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

[Xin Liu]

> 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.

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/16728/files
  - new: https://git.openjdk.org/jdk/pull/16728/files/1c1dca2e..454304a0

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=16728&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=16728&range=00-01

  Stats: 2 lines in 1 file changed: 1 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/16728.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16728/head:pull/16728

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