Add perfetto for rust

[jasnell]

Make perfetto available to rust

[ask-bonk]

Since last review: 0 resolved, 0 still open, 0 new.
LGTM!

Not re-run: tests, docs, compat-flags, design-simplicity, kj-style, memory-safety, rust-first (no author changes in their files since the last review)

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_{Reviewed commit: 563c979f · github run}

[codecov-commenter]

Codecov Report

❌ Patch coverage is 4.60251% with 228 lines in your changes missing coverage. Please review.
✅ Project coverage is 41.79%. Comparing base (3d28dc6) to head (563c979).
⚠️ Report is 7 commits behind head on main.

Files with missing lines	Patch %	Lines
src/rust/perfetto/lib.rs	1.91%	154 Missing ⚠️
src/rust/perfetto/enabled.rs	6.32%	74 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7528      +/-   ##
==========================================
- Coverage   41.82%   41.79%   -0.03%     
==========================================
  Files        1015     1017       +2     
  Lines      292015   292254     +239     
  Branches    28198    28198              
==========================================
+ Hits       122133   122147      +14     
- Misses     153387   153613     +226     
+ Partials    16495    16494       -1     

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[npaun]

This is kind of funny but I don't quite get why we need this really option.

[npaun]

Looks perfetto to me once the Bonk issues are addressed. My clanker also brings up the exact same concerns. No further concerns from me as a human.

[clanky-the-clanker]

💬 Approved with comments — No correctness or security problems found. The only item is an INFO note about per-event locking and allocation in the enabled Rust tracing backend, which costs nothing unless a tracing session is active.

Findings: 1 info

_{The decision is advisory: this comment never approves, blocks or requests changes on the pull request.}

🔧 Fix with your agent

Paste the following into your coding agent to investigate and fix these issues.

Fix the following code review issues found in pull request #7528.

Before making changes, review each issue and present a brief summary table:
- For each finding, state whether you agree, disagree, or need clarification
- If you disagree (e.g., the fix would require disproportionate changes
  for minimal ROI, or the finding is factually incorrect), explain why
- If you need to clarify something before deciding, ask those questions
- Then share your plan for which issues to tackle and in what order

## INFO (1)

### [Performance] Enabled Rust trace points take a process-wide mutex and allocate on every event, and counters recompute their track UUID each call. This costs nothing unless tracing is active, but it could cause contention if trace points are added to multi-threaded hot paths.
- **File:** src/rust/perfetto/enabled.rs:129

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⚡ Performance: 💬 1 info

No hot-path regressions; one note on per-event cost while tracing is enabled

Recommendation: 💬 Optional Nits

• [INFO] Enabled Rust trace points take a process-wide mutex and allocate on every event, and counters recompute their track UUID each call. This costs nothing unless tracing is active, but it could cause contention if trace points are added to multi-threaded hot paths. (src/rust/perfetto/enabled.rs:129)

View full review

• src/rust/perfetto/enabled.rs:90-94, 129-142: Every emitted event takes the SDK's process-wide lock, as the doc comment in lib.rs says. It also builds a fresh SdkContext and allocates for the extras. emit_counter recomputes the counter track UUID on every call. This only costs anything while a tracing session has the category enabled. The disabled path is two atomic loads.
• Optional: keep Rust trace points off multi-threaded hot paths, as lib.rs already advises. If counters become frequent, cache the counter track UUID per call site.

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✅ All other checks passed

🔒 Security: ✅ LGTM

This is internal tracing plumbing: a patch to the Perfetto build, a Rust SDK wrapper and a cxx bridge. None of it is reachable from user input.

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📖 AGENTS.md: ✅ LGTM

The AGENTS.md updates in the root, src/rust/ and src/workerd/util/ match the new crate and build flag.

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📋 Code Quality: ✅ LGTM

The Bazel select gating, the cfg-gated enabled and disabled backends, and the tests are consistent. //src/workerd/util:perfetto includes <workerd/rust/perfetto/ffi.rs.h> and depends only on //src/rust/perfetto. Other C++ targets do the same: async-trace depends on //src/rust/async-trace, and bench.c++ includes histogram/ffi.rs.h the same way. wd_rust_crate adds the bridge library to the crate's link_deps.

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📝 Documentation: ✅ LGTM

The documentation updates in docs/v8-updates.md and the crate's doc comments match the change.

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_{Reviewed commit: 563c979f}