Severe but difficult to diagnose lock error in qpid::sys::StateMonitor
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Key: QPID-3199
URL: https://issues.apache.org/jira/browse/QPID-3199
Project: Qpid
Issue Type: Bug
Components: C++ Broker, C++ Client
Affects Versions: 0.8, 0.9
Reporter: Andrew Stitcher
Assignee: Andrew Stitcher
Fix For: 0.10
The qpid::sys::StateMonitor class has 4 member functions liker this:
void waitFor(Enum s) { ScopedWait(*this); while (s != state) wait(); }
However the ScopeWait(*this); is in error, because it creates a temporary
ScopedWait and then immediately destroys it. Instead of locking this and then
unlocking it at the end of the function execution.
What was meant was:
void waitFor(Enum s) { ScopedWait w(*this); while (s != state) wait(); }
which creates a local variable w which is destroyed at the end of the function
execution unlocking this.
It is possible that the compiler/options we use do not actually exhibit this
bug or at least only with some options as this correct behaviour, may not
actually be implemented by all compilers.
If this bug exhibits it might show as StateMonitor::waitFor() never detecting
the desired state for instance. It is remarkable that this code has been in the
codebase for 3.5 years and we've not noticed any bug caused by it.
This bug was detected by the work on QPID-3004: The clang++ compiler detected
that the value being returned by the constructor was not being used at all.
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