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Avoid deadlock in syncer.Close #92

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Avoid deadlock in syncer.Close #92

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84396b0
syncer: rename Close to Shutdown
peterjan Aug 29, 2024
2c7ca17
Merge branch 'master' of https://github.com/SiaFoundation/coreutils i…
peterjan Sep 11, 2024
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29 changes: 23 additions & 6 deletions syncer/syncer.go
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Original file line number Diff line number Diff line change
Expand Up @@ -285,14 +285,19 @@ func (s *Syncer) runPeer(p *Peer) error {
p.setErr(err)
return fmt.Errorf("failed to accept rpc: %w", err)
}

// set a generous deadline
err = stream.SetDeadline(time.Now().Add(10 * time.Minute))
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This is not really a fix I'm afraid. If we ever run into this on shutdown, the deadlock still exists. We should figure out what is causing the actual deadlock since the stream should unblock after the listener is closed.

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Other than this line I couldn't really identify another spot where it might potentially deadlock. @lukechampine do you mind taking a look? I commented on the PR with a link to the stack trace. I figured the stream could live on even after the listener got closed.

if err != nil {
p.setErr(err)
return fmt.Errorf("failed to set deadline: %w", err)
}

inflight <- struct{}{}
s.wg.Add(1)
go func() {
defer s.wg.Done()
defer stream.Close()
// NOTE: we do not set any deadlines on the stream. If a peer is
// slow, fine; we don't need to worry about resource exhaustion
// unless we have tons of peers.
if err := s.handleRPC(id, stream, p); err != nil {
s.log.Debug("rpc failed", zap.Stringer("peer", p), zap.Stringer("rpc", id), zap.Error(err))
}
Expand Down Expand Up @@ -675,10 +680,22 @@ func (s *Syncer) Run(ctx context.Context) error {
return err
}

// Close closes the Syncer's net.Listener.
func (s *Syncer) Close() error {
// Shutdown closes the Syncer's net.Listener.
func (s *Syncer) Shutdown(ctx context.Context) error {
err := s.l.Close()
s.wg.Wait()

waitChan := make(chan struct{})
go func() {
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Similar thing here. This shouldn't really be necessary. If we run into this, we still got deadlock issues.

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I definitely agree on the deadlock still being an issue, I wasn't aware the stream would automatically unblock if the listener got closed. That aside though, I always do the waitChan pattern to avoid a naked s.wg.Wait and consider it good practice?

s.wg.Wait()
close(waitChan)
}()

select {
case <-ctx.Done():
return errors.Join(err, ctx.Err())
case <-waitChan:
}

return err
}

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4 changes: 2 additions & 2 deletions syncer/syncer_test.go
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Expand Up @@ -47,15 +47,15 @@ func TestSyncer(t *testing.T) {
UniqueID: gateway.GenerateUniqueID(),
NetAddress: l1.Addr().String(),
}, syncer.WithLogger(log.Named("syncer1")))
defer s1.Close()
defer s1.Shutdown(context.Background())
go s1.Run(context.Background())

s2 := syncer.New(l2, cm2, testutil.NewMemPeerStore(), gateway.Header{
GenesisID: genesis.ID(),
UniqueID: gateway.GenerateUniqueID(),
NetAddress: l2.Addr().String(),
}, syncer.WithLogger(log.Named("syncer2")), syncer.WithSyncInterval(10*time.Millisecond))
defer s2.Close()
defer s2.Shutdown(context.Background())
go s2.Run(context.Background())

// mine a few blocks on cm1
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