C2a: Validate live ClrMD and layout-sensitive drilldowns across .NET 8/9/10

[pedrosakuma]

Parent and outcome

Parent: #919, Phase 3 / C2a. Independently deliverable compatibility coverage.

Add representative live ClrMD and runtime-layout-sensitive evidence across .NET 8/9/10, not an exhaustive collector/runtime/OS matrix.

Current evidence

Core CrossVersionTargetTests.cs covers .NET 8/9 counters, GC events and dump-based ClrMD, not live attach. samples/MultiVersionSample/Program.cs, DotnetDiagnostics.TestSupport/MultiVersionSampleProcess.cs and pinned-target LiveCoreClrProcessTests.cs provide reusable fixture/readiness patterns. ClrMdAsyncStateMachineWalker.cs represents layout-sensitive analysis.

docs/runtime-version-compat-matrix.md explicitly identifies live ClrMD, async and closed-generic gaps; reconcile its CI OS narrative with actual workflow/runtime installation.

Scope and acceptance

• Provide bounded fixture modes for a recognizable retained object, pending async operation and known closed generic instantiation.
• Exercise live heap/thread inspection plus representative async and closed-generic resolution against actual .NET 8/9/10 targets in one documented Linux x64 sidecar topology.
• Assert actual target runtime major and expected fixture identities/results, not merely successful attach or nonempty JSON.
• Reuse applicable Windows net10 controls; explicitly leave unexercised Windows cross-version cells unclaimed.
• Use matching UID, shared diagnostic socket visibility and pod-like PID visibility; grant SYS_PTRACE only to the sidecar where required. No shared-host Yama relaxation, privileged container or host-PID topology.
• Distinguish pass, unsupported/missing prerequisite, assertion failure and timeout/abort. Expected cases on the provisioned compatibility job must execute and pass; missing discovery/skips are not coverage.
• Start new topology automation advisory. Required-check promotion needs a separate stability/readiness decision.
• Update documentation by exact mechanism, target version, OS, architecture and topology, linking successful evidence and unresolved gaps.

Validation and closure evidence

Use the pinned SDK, targeted new selectors alongside CrossVersionTargetTests, fixed deadlines and no pass-seeking retries. Reuse scripts/test_evidence.py inventory patterns.

Preserve commit, SDK/target runtime/ClrMD/DAC versions, kernel/container identities, UID/capability/PID setup, expected/executed cases, actual drilldown results, TRX/logs/exits, timeout and cleanup outcomes. Publish a bounded result table and durable artifacts.

Requires supported target runtimes and safely scoped ptrace. Coordinate shared sample/workflow edits with C2b without inventing a hard dependency. No all-reader/all-drilldown matrix, NativeAOT parity, dependency upgrade or #879 fix.

[pedrosakuma]

Delivered: bounded representative local compatibility coverage

#996 delivers the bounded C2a outcome: fixture modes, actual live
heap/thread/async/concrete-generic checks, reproducible local advisory
automation, strict inventory and cleanup accounting, and updated compatibility
documentation.

The durable evidence report
records all twelve required facts passing on actual .NET 8.0.31, 9.0.20 and
10.0.7 targets in one local Linux x64 shared-PID sidecar topology. Runtime
identity is corroborated by diagnostic IPC and the mapped module; raw ClrMD
0.0 remains explicitly unknown. The tests assert the retained population,
pending async shape and actual production generic resolver output.

Matching UID/socket/PID visibility and inspector-only SYS_PTRACE were used,
without host security relaxation. All owned containers, cleanup helpers and
scratch/runtime copies were removed. Failed development attempts remain
documented as failures, not retroactively reclassified.

Existing Windows net10 and ordinary cross-version controls remain separate
evidence. The final normal CI/Kind checks passed on the reviewed head, including
actual Windows Core execution after correcting the new Python mock portability
defect. Ordinary CI explicitly skips the four unprovisioned sidecar facts and
does not supply hosted live-sidecar coverage.

This completes the issue's representative, advisory, single-topology scope.
It does not establish universal patch/OS/architecture coverage, repeated-run
reliability, Windows cross-version live coverage, all-reader parity, NativeAOT
or method-parameter support. No hosted compatibility workflow was provisioned;
the unprovisioned proposal was removed rather than shipped as operational.
Hosted integration or required-check promotion needs separately scoped work.

#921 remains at its genuine human-review gate; #982/#983 remain parked.