Wire reference chains into profiler lifecycle and JNI API by jbachorik · Pull Request #798 · DataDog/java-profiler · GitHub

What does this PR do?:

Hooks the reference-chain engine into the profiler lifecycle. Profiler starts and stops ReferenceChainTracker and LivenessTracker together with the recording, writes resolved chains into JFR on dump() and emits abandoned-search events. vmEntry GC hooks feed the tracker, and there are new JFR event types for the chains.

ObjectSampler now always passes the recording's flags to LivenessTracker, so they can't go stale between recordings. LivenessTracker is started last; if it fails to start, the sampler is deactivated and SAMPLED_OBJECT_ALLOC is disabled again.

javaApi.cpp adds the reference-chain natives and test seams, declared in JavaProfiler.java. The seams are real only in debug builds - release builds export no-op stubs, so calling them never throws UnsatisfiedLinkError.

Motivation:

Makes the reference-chain tracker reachable from a live recording.

Additional Notes:

A few engine fixes ride along:

The heap callback no longer takes _resolved_chains_lock inside the FollowReferences pause, where it could stall the walk behind the JFR dump thread copying the cache. Evictions are queued into a preallocated buffer and applied in one batch on the BFS thread, outside the walk.
PriorityExpandSet::insert() returns false on a full table instead of probing forever.
LivenessTracker::cleanup_table() takes an account_epoch flag, so the overflow sweep in track() doesn't claim the GC epoch the background sweep needs. Entries remember the epoch they were admitted in and are only aged for GCs they actually survived.
New counters for urgency-boost admissions and back-offs (LIVENESS_URGENT_BOOST_ADMITS / LIVENESS_URGENT_BOOST_BACKED_OFF); secondsToOOM() no longer divides by zero.
The debug test seams prime ProfiledThread TLS on entry, like the other JNI entry points.

How to test the change?:

:ddprof-lib:gtestDebug / :ddprof-lib:gtestRelease - the referenceChains and liveness suites, incl. new tests for the PriorityExpandSet full table, ClassTagAllocator, the urgency-boost backstop and account_epoch.
:ddprof-lib:assembleReleaseJar builds the release library with the seam stubs.
Java integration tests via testDebug / testRelease.

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 security review (run the dd:platform-security-review
skill, or file a request via the PSEC review form).
bewaire also runs automatically on every PR.
This PR doesn't touch any of that.
JIRA: PROF-15341

🤖 Generated with Claude Code

Scan-Build Report

User:	runner@runnervmlun5p
Working Directory:	/home/runner/work/java-profiler/java-profiler/ddprof-lib/src/test/make
Command Line:	make -j4 all
Clang Version:	Ubuntu clang version 18.1.3 (1ubuntu1)
Date:	Thu Sep 17 20:07:57 2026

Bug Summary

C++ move semantics
Bug Type	Quantity	Display?
All Bugs	4
Use-after-move	1
Logic error
Dereference of null pointer	1
Result of operation is garbage or undefined	1
Unused code
Dead increment	1

Reports

Bug Group	Bug Type ▾	File	Function/Method	Line	Path Length
Unused code	Dead increment	referenceChains.cpp	collectStaticFieldAnchorsForRotation	3677	1
Logic error	Dereference of null pointer	faultInjection.cpp	crashNow	24	2
Logic error	Result of operation is garbage or undefined	livenessTracker.cpp	secondsToOOM	1383	24
C++ move semantics	Use-after-move	referenceChains.cpp	buildCanaryChainEvent	6083	68

CI Test Results

Run: #37616728506 | Commit: e9e835c | Duration: 17m 28s (longest job)

✅ All 32 test jobs passed

Status Overview

JDK	glibc-aarch64/debug	glibc-amd64/debug	musl-aarch64/debug	musl-amd64/debug
8	-	✅	-	-
8-ibm	-	✅	-	-
8-j9	✅	✅	-	-
8-librca	-	-	✅	✅
8-orcl	-	✅	-	-
11	-	✅	-	-
11-j9	✅	✅	-	-
11-librca	-	-	✅	✅
17	✅	✅	-	-
17-graal	✅	✅	-	-
17-j9	✅	✅	-	-
17-librca	-	-	✅	✅
21	✅	✅	-	-
21-graal	✅	✅	-	-
21-librca	-	-	✅	✅
25	✅	✅	-	-
25-graal	✅	✅	-	-
25-librca	-	-	✅	✅

Legend: ✅ passed | ❌ failed | ⚪ skipped | 🚫 cancelled

Summary: Total: 32 | Passed: 32 | Failed: 0

Updated: 2026-10-07 12:10:47 UTC

✅ All 40 integration tests passed

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📝 Code Review	✅ Completed 2026-10-06T14:34:28.345419Z	d21eb08	Draft marked ready

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Stopping a recording can omit all reference-chain events since the last dump. The lifecycle also keeps strong thread references and stale chain data across recording sessions.

