RFR: 8309067: gtest/AsyncLogGtest.java fails again in stderrOutput_vm [v2]

[David Holmes]

On Mon, 20 Nov 2023 17:29:49 GMT, Xin Liu <xliu at openjdk.org> wrote:

>> This patch fixes the flaky test AsyncLogTest. The race condition only occurs in async 
>> logging mode in combination of BufferUpdater. BufferUpdater is introduced for testing 
>> only. It installs smaller buffers so test code can reliably observe messages dropped.
>> AsyncLog drops messages when the current buffer is overflown. 
>> 
>> In stdout/stderrOutput testcases, we used to mix test_asynclog_ls() and 
>> test_asynclog_drop_messages(). The later function uses BufferUpdater. As a result, log
>> messages distribute in both old buffer and new buffer. 
>> 
>> A race condition unfolds in the sequences. Thread names are in brackets.
>> 1. [AsyncLogWriter] just calls write().
>> 2. [Test] calls the constructor of BufferUpdater. It updates AsyncLogWriter->_buffer_staging.
>> 3. [AsyncLogWriter] sees the latest _buffer_staging. It emits nothing. 
>> 4. [Test] call the destructor of BufferUpdater. _buffer_staging resumes to 
>> the original buffer.
>> 5. [AsyncLogWriter] is waken up from _lock.wait. It proceeds to swaps _buffer and _buffer_staging.
>> at this point, we lost the log messages in the original buffer. AsyncLogWriter was about to dump them
>> at 1.
>> 
>> Only stdout/stderrOutput testcases interleave the original buffer and small buffer. That's why
>> we only encounter flaky failures on them. It's up to the OS scheduling. 
>> 
>> To eliminate the race condition, we must ensure that all messages write after BufferUpdater.
>> test_asynclog_drop_messages() fuses test_asynclog_ls().
>
> Xin Liu has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Update per reviewer's feedback.

Seems reasonable. Thanks for getting to the bottom of this. I too am glad to see it is a test issue.

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Marked as reviewed by dholmes (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/16728#pullrequestreview-1743399753