Unable to detect Unified-Parallel logs that don't start with "Using Parallel"

[jlittle-ptc]

Describe the bug
Discovered while fixing #457, the diarizer is not able to reliably identify Unified-Parallel logs that don't start with "Using Parallel" at the beginning of the file. This means that rollover logs using Unified-Parallel cannot be reliably identified and processed.

To Reproduce
Steps to reproduce the behavior:
Comment out the first line in UnifiedGenerationalEventsTest.testParallelNoDetails() (added in PR for #457) and run the test. (Lines were taken from file attached to #457)

The test does not generate the correct number of events as it was not able to select the Parallel collector from the information provided.

Expected behavior
Parser is able to successfully identify the garbage collector so that it can parse the events properly.

Additional context
Unfortunately, many of the logged events in Parallel logs are also used in other collectors which makes this difficult.

[brunoborges]

/relevance-check

[github-actions]

Relevance Assessment: Still Relevant

• Summary: This bug is still present in the current codebase. The diarizer's Unified-Parallel detection logic has not been improved since this issue was filed, and the limitation is explicitly documented in both code comments and the related PR.

• Evidence:

  • The discoverCollector() method in UnifiedDiarizer.java (lines 207-213) still only checks for "Using Parallel", "ParOldGen", or "PSYoungGen" — none of which appear in Parallel GC logs with minimal (-Xlog:gc) logging unless the "Using Parallel" startup line is present.
  • The test testParallelNoDetails() in UnifiedGenerationalEventsTest.java (lines 89-91) still contains the comment: "This test case will fail if the 'Using Parallel' line is not included! There is no unique way to identify these lines as being Unified-Parallel on their own. (Collides with Serial)"
  • PR Fix #457 - create missing pauseEvent and publish when appropriate. #461 (which fixed the related issue UnifiedGenerationalParser does not create ForwardReference for Parallel file #457) explicitly documented this as a remaining unresolved issue: "If the file being scanned does not contain the 'Using Parallel' text, there is no way for the engine to properly identify the file."
  • No commits since fadbf1f9 (Nov 2025, the UnifiedGenerationalParser does not create ForwardReference for Parallel file #457 fix) have touched the diarizer's collector detection logic. Recent commits are dependency bumps and CI changes.
  • No open PRs address this issue.

• Recommendation: ✅ Keep open — The bug is confirmed present in the current code, well-documented, and directly impacts users with Parallel GC rollover logs (which lack the "Using Parallel" startup line). The issue is assigned to jlittle-ptc who originally identified it.

Generated by Relevance Check Agent for issue #462