The capture cpal callback (CaptureState::ingest) ran two heap
allocations per callback on the realtime audio thread:
1. engine.rs:1196-1202 — fresh `mono: Vec<f32>` for the downmix
output, once per cpal callback (50–100 Hz).
2. engine.rs:1217-1218 — `pcm_accum.drain(..FRAME_SAMPLES).collect()`
building a fresh Vec<f32> of 960 samples per Opus frame.
Both sites mirror the pattern already fixed for the output side at
engine.rs:1389-1397, where allocating per callback on glibc malloc
was correlated with user-perceptible audio popping. The output-side
fix replaced the per-callback allocation with a pre-allocated
`scratch` Vec that is cleared and resized in place; this commit
applies the same template to the capture side.
Changes:
- Add `mono_scratch: Vec<f32>` and `frame_scratch: Vec<f32>` to
CaptureState. Initialised with Vec::with_capacity(4096) and
Vec::with_capacity(FRAME_SAMPLES=960) respectively in
CaptureState::new.
- Replace the downmix Vec construction with in-place push into
`self.mono_scratch`; `clear()` retains capacity across callbacks.
- Replace the drain().collect() with `self.frame_scratch.extend(
self.pcm_accum.drain(..FRAME_SAMPLES))`; same capacity-retention.
- The resampler call uses std::mem::take to swap the scratch buffer
out for the duration of the &mut self call, then moves it back —
the backing allocation is preserved across callbacks.
Algorithm semantics are unchanged: same downmix arithmetic, same
clamp loop, same Opus encode call sequence. Only the storage
strategy differs.
Out of scope (intentionally not touched):
- Android audio path (android_voice_unit.rs, mobile_voice_backend.rs):
researcher constraint C-4 — the Android cpal data path is mid-
migration and being replaced.
- Output callback at engine.rs:1409+: the only obvious per-callback
allocation there (`scratch`) was already fixed; a fuller audit
is a separate scope decision.
- The `scratch` buffer at engine.rs:1389-1397 — already correct.
Verification:
- cargo check --workspace --all-targets: passes.
- cargo test --workspace: 106 passed / 0 failed / 3 ignored.
- cargo clippy --workspace --all-targets: no new lints introduced;
the one warning inside the edited region (clamp-like pattern at
line 1266) was pre-existing on the copied clamp loop.
EdisonJwa
d13b56d379