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bd031a05af
Replaces #9366 on a branch in this repository and resolves its conflicts
with main.
## Summary
The ACP observer kept up to 1,000 raw events in both replay and live
buffers. Large valid messages could retain gigabytes before downstream
trimming ran.
Fit telemetry before either buffer retains it. Cap replay at 4 MiB of
serialized JSON and keep the existing live window. Its per-event limit
bounds live JSON below 64 MiB.
Preserve the CPU and locking improvements from #9381: prepare large
payloads and copy snapshots outside the replay lock. Finalize sequence
and timestamp with publication. Account for that final metadata,
including sequence digit-width changes, in the byte limit.
The tradeoffs are extra work per emit, shorter replay history for large
events, and elided oversized telemetry. Agent protocol messages remain
unchanged.
### Related issue
Fixes #7987. Existing publisher byte limits act downstream of these
buffers.
### Testing
The opt-in probe exercises the real in-process emit, snapshot and
stalled-subscriber paths:
```sh
CARGO_INCREMENTAL=0 CARGO_PROFILE_RELEASE_LTO=thin CARGO_PROFILE_RELEASE_CODEGEN_UNITS=4 \
cargo test --release -p buzz-acp --lib observer::tests::retention_probe -- --exact --ignored --nocapture
```
Compared baseline `a314975ce` with candidate `c2df3cb56` on macOS arm64,
Rust 1.98.1. The baseline observer, fitter, workspace manifest and
lockfile match the candidate's parent `8f3f4e266`. Both arms use
`opt-level=3`, thin LTO and four codegen units, with incremental
compilation and compiler-cache wrappers off. This profile is fixed for
the comparison; this PR does not change build settings.
Values are medians of three fresh-process runs per version and workload,
in alternating order. Peak RSS comes from `/usr/bin/time -l` around each
test executable; compilation is excluded. Each version was rebuilt
separately and its executable and workload identity verified.
| Workload | Peak RSS before → after | Emit time per event before →
after |
| --- | --- | --- |
| 10,000 × 1 KiB | 14.0 → 14.1 MiB | 0.47 → 0.89 µs |
| 1,000 × 60,000 bytes | 199.4 → 80.8 MiB | 8.54 → 22.53 µs |
| 1,000 × 1 MiB | 3,025.8 → 26.4 MiB | 123.50 → 307.89 µs |
For the 1 MiB workload, replay JSON fell from 1,000.2 MiB to 3.997 MiB.
Live JSON fell from 1,000.2 MiB to 5.91 MiB. Replay retained 676 fitted
events; the live subscriber received all 1,000.
Emit time includes payload construction. Peak RSS includes snapshot and
receive copies made by the probe. JSON budgets are not exact heap
limits. These local measurements show the memory/CPU tradeoff;
concurrent producer throughput and native app latency were not
benchmarked. See [the probe
instructions](c2df3cb567/crates/buzz-acp/TESTING.md (observer-retention-measurement))
for workloads, controls and limits.
Human verification remains pending.
Generated with AI assistance
Signed-off-by: Sanket Kanjalkar <sanketk@squareup.com>
Co-authored-by: Codex <codex@openai.com>