feat: add caller owned process/thread context storage

[morrisonlevi]

What does this PR do?

This PR supports the dd-trace-php PR needed to add Process and Thread contexts. This includes adjusting visibility and storage and a few similar things.

Motivation

PHP needs to support allocating its own storage. It also supports these contexts on macOS and Windows even though it's non-standard (without discovery, just the in-process mechanism). So we needed to change some things.

Additional Notes

I have reviewed the code just a tiny bit. I've been focused on the observable behaviors so far.

How to test the change?

Mostly, this should be tested through PHP: DataDog/dd-trace-php#4062.

[pr-commenter]

Benchmarks

Comparison

Candidate

Candidate benchmark details

Baseline

Baseline benchmark details

[github-actions]

📚 Documentation Check Results

⚠️ 564 documentation warning(s) found

📦 libdd-library-config - 272 warning(s)

📦 libdd-otel-thread-ctx-ffi - 287 warning(s)

📦 libdd-otel-thread-ctx - 5 warning(s)

---

Updated: 2026-07-24 15:29:18 UTC | Commit: 0f2c335 | missing-docs job results

[github-actions]

🔒 Cargo Deny Results

⚠️ 2 issue(s) found, showing only errors (advisories, bans, sources)

📦 libdd-library-config - 1 error(s)

Show output

error[unsound]: Rand is unsound with a custom logger using `rand::rng()`
   ┌─ /home/runner/work/libdatadog/libdatadog/Cargo.lock:46:1
   │
46 │ rand 0.8.5 registry+https://github.com/rust-lang/crates.io-index
   │ ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ unsound advisory detected
   │
   ├ ID: RUSTSEC-2026-0097
   ├ Advisory: https://rustsec.org/advisories/RUSTSEC-2026-0097
   ├ It has been reported (by [@lopopolo](https://github.com/lopopolo)) that the `rand` library is [unsound](https://rust-lang.github.io/unsafe-code-guidelines/glossary.html#soundness-of-code--of-a-library) (i.e. that safe code using the public API can cause Undefined Behaviour) when all the following conditions are met:
     
     - The `log` and `thread_rng` features are enabled
     - A [custom logger](https://docs.rs/log/latest/log/#implementing-a-logger) is defined
     - The custom logger accesses `rand::rng()` (previously `rand::thread_rng()`) and calls any `TryRng` (previously `RngCore`) methods on `ThreadRng`
     - The `ThreadRng` (attempts to) reseed while called from the custom logger (this happens every 64 kB of generated data)
     - Trace-level logging is enabled or warn-level logging is enabled and the random source (the `getrandom` crate) is unable to provide a new seed
     
     `TryRng` (previously `RngCore`) methods for `ThreadRng` use `unsafe` code to cast `*mut BlockRng<ReseedingCore>` to `&mut BlockRng<ReseedingCore>`. When all the above conditions are met this results in an aliased mutable reference, violating the Stacked Borrows rules. Miri is able to detect this violation in sample code. Since construction of [aliased mutable references is Undefined Behaviour](https://doc.rust-lang.org/stable/nomicon/references.html), the behaviour of optimized builds is hard to predict.
   ├ Announcement: https://github.com/rust-random/rand/pull/1763
   ├ Solution: Upgrade to >=0.10.1 OR <0.10.0, >=0.9.3 OR <0.9.0, >=0.8.6 (try `cargo update -p rand`)
   ├ rand v0.8.5
     └── libdd-library-config v3.0.0

advisories FAILED, bans ok, sources ok

📦 libdd-otel-thread-ctx-ffi - 1 error(s)

Show output

error[unsound]: Rand is unsound with a custom logger using `rand::rng()`
    ┌─ /home/runner/work/libdatadog/libdatadog/Cargo.lock:109:1
    │
109 │ rand 0.8.5 registry+https://github.com/rust-lang/crates.io-index
    │ ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ unsound advisory detected
    │
    ├ ID: RUSTSEC-2026-0097
    ├ Advisory: https://rustsec.org/advisories/RUSTSEC-2026-0097
    ├ It has been reported (by [@lopopolo](https://github.com/lopopolo)) that the `rand` library is [unsound](https://rust-lang.github.io/unsafe-code-guidelines/glossary.html#soundness-of-code--of-a-library) (i.e. that safe code using the public API can cause Undefined Behaviour) when all the following conditions are met:
      
      - The `log` and `thread_rng` features are enabled
      - A [custom logger](https://docs.rs/log/latest/log/#implementing-a-logger) is defined
      - The custom logger accesses `rand::rng()` (previously `rand::thread_rng()`) and calls any `TryRng` (previously `RngCore`) methods on `ThreadRng`
      - The `ThreadRng` (attempts to) reseed while called from the custom logger (this happens every 64 kB of generated data)
      - Trace-level logging is enabled or warn-level logging is enabled and the random source (the `getrandom` crate) is unable to provide a new seed
      
      `TryRng` (previously `RngCore`) methods for `ThreadRng` use `unsafe` code to cast `*mut BlockRng<ReseedingCore>` to `&mut BlockRng<ReseedingCore>`. When all the above conditions are met this results in an aliased mutable reference, violating the Stacked Borrows rules. Miri is able to detect this violation in sample code. Since construction of [aliased mutable references is Undefined Behaviour](https://doc.rust-lang.org/stable/nomicon/references.html), the behaviour of optimized builds is hard to predict.
    ├ Announcement: https://github.com/rust-random/rand/pull/1763
    ├ Solution: Upgrade to >=0.10.1 OR <0.10.0, >=0.9.3 OR <0.9.0, >=0.8.6 (try `cargo update -p rand`)
    ├ rand v0.8.5
      └── (dev) libdd-common v5.1.0
          └── libdd-common-ffi v37.0.0
              └── libdd-otel-thread-ctx-ffi v1.0.0

advisories FAILED, bans ok, sources ok

📦 libdd-otel-thread-ctx - ✅ No issues

---

Updated: 2026-07-24 15:31:05 UTC | Commit: 0f2c335 | dependency-check job results

[datadog-official]

🎉 All green!

