Fix continuation metadata lookup and frame alignment

[kaahos]

What does this PR do?

Fix two errors in HotSpot continuation walking:

• Allow metadata lookup to reach later field groups for the same VM type. The previous type-level continue skipped JavaThread::_cont_entry and StubRoutines::_cont_returnBarrier.
• Round the exported ContinuationEntry size to two machine words before computing its entry frame pointer.
• Add eight native regression cases using the production metadata dispatcher and frame-pointer calculation, including missing-metadata and already-aligned controls.

Motivation

On JVM builds exporting continuation metadata, cstack=vmx could discover the entry type size but fail to resolve the current thread's entry, truncating samples at break_cont_entry_null. Fixing that lookup exposes the second error: HotSpot's exported sizeof(ContinuationEntry) can be smaller than its stack allocation. The tested JDK 27 exports 56 bytes and requires 64, so adding the raw size places the frame pointer eight bytes too low.

Additional Notes

The alignment matches HotSpot's shared ContinuationEntry::size() and the entry-frame calculation on both x86 and aarch64. It adds no allocations. Missing metadata remains optional, and already-aligned sizes remain unchanged.

How to test the change?

./.claude/commands/build-and-summarize :ddprof-lib:gtestDebug_continuationMetadata_ut

JAVA_TEST_HOME=/path/to/hotspot-jdk-27 \
  ./.claude/commands/build-and-summarize :ddprof-test:testDebug \
  -Ptests=VirtualThreadWallClockTest.samplesCarrierFramesFromCpuBoundVT -Pskip-gtest

For Datadog employees:

• This PR doesn't touch code that signs or publishes builds or packages, or handles credentials.
• JIRA: N/A.

[dd-octo-sts]

CI Test Results

Run: #38913801280 | Commit: 47cd98a | Duration: 18m 18s (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

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Updated: 2026-10-16 14:33:53 UTC