ComplexCancellable: Fix race condition #597
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I discovered a race condition while investigating a sporadic test failure in the client: ``` [ERROR] HttpMinimalIntegrationTests$H2Tls>AbstractHttpAsyncFundamentalsTest.testSequentialGetRequests:86 » Execution org.apache.hc.core5.http2.H2StreamResetException: Cancelled ``` I discovered that two threads are somehow racing when calling `ComplexCancellable.setDependency()`: ``` setDependency dep=org.apache.hc.client5.http.impl.async.MinimalH2AsyncClient$$Lambda$655/251536271@633478cb ct=ForkJoinPool-1-worker-2 setDependency dep=[id=1, reserved=false, removeClosed=false, localClosed=false, localReset=false] ct=httpclient-dispatch-1 setDependency dep=org.apache.hc.client5.http.impl.async.MinimalH2AsyncClient$$Lambda$655/251536271@2e66671a ct=ForkJoinPool-1-worker-2 setDependency dep=[id=3, reserved=false, removeClosed=false, localClosed=false, localReset=false] ct=httpclient-dispatch-1 setDependency dep=org.apache.hc.client5.http.impl.async.MinimalH2AsyncClient$$Lambda$655/251536271@32f4e96 ct=ForkJoinPool-1-worker-2 setDependency dep=[id=5, reserved=false, removeClosed=false, localClosed=false, localReset=false] ct=httpclient-dispatch-1 ``` The implementation of `setDependency` was performing a single CAS and interpreting failure to mean that the mark bit had been set -- in other words, that another thread had called `cancel()`, rather than `setDependency()`. This change corrects the implementation of `setDependency` by having it run the CAS in a loop. On each update failure, it checks if the mark bit has been set (nothing ever unsets it). If so, it calls `dependency.cancel()` and returns; else, it just tries again.
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I discovered a race condition while investigating a sporadic test failure in the client:
It turned out that two threads are somehow racing when calling
ComplexCancellable.setDependency():The implementation of
setDependencywas performing a single CAS and interpreting failure to mean that the mark bit had been set -- in other words, that another thread had calledcancel(), rather thansetDependency().This change corrects the implementation of
setDependencyby having it run the CAS in a loop. On each update failure, it checks if the mark bit has been set (nothing ever unsets it). If so, it callsdependency.cancel()and returns; else, it just tries again.