🧪 All tests passed
❄️ No new flaky tests detected

🎯 Code Coverage (details)
• Patch Coverage: 83.51%
• Overall Coverage: 74.69% (+0.01%)

_{This comment will be updated automatically if new data arrives.
🔗 Commit SHA: 6234054 | Docs | Datadog PR Page | Give us feedback!}

[dd-octo-sts]

Artifact Size Benchmark Report

aarch64-alpine-linux-musl

Artifact	Baseline	Commit	Change
/aarch64-alpine-linux-musl/lib/libdatadog_profiling.a	88.33 MB	88.33 MB	+0% (+3.85 KB) 👌
/aarch64-alpine-linux-musl/lib/libdatadog_profiling.so	8.13 MB	8.13 MB	0% (0 B) 👌

aarch64-unknown-linux-gnu

Artifact	Baseline	Commit	Change
/aarch64-unknown-linux-gnu/lib/libdatadog_profiling.so	10.94 MB	10.94 MB	+0% (+80 B) 👌
/aarch64-unknown-linux-gnu/lib/libdatadog_profiling.a	99.62 MB	99.62 MB	+0% (+3.84 KB) 👌

libdatadog-x64-windows

Artifact	Baseline	Commit	Change
/libdatadog-x64-windows/debug/dynamic/datadog_profiling_ffi.dll	26.65 MB	26.65 MB	0% (0 B) 👌
/libdatadog-x64-windows/debug/dynamic/datadog_profiling_ffi.lib	89.18 KB	89.18 KB	0% (0 B) 👌
/libdatadog-x64-windows/debug/dynamic/datadog_profiling_ffi.pdb	191.80 MB	192.16 MB	+.18% (+368.00 KB) 🔍
/libdatadog-x64-windows/debug/static/datadog_profiling_ffi.lib	1.04 GB	1.04 GB	+.24% (+2.57 MB) 🔍
/libdatadog-x64-windows/release/dynamic/datadog_profiling_ffi.dll	8.68 MB	8.68 MB	0% (0 B) 👌
/libdatadog-x64-windows/release/dynamic/datadog_profiling_ffi.lib	89.18 KB	89.18 KB	0% (0 B) 👌
/libdatadog-x64-windows/release/dynamic/datadog_profiling_ffi.pdb	25.40 MB	25.40 MB	0% (0 B) 👌
/libdatadog-x64-windows/release/static/datadog_profiling_ffi.lib	50.65 MB	50.65 MB	--.01% (-5.61 KB) 💪

libdatadog-x86-windows

Artifact	Baseline	Commit	Change
/libdatadog-x86-windows/debug/dynamic/datadog_profiling_ffi.dll	23.21 MB	23.21 MB	0% (0 B) 👌
/libdatadog-x86-windows/debug/dynamic/datadog_profiling_ffi.lib	90.58 KB	90.58 KB	0% (0 B) 👌
/libdatadog-x86-windows/debug/dynamic/datadog_profiling_ffi.pdb	196.50 MB	196.87 MB	+.18% (+376.00 KB) 🔍
/libdatadog-x86-windows/debug/static/datadog_profiling_ffi.lib	1.03 GB	1.03 GB	+.24% (+2.57 MB) 🔍
/libdatadog-x86-windows/release/dynamic/datadog_profiling_ffi.dll	6.70 MB	6.70 MB	0% (0 B) 👌
/libdatadog-x86-windows/release/dynamic/datadog_profiling_ffi.lib	90.58 KB	90.58 KB	0% (0 B) 👌
/libdatadog-x86-windows/release/dynamic/datadog_profiling_ffi.pdb	27.32 MB	27.31 MB	--.02% (-8.00 KB) 💪
/libdatadog-x86-windows/release/static/datadog_profiling_ffi.lib	48.23 MB	48.22 MB	--.01% (-6.38 KB) 💪

x86_64-alpine-linux-musl

Artifact	Baseline	Commit	Change
/x86_64-alpine-linux-musl/lib/libdatadog_profiling.a	78.78 MB	78.79 MB	+0% (+7.53 KB) 👌
/x86_64-alpine-linux-musl/lib/libdatadog_profiling.so	9.07 MB	9.07 MB	0% (0 B) 👌

x86_64-unknown-linux-gnu

Artifact	Baseline	Commit	Change
/x86_64-unknown-linux-gnu/lib/libdatadog_profiling.a	94.28 MB	94.29 MB	+0% (+7.57 KB) 👌
/x86_64-unknown-linux-gnu/lib/libdatadog_profiling.so	11.04 MB	11.04 MB	+0% (+368 B) 👌