feat(aspire): opt-in live forwarding of resource logs into test output (#6341)

[thomhurst]

Closes #6341.

Problem

AspireFixture<TAppHost> captures OpenTelemetry but never surfaces a resource's raw stdout/stderr. The blind spot is worst at startup: a resource that throws during boot exits before its OTel exporter flushes, and host-builder/DI/config failures go to stderr, not OTel. So "a resource won't come up" — the single most common Aspire-test failure — shows only a generic startup timeout, never the cause.

Main already has a post-mortem error-log dump on failure (DumpResourceLogsOnFailure, #6323), but it fetches after the fact and races to "(no logs available)" for already-exited resources. This adds a live, opt-in forward.

What changed

• New AspireFixtureOptions bag, exposed via protected virtual AspireFixtureOptions Options. Existing virtual knobs untouched.

  protected override AspireFixtureOptions Options => new() { ForwardResourceLogs = true };

• When on, subscribes to each selected resource's ResourceLoggerService.WatchAsync and forwards every line — prefixed [name] line — into the owning test's captured output.
• ResourceLogNames selects a subset; null/empty = every resource ShouldWaitForResource picks.

Key correctness points

• Subscribe before StartAsync, not after. A boot crash makes StartAsync throw/hang (that is the failure), so the issue's post-start reference impl would never subscribe. WatchAsync replays the buffered backlog, so the earliest lines are still delivered.
• The pump runs on a background thread where TestContext.Current (an AsyncLocal) does not flow. Capture the owner context once at subscribe time; fall back to stderr (LogProgress) when init runs outside a test (session-shared).
• Cancel pumps first in teardown so nothing writes to an already-finished test's output.
• Refactored WatchResourceLogs to share the new PumpResourceLogsAsync.

Acceptance criteria

• ✅ Opt-in, default off — no behaviour change for existing users.
• ✅ Forwarded logs appear in the fixture-owner test's captured output.
• ✅ Resources that exit during startup are still captured (live sub + backlog replay).
• ✅ Resource-name prefix + include filter.

Known limitation

Logs attach to the fixture-owner test (the test whose execution triggered init) for the subscription's life — matching the issue's "test that owns the fixture lifecycle" wording and the AsyncLocal constraint. Per-request correlation remains the OTLP receiver's job.

Tests

• New pure (no-Docker) ResourceLogSelectionTests for SelectResourceLogNames (explicit-subset, case-sensitivity, ShouldWaitForResource fallback, empty-names). 4/4 pass.
• TUnit.Aspire.Core builds across net8.0/net9.0/net10.0.
• Full live forwarding needs a real AppHost + Docker (not run in this environment).

[codacy-production]

Up to standards ✅

🟢 Issues 0 issues

Results:
0 new issues

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🟢 Metrics 11 complexity

Metric	Results
Complexity	11

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[claude]

Code review

No issues found. Checked for bugs and CLAUDE.md compliance.

Reviewed AspireFixture.cs's new opt-in resource-log forwarding (subscribe-before-StartAsync, background pump tasks per resource, teardown ordering that cancels pumps before stopping the app), the new AspireFixtureOptions bag, and ResourceLogSelectionTests.cs. Specifically verified:

• Concurrent writes from multiple resource pumps to the shared TestContext.Output are safe (ConcurrentStringWriter is lock-guarded).
• Pump tasks are joined via Task.WhenAll/awaited in teardown, so no orphaned tasks or unobserved faults.
• CTS linked to the run-abort token, cancelled before app stop, disposed and nulled correctly.
• The new test fakes (FakeResource, FakeCompute, FakeChildCompute) correctly implement IResource/IComputeResource/IResourceWithParent and compile against Aspire.Hosting.
• No dual-mode (source-gen/reflection), snapshot, VSTest, or AOT/reflection-annotation concerns — scoped entirely to TUnit.Aspire.Core, outside those rules' stated scope.

🤖 Generated with Claude Code

[claude]

Code review (updated after latest push)

Re-reviewed the latest commit (f72a7b8d, "tidy resource-log forwarding after review") against the prior review of 92350c68. No issues found.

The follow-up commit is a clean simplification, verified against the surrounding code:

• Dropped the _options field in favor of a local var options = Options; — it was only ever read within the same InitializeAsync call, so caching it in a field was unnecessary.
• Dropped the extra Task.Run wrapper around each pump — PumpResourceLogsAsync is async and yields at its first await (the WatchAsync enumeration), so invoking it directly still runs the pumps concurrently without an extra thread-pool hop.
• Inlined WriteForwardedLine into a per-resource Action<string> bound once at subscribe time — same behavior, one fewer indirection per line.
• Removed the _logForwardCts = null; _logForwardTask = null; resets after teardown. Confirmed safe: StopAndDisposeCoreAsync is only ever invoked once, via StopAndDisposeAsync's lock (_teardownGate) { return _teardownTask ??= StopAndDisposeCoreAsync(); } single-shot gate, so there's no risk of these fields being read again in a stale state.
• await _logForwardTask! (non-null-forgiving) is sound: _logForwardCts and _logForwardTask are always assigned together with no branch between them in StartForwardingResourceLogs, and the guard if (_logForwardCts is not null) covers the only path where they're set.
• Test helper consolidation (FakeResources.cs shared by ResourceLogSelectionTests and WaitForHealthyReproductionTests) removes prior duplication with no behavior change — confirmed via InternalsVisibleTo wiring in TUnit.Aspire.Core.csproj that the shared internal fakes are still accessible from both test files.

No dual-mode, snapshot, VSTest, or AOT/reflection-annotation concerns — this stays scoped to TUnit.Aspire.Core/TUnit.Aspire.Tests, outside those rules.

🤖 Generated with Claude Code