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🤖 Datadog Autotest · Commit 93f2dbf · What is Autotest? · @DataDog review to ask questions · Any feedback? Reach out in #autotest

Introduces the ReferenceChainEvent/ReferenceChainAbandonedEvent payloads (event.h), their JFR metadata (jfrMetadata.*), and the FlightRecorder emission paths that serialize chain events into JFR recording buffers, including the constant-pool handling for per-hop edge labels. Emission is pull-style: profiler.cpp snapshots events and hands them to FlightRecorder; this layer does not depend on the tracker itself.

- ReferenceChainEvent carries one vector of ReferenceChainHop (klass id + retention-edge label) instead of two parallel vectors - Compress the sub-option floor/ceiling rationale and the provisional default constant comments to one concise statement each - Drop design-doc and Jira references from code comments; revert the unrelated LineNumberTable comment rewrite

- cleanup_table: account_epoch flag; track() overflow sweep skips epoch claim - admitForTracking: LIVENESS_URGENT_BOOST_ADMITS/BACKED_OFF counters - secondsToOOM: zero-denominator guard; ringThirdsStats -> ringWindowStats - JNI poll seam calls finishLoopIterationSerialized - gtests: DipThenRecover, OddLengthRisingWindow

…ckage-private

…rences pause - heapReferenceCallback ran invalidateResolvedChain() inside the FollowReferences STW pause, taking _resolved_chains_lock, which drainPendingChainEvents() holds across its cache copy on the JFR dump thread. Evictions are now queued and applied on the BFS thread outside any walk. - PriorityExpandSet::insert() had no occupancy bound and could spin forever on a full table; it now returns false.

… seam - referenceChainsPesTests.inc: PriorityExpandSet round-trip, rebuildFrom, full-table self-termination; ClassTagAllocator negative-tag minting - referenceChainsTestAccessors.h: cacheChain() passes source_tag and source_tag_val through unchanged - referenceChainsOomTests.inc: setCanaryBackoffForTest() takes ns

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The shared accessor's gtest include breaks the reference-chain fuzz build. Deferred overflow accounting also allows newly admitted objects to receive ages for GCs that occurred before their allocation.

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Allocation callbacks can reach an uninitialized liveness table during recording startup and crash the JVM. Queue overflow and reap-only cleanup also introduce stale reference-chain and leak-population results.

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Fixed in 0668fa1 (addressing general review comment).

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Urgent liveness admission races with table cleanup, and debug JNI queue inspection races with the BFS worker, potentially crashing the JVM.

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Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

jbachorik added AI AI: Author Signed-off labels Sep 17, 2026

jbachorik mentioned this pull request Sep 17, 2026

C++ unit tests for reference-chain tracking #799

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jbachorik added this pull request to stack #803 September 17, 2026 20:02

jbachorik marked this pull request as ready for review September 17, 2026 20:03

jbachorik requested a review from a team as a code owner September 17, 2026 20:03

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jbachorik force-pushed the jb/rc-4-profiler-wiring branch from 217a7ab to b5588b7 Compare September 17, 2026 20:19

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jbachorik mentioned this pull request Sep 17, 2026

JFR event types and emission for reference chains #796

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jbachorik force-pushed the jb/rc-4-profiler-wiring branch from 5f7ef97 to 2fbc79b Compare September 18, 2026 07:06

Base automatically changed from jb/rc-3-refchain-tracker to main October 6, 2026 09:03

jbachorik and others added 12 commits October 6, 2026 11:54

Wire reference chains into the profiler lifecycle and JNI API

a2b0120

Drop remaining Jira and cross-layer test references

e9e0d69

misc: objectSampler start order, dump-failure test log, Java seams pa…

216abc9

…ckage-private

objectSampler: roll back on LivenessTracker start failure

4169c73

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

javaApi: release stubs for debug test seams, null-check jvmti

97ecc3c

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referenceChains: batch deferred invalidations, cache probe start

af7af58

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tests: cover urgency backstop and account_epoch, tidy PES tests

d21eb08

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jbachorik force-pushed the jb/rc-4-profiler-wiring branch from 47c0511 to d21eb08 Compare October 6, 2026 12:58

Trim comments and test-seam Javadocs

e8c79ca

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

jbachorik added the test:reliability label Oct 6, 2026

jbachorik marked this pull request as ready for review October 6, 2026 14:28

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sphinx: address review feedback on PR #798

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jbachorik and others added 2 commits October 7, 2026 13:44

sphinx: address review feedback on PR #798

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Wire reference chains into profiler lifecycle and JNI API by jbachorik · Pull Request #798 · DataDog/java-profiler · GitHub

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Wire reference chains into profiler lifecycle and JNI API - #798

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Wire reference chains into profiler lifecycle and JNI API#798
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