[KowalskiThomas]

Backport of PR #18599 (branch kowalski/fix-fancy-rust-fix-for-hamt-crash) to branch 4.9.

Description

This is an attempt to fix a customer-reported crash (Jira).

• The customer is using a version that already includes this ContextVar-related crash: fix(internal): crash in ContextVar #17407
• It's likely gh-148891: prevent crash in HAMT python/cpython#148928 would fix the crash as well but (1) it would only land in recent versions of CPython -- the customer is using 3.11 -- and (2) I don't have a review/approval yet.

This fix addresses the issue in a sort of makeshift way like the CPython upstream fix would. Since we can't bump reference counts directly from CPython, the PR adds a Rust helper which takes care of it. Not great, not terrible.

Note the bug only happens in versions prior to 3.12 (which made significant changes to the GC and doesn't trigger that HAMT crash anymore), so the fix is gated to them.

Notes on conflict resolution

Applied manually to the release branch. contextvar::register_contextvar is wired into each branch's src/native/lib.rs layout (4.8 uses utils instead of ddtrace_utils; 4.11 registers after event_hub).

[cit-pr-commenter-54b7da]

Codeowners resolved as

ddtrace/_trace/provider.py                                              @DataDog/apm-sdk-capabilities-python
ddtrace/internal/native/_native.pyi                                     @DataDog/apm-core-python
releasenotes/notes/internal-add-contextvar-helper-d73542f1c6cfceac.yaml  @DataDog/apm-python
src/native/contextvar.rs                                                @DataDog/apm-core-python
src/native/lib.rs                                                       @DataDog/apm-core-python

[datadog-datadog-prod-us1]

🎉 All green!

🧪 All tests passed
❄️ No new flaky tests detected

_{This comment will be updated automatically if new data arrives.
🔗 Commit SHA: 993928d | Docs | Datadog PR Page | Give us feedback!}

[pr-commenter]

Benchmarks

Benchmark execution time: 2026-06-23 16:32:08

Comparing candidate commit 993928d in PR branch kowalski/backport-18599-to-4.9 with baseline commit 89ff0ed in branch 4.9.

Found 0 performance improvements and 5 performance regressions! Performance is the same for 608 metrics, 9 unstable metrics.

scenario:iastaspects-lstrip_aspect

• 🟥 execution_time [+43.982µs; +51.250µs] or [+15.659%; +18.246%]

scenario:iastaspects-translate_aspect

• 🟥 execution_time [+47.530µs; +58.650µs] or [+9.424%; +11.629%]

scenario:iastaspectsospath-ospathbasename_aspect

• 🟥 execution_time [+97.875µs; +105.429µs] or [+22.475%; +24.210%]

scenario:span-start

• 🟥 execution_time [+1.443ms; +1.615ms] or [+9.214%; +10.311%]

scenario:telemetryaddmetric-1-count-metric-1-times

• 🟥 execution_time [+164.146ns; +194.684ns] or [+7.699%; +9.131%]