison Jones, Hazem Krimi, Heiko Zuerker, Hireworks, Hugo Locurcio, Iain Barnett, Ian Johnson, ignacy123, Iskander Sharipov, Jaakko Hannikainen, Jack Croft, Jacob, Jake Peterson, James O'Beirne, James Patterson, Jaroslav Lichtblau, Jaroslav Malec, Jaspitta, Jaya Chithra, Jaya Kumar, Jeffery To, jelle van der Waa, Jens Diemer, Jochen Voss, Johan Vromans, John Rinehart, Jonas Thelemann, Jonathan, Jose Manuel Delicado, JRNitre, jtagcat, Julian Lehrhuber, Jörg Thalheim, Jędrzej Kula, Kapil Sareen, Karol Różycki, Kebin Liu, Keith Harrison, Kelong Cong, Ken'ichi Kamada, Kevin Allen, Kevin Bushiri, Kevin White, Jr., klemens, Kurt Fitzner, kylosus, Lars Lehtonen, Laurent Etiemble, Leo Arias, Liu Siyuan, Lord Landon Agahnim, LSmithx2, Luiz Angelo Daros de Luca, Lukas Lihotzki, Luke Hamburg, luzpaz, Majed Abdulaziz, Marc Laporte, Marcel Meyer, Marcin Dziadus, Marcus Legendre, Mario Majila, Mark Pulford, Martchus, Mateusz Naściszewski, Mateusz Ż, mathias4833, Matic Potočnik, Matt Burke, Matt Robenolt, Matteo Ruina, mattn, Maurizio Tomasi, Max, Max Schulze, MaximAL, Maximilian, Maxwell G, Michael Jephcote, Michael Rienstra, Michael Wang 汪東陽, MichaIng, Migelo, Mike Boone, MikeLund, MikolajTwarog, Mingxuan Lin, mv1005, Nate Morrison, nf, Nicholas Rishel, Nick Busey, Nico Stapelbroek, Nicolas Braud-Santoni, Nicolas Perraut, Niels Peter Roest, Nils Jakobi, NinoM4ster, Nitroretro, NoLooseEnds, Oliver Freyermuth, orangekame3, otbutz, overkill, Oyebanji Jacob Mayowa, Pablo, Pascal Jungblut, Paul Brit, Paul Donald, Pawel Palenica, perewa, Peter Badida, Peter Dave Hello, Peter Hoeg, Peter Marquardt, Phani Rithvij, Phil Davis, Philippe Schommers, Phill Luby, Piotr Bejda, polyfloyd, Prathik P Kulkarni, pullmerge, Quentin Hibon, Rahmi Pruitt, RealCharlesChia, red_led, Robert Carosi, Roberto Santalla, Robin Schoonover, Roman Zaynetdinov, rubenbe, Ruslan Yevdokymov, Ryan Qian, Ryan Sullivan, Sacheendra Talluri, Scott Klupfel, sec65, Sergey Mishin, Sertonix, Severin von Wnuck-Lipinski, Shaarad Dalvi, Shivam Kumar, Simon Mwepu, Simon Pickup, Sly_tom_cat, Sonu Kumar Saw, Stefan Kuntz, Steven Eckhoff, Suhas Gundimeda, Sven Bachmann, Sébastien WENSKE, Tao, Taylor Khan, Terrance, TheCreeper, Thomas, Thomas Hipp, Tim Abell, Tim Howes, Tobias Frölich, Tobias Klauser, Tobias Nygren, Tobias Tom, Tom Jakubowski, Tully Robinson, Tyler Brazier, Tyler Kropp, Umer-Azaz, Unrud, Val Markovic, vapatel2, Veeti Paananen, Victor Buinsky, Vik, Vil Brekin, villekalliomaki, Vladimir Rusinov, vvaswani, wangguoliang, WangXi, Will Rouesnel, William A. Kennington III, wouter bolsterlee, xarx00, Xavier O., xjtdy888, Yannic A., yparitcher, 佛跳墙, 落心
           </div>
         </div>
       </div>

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()