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Fix a deadlock in TransportSet. #2258

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Sep 12, 2016
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Fix a deadlock in TransportSet. #2258

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7 changes: 6 additions & 1 deletion core/src/main/java/io/grpc/internal/TransportSet.java
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Original file line number Diff line number Diff line change
Expand Up @@ -249,9 +249,14 @@ public void run() {
delayedTransport.endBackoff();
boolean shutdownDelayedTransport = false;
Runnable runnable = null;
// TransportSet as a channel layer class should not call into transport methods while
// holding the lock, thus we call hasPendingStreams() outside of the lock. It will cause
// a _benign_ race where the TransportSet may transition to CONNECTING when there is not
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I believe the race is also pre-existing, since at any time obtainActiveTransport() can return the delayed transport and a new stream be created.

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That sounds a real issue. If EndOfCurrentBackoff wins in the race with newStream(), the application will see a spurious shutdown error.

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I don't see where the shutdown error would come from because the delayed transport becomes pass-through to the supplier for newStream(), instead of failing new streams after shutdown.

Instead of calling startNewTransport() here, we would instead enter IDLE. But since we call setTransportSupplier with a supplier that calls obtainActiveTransport, then if the race occurs there will just be a call to obtainActiveTransport that will trigger entering CONNECTING.

The only issue is that the stream will wrapped twice with a DelayedStream. But since the race should only happen at most once per stream, we decided that it wasn't a big deal.

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You are right... I lost my touch with delayed transport after not touching it for a while

// pending stream.
boolean hasPendingStreams = delayedTransport.hasPendingStreams();
synchronized (lock) {
reconnectTask = null;
if (delayedTransport.hasPendingStreams()) {
if (hasPendingStreams) {
// Transition directly to CONNECTING
runnable = startNewTransport(delayedTransport);
} else {
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