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Capture a timer's locker for each instance of a CancellableTimer so that
reassigning a tcpip.CancellableTimer does not cause a data race.
Reassigning a tcpip.CancellableTimer updates its underlying locker. When
a timer fires, it does a read of the timer's locker variable to lock it.
This read of the locker was not synchronized so a race existed where one
goroutine may reassign the timer (updating the locker) and another
handles the timer firing (attempts to lock the timer's locker).
Test: tcpip_test.TestCancellableTimerReassignment
PiperOrigin-RevId: 307499822
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CancellableTimer tests were in a timer_test package but lived within the
tcpip directory. This caused issues with go tools.
PiperOrigin-RevId: 289166345
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Add a new CancellableTimer type to encapsulate the work of safely stopping
timers when it fires at the same time some "related work" is being handled. The
term "related work" is some work that needs to be done while having obtained
some common lock (L).
Example: Say we have an invalidation timer that may be extended or cancelled by
some event. Creating a normal timer and simply cancelling may not be sufficient
as the timer may have already fired when the event handler attemps to cancel it.
Even if the timer and event handler obtains L before doing work, once the event
handler releases L, the timer will eventually obtain L and do some unwanted
work.
To prevent the timer from doing unwanted work, it checks if it should early
return instead of doing the normal work after obtaining L. When stopping the
timer callers must have L locked so the timer can be safely informed that it
should early return.
Test: Tests that CancellableTimer fires and resets properly. Test to make sure
the timer fn is not called after being stopped within the lock L.
PiperOrigin-RevId: 288806984
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