spike: Activity-based tracing — SentrySpanProcessor as a raw ActivityListener in core (experiment, do not merge)

[jamescrosswell]

⚠️ Experiment — not intended to be merged

This is part (a) of a two-part spike exploring whether System.Diagnostics.Activity can become the single source of truth for tracing in the .NET SDK (context: #3238, #3288, #5350, #4859, and the internal discussion about the Sentry-vs-OTel instrumentation ecosystems bifurcating). It exists to inform that discussion with working code; it will be closed, not merged.

Question this spike answers

Can the Activity→Sentry conversion logic in Sentry.OpenTelemetry's SentrySpanProcessor live in the core Sentry package, driven by a raw System.Diagnostics.ActivityListener, with zero dependency on the OpenTelemetry SDK — on all TFMs — and pass the existing SentrySpanProcessorTests suite unchanged?

Answer: yes. The port was near-verbatim. Exactly two things needed to change, both predicted in advance:

1. No BaseProcessor<Activity> base class — ActivityListener.ActivityStarted/ActivityStopped are drop-in replacements for OnStart/OnEnd.
2. OTel Resource detection (ParentProvider.GetResource()) has no System.Diagnostics equivalent — replaced with an injectable resourceAttributeResolver (defaults to empty).

Everything else — the shadow span map, transaction-vs-child parent inference (including mixed Sentry/OTel parenting), DSC-from-baggage, DSN self-request filtering, scope restoration across async, pruning/leak guards, error generation from exception events, op/description/status mapping — is pure System.Diagnostics code and ported as-is.

Packaging model: all TFMs, no new core dependency

The sources live in src/Sentry.DiagnosticSource/Internal/Tracing/ and ship through the same split the EF/SqlClient DiagnosticSource integration already uses:

• Modern TFMs: compiled directly into the core Sentry package via the existing <Compile Include> in Sentry.csproj — ActivityListener is part of the shared framework there, so core takes no new dependency.
• Legacy TFMs (netstandard2.0/2.1, net462): the code ships in the Sentry.DiagnosticSource package, which carries the System.Diagnostics.DiagnosticSource package reference. Legacy apps opt in by adding that package — exactly as they do today for EF/SQL instrumentation.

📌 Version finding: ActivityListener needs DiagnosticSource 5.0 and ActivityStatusCode needs 6.0, but the netstandard2.0/net462 assets of the 6.0 package lag the modern API surface (no Activity.HasRemoteParent), so 8.0.1 is the effective minimum (bumped from 4.5.0).

A HAS_ACTIVITY_LISTENER constant (defined in both csproj files) gates compilation; only two small source concessions were needed for the legacy TFMs (hex-string id conversion branches, no KeyValuePair deconstruction).

What's here

File	What it is
src/Sentry.DiagnosticSource/Internal/Tracing/SentryActivityProcessor.cs	Port of SentrySpanProcessor (kept diffable against the original)
src/Sentry.DiagnosticSource/Internal/Tracing/SentryActivityListener.cs	Replaces the OTel TracerProvider registration glue with a raw ActivityListener
src/Sentry.DiagnosticSource/Internal/Tracing/ActivityIdExtensions.cs, ActivityAttributeExtensions.cs, ISentryActivityEnricher.cs	Ports of the small helpers the processor needs (distinct namespace to avoid InternalsVisibleTo collisions with the copies in the OTel packages)
test/Sentry.DiagnosticSource.Tests/Tracing/SentryActivityProcessorTests.cs	All 33 tests from SentrySpanProcessorTests ported 1:1 — bodies kept diffable — passing against the core port
test/Sentry.DiagnosticSource.Tests/Tracing/SentryActivityListenerTests.cs	4 new end-to-end tests: root activity → captured transaction, child activity → correctly-parented span, unsampled → dropped, unmatched source → StartActivity returns null (the zero-cost HasListeners property). No OTel SDK anywhere in the pipeline.

The tests live in Sentry.DiagnosticSource.Tests deliberately: modern TFMs exercise the compiled-into-core path, while net48 (Windows CI) exercises the standalone-package path — one suite covers both packagings.

Test evidence (local, macOS)

• Original suite: SentrySpanProcessorTests 33/33 ✅
• Ported suite against the core processor: 33/33 ✅ (+ 4/4 new listener e2e tests; Sentry.DiagnosticSource.Tests 119/119 on net10.0)
• Full Sentry.Tests (net10.0): 2402 passed, 0 failed ✅
• src/Sentry builds clean on all TFMs ✅; Sentry.DiagnosticSource builds clean on netstandard2.0/2.1/net462 with DiagnosticSource 8.0.1 ✅
• OTel packages build unchanged ✅
• net48 test execution happens in Windows CI (not runnable locally on macOS)

What this spike deliberately does NOT address

• Sampling at creation time — the listener's Sample callback returns AllDataAndRecorded for parity with today's POTEL behaviour (record everything, Sentry samples at conversion). Moving Sentry's sampling into the Sample callback is future work and needs answers for the TracesSampler customSamplingContext gap and multi-listener composition (the runtime takes the most permissive result).
• The Instrumenter.OpenTelemetry constructor guard is retained as-is for test parity; in a real implementation it inverts (Activity-based becomes the default, not the opt-in).
• Idle/deadline transactions (MAUI, feat: Automatic trace instrumentation for MAUI #5138) — that's part (b) of the spike.
• Migrating any integration to emit Activities, propagation, or public API shape.

#skip-changelog

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 70.68966% with 85 lines in your changes missing coverage. Please review.
✅ Project coverage is 73.98%. Comparing base (11068e0) to head (5d22058).
⚠️ Report is 42 commits behind head on main.

Files with missing lines	Patch %	Lines
...Source/Internal/Tracing/SentryActivityProcessor.cs	70.00%	52 Missing and 17 partials ⚠️
...ticSource/Internal/Tracing/ActivityIdExtensions.cs	12.50%	14 Missing ⚠️
...cSource/Internal/Tracing/SentryActivityListener.cs	90.47%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5382      +/-   ##
==========================================
- Coverage   74.15%   73.98%   -0.18%     
==========================================
  Files         508      513       +5     
  Lines       18353    18710     +357     
  Branches     3586     3673      +87     
==========================================
+ Hits        13610    13842     +232     
- Misses       3870     3970     +100     
- Partials      873      898      +25     

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