drop bad queue time events

[richardstartin]

What does this PR do?:
Queue time events which are committed during a profiler dump may end up with null scheduler and task attributes, if they are committed just as the class map is being cleared, because the lock can't be acquired when looking up the class name. These events aren't very useful so we'll drop them for now.

Motivation:

Additional Notes:

How to test the change?:

For Datadog employees:

• If this PR touches code that signs or publishes builds or packages, or handles
  credentials of any kind, I've requested a review from @DataDog/security-design-and-guidance.
• This PR doesn't touch any of that.

Unsure? Have a question? Request a review!

[github-actions]

🔧 Report generated by pr-comment-scanbuild

Scan-Build Report

User:	runner@fv-az770-114
Working Directory:	/home/runner/work/java-profiler/java-profiler/ddprof-lib/src/test/make
Command Line:	make -j4 clean all
Clang Version:	Ubuntu clang version 14.0.0-1ubuntu1.1
Date:	Mon Apr 29 17:47:10 2024

Bug Summary

Bug Type	Quantity	Display?
All Bugs	6
Logic error
Assigned value is garbage or undefined	1
Dereference of null pointer	2
Result of operation is garbage or undefined	1
Unused code
Dead initialization	1
Dead nested assignment	1

Reports

Bug Group	Bug Type ▾	File	Function/Method	Line	Path Length
Logic error	Assigned value is garbage or undefined	dwarf.cpp	parseInstructions	235	20
Unused code	Dead initialization	livenessTracker.cpp	cleanup_table	45	1
Unused code	Dead nested assignment	vmStructs.cpp	checkNativeBinding	818	1
Logic error	Dereference of null pointer	flightRecorder.cpp	flush	1410	8
Logic error	Dereference of null pointer	safeAccess.h	load	35	18
Logic error	Result of operation is garbage or undefined	dwarf.h	getSLeb	142	25

[github-actions]

🔧 Report generated by pr-comment-cppcheck

CppCheck Report

Warnings (5)

• Obsolete function 'alloca' called.
• Member variable 'ScopeDesc::_stream' is not initialized in the constructor.
• Member variable 'ScopeDesc::_method_offset' is not initialized in the constructor.
• Member variable 'ScopeDesc::_bci' is not initialized in the constructor.
• Member variable 'ElfParser::_vaddr_diff' is not initialized in the constructor.

Style Violations (162)

• Local variable 'jni' shadows outer function
• Local variable 'size' shadows outer function
• Local variable 'prev' shadows outer function
• The class 'NativeFunc' does not declare a constructor although it has private member variables which likely require initialization.
• Local variable 'count' shadows outer function
• Local variable 'i' shadows outer variable
• Consider using std::transform algorithm instead of a raw loop.
• struct member 'Tag::value' is never used.
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• struct member 'QueueTimeEvent::_start' is never used.
• struct member 'QueueTimeEvent::_end' is never used.
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• struct member 'QueueTimeEvent::_scheduler' is never used.
• struct member 'QueueTimeEvent::_origin' is never used.
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• struct member 'jvmtiStackInfoExtended::frame_buffer' is never used.
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• Variable 'error' is assigned a value that is never used.
• Unused private function: 'StackFrame::withinCurrentStack'
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• Variable 'ctx' can be declared with const
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[jbachorik]

Do we want to track the number of dropped events as a counter or we mostly don't care?

[richardstartin]

Dropping an event isn't too significant except at the trace level - it doesn't change sampling statistics, the events are latency thresholded and tend to occur in batches, so if we drop some during clearing the class map, we'll usually get some just before or after. We can revisit this if we want to change the way buffering works during dumping the recording.