This allows to call cancelPending() and waitAll() concurrently with no
additional synchronization. While calling these two functions
concurrently may not be useful often, supporting it costs little in
terms of performance and code complexity. Furthermore, a completely
thread-safe class is easier to document and use correctly.
Optimize job finalization by notifying _waitVar only if jobsLeft is
reduced to 0.
Optimize cancelPending() by not locking jobsLeftMutex when no job is
canceled (if the queue is empty when this function is called).
Add assertions that verify invariants.
The output of ConcurrentQueueTest::randomCalls() reflects the fact that
a caller of waitAll() can be blocked indefinitely when another thread
cancels all queued jobs while no job is being executed. The test output
snippet below omits repetitive information: the
"QINFO : ConcurrentQueueTest::randomCalls(queue{1}; 2 user thread(s))"
prefix of each line, current thread id and the common hh:mm:ss current
time part (leaving only the millisecond part). Note that thread #1
begins waiting at the 1st line of the snippet. "=> -8" means that total
equals -8 and thus ConcurrentQueue::jobsLeft equals 8. Thread #0 then
cancels all 8 queued jobs, then enqueues and cancels 3 jobs twice, then
momentarily waits 3 times. #1 is not waked until #0 enqueues 8 more jobs
and starts waiting for them too. Once these new 8 jobs are completed,
both #0 and #1 end waiting.
1. [ms] 311 | #1 begin waiting for 1 thread => -8
2. [ms] 311 | #0 enqueuing complete.
3. [ms] 311 | #0 canceled 8 jobs => -8
4. [ms] 311 | #0 enqueuing 3 jobs...
5. [ms] 312 | #0 enqueuing complete.
6. [ms] 312 | #0 canceled 3 jobs => -3
7. [ms] 312 | #0 enqueuing 3 jobs...
8. [ms] 312 | #0 enqueuing complete.
9. [ms] 312 | #0 canceled 3 jobs => -3
10. [ms] 312 | #0 begin waiting for 1 thread => 0
11. [ms] 312 | #0 end waiting for 1 thread => 0
12. [ms] 312 | #0 begin waiting for 1 thread => 0
13. [ms] 312 | #0 end waiting for 1 thread => 0
14. [ms] 312 | #0 begin waiting for 1 thread => 0
15. [ms] 312 | #0 end waiting for 1 thread => 0
16. [ms] 312 | #0 canceled 0 jobs => 0
17. [ms] 312 | #0 canceled 0 jobs => 0
18. [ms] 312 | #0 enqueuing 3 jobs...
19. [ms] 312 | #0 enqueuing complete.
20. [ms] 312 | #0 enqueuing 3 jobs...
21. [ms] 312 | #0 enqueuing complete.
22. [ms] 312 | #0 enqueuing 2 jobs...
23. [ms] 312 | #0 enqueuing complete.
24. [ms] 312 | #0 begin waiting for 1 thread => -8
25. [ms] 312 | [0.1] sleep 0.003 ms...
26. [ms] 312 | [0.1] +1 => -7
27. [ms] 312 | [0.2] sleep 0.003 ms...
28. [ms] 312 | [0.2] +1 => -6
29. [ms] 312 | [0.3] sleep 0.003 ms...
30. [ms] 312 | [0.3] +1 => -5
31. [ms] 312 | [0.1] sleep 0 ms...
32. [ms] 313 | [0.1] +1 => -4
33. [ms] 313 | [0.2] sleep 0.005 ms...
34. [ms] 313 | [0.2] +1 => -3
35. [ms] 313 | [0.3] sleep 0 ms...
36. [ms] 313 | [0.3] +1 => -2
37. [ms] 313 | [0.1] sleep 0.001 ms...
38. [ms] 313 | [0.1] +1 => -1
39. [ms] 313 | [0.2] sleep 0.001 ms...
40. [ms] 313 | [0.2] +1 => 0
41. [ms] 313 | #0 end waiting for 1 thread => 0
42. [ms] 313 | #1 end waiting for 1 thread => 0
The new test is a randomized stress test. It consistently passes right
now. The test's detailed output can be analyzed to reveal anomalies and
bugs. The test can catch various bugs that future changes to
ConcurrentQueue's code may introduce.
Replace _waitVar.notify_one() with _waitVar.notify_all(). This was the
only hurdle to documenting ConcurrentQueue::waitAll() as thread-safe.
ConcurrentQueueTest::waitAllFromMultipleThreads() passes instead of
hanging now.
ConcurrentQueueTest::cancelPending1UserThread() often fails when
ConcurrentQueueTest is launched from Qt Creator immediately after
switching between Debug and Release YACReader build configurations.
The CPU busyness must be affecting the thread scheduling timing, which
breaks the test's timing assumptions in this case.
Use the return value of ConcurrentQueue::cancelPending() instead of
relying on the timing of thread scheduling to determine the number of
canceled jobs.
The return value can be used to make
ConcurrentQueueTest::cancelPending1UserThread() non-flaky. It may also
be useful to non-testing code.
Improve the performance of cancelPending() by locking the two mutexes
separately and minimizing the locking time.
Worker threads may well be executing jobs while this function is being
called. If ConcurrentQueue::waitAll() is called soon enough after
cancelPending(), the worker threads may still be running, but waitAll()
would return immediately as jobsLeft would be nonpositive.
Subtracting _queue.size() from jobsLeft sets this variable to the number
of worker threads that are executing jobs at the moment.
ConcurrentQueueTest::cancelPending1UserThread() passes most of the time
now. But it still fails occasionally because it depends on the timing of
thread scheduling, which is unreliable.
Place common Qt Test qmake code into tests/qt_test.pri.
Build tests as part of top-level YACReader project unless no_tests
CONFIG option is set. This way the tests are built by default during
development. Packagers can skip building tests by running
`qmake "CONFIG+=no_tests"`.
Both ConcurrentQueueTest::singleUserThread() and
ConcurrentQueueTest::multipleUserThreads() pass. Evidently
ConcurrentQueue::enqueue() can be safely called from multiple threads on
the same ConcurrentQueue object with no additional synchronization. Once
each thread enqueues all its jobs, one thread can safely call waitAll().
