[QPID-3199] Severe but difficult to diagnose lock error in qpid::sys::StateMonitor

• Type: Bug
• Status: Closed
• Priority: Major
• Resolution: Fixed
• Affects Version/s: 0.8, 0.9
• Fix Version/s: 0.10, 0.11
• Component/s: C++ Broker, C++ Client
• Labels: clang
• Assignee: Andrew Stitcher
• Reporter: Andrew Stitcher
• Created: 11/Apr/11 20:58
• Updated: 29/Jul/13 18:52
• Resolved: 11/Apr/11 21:47

Description

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. As the 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.