Integrated: 8309067: gtest/AsyncLogGtest.java fails again in stderrOutput_vm

[Xin Liu]

On Mon, 20 Nov 2023 08:27:49 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().

This pull request has now been integrated.

Changeset: d1af748a
Author:    Xin Liu <xliu at openjdk.org>
URL:       https://git.openjdk.org/jdk/commit/d1af748ab470e1068d03a8f8bc1a7445eff721fa
Stats:     51 lines in 1 file changed: 19 ins; 17 del; 15 mod

8309067: gtest/AsyncLogGtest.java fails again in stderrOutput_vm

Reviewed-by: jsjolen, dholmes

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PR: https://git.openjdk.org/jdk/pull/16728