The human identified that both agents were fighting the wrong subsystem, and
testing it here confirms the diagnosis and finds the limit.
A PulseAudio null sink s MONITOR is sampled on a wall clock: when the client is
late PulseAudio does not wait, it emits silence to keep its own timeline. So the
39.3 percent silence in the take-2 capture was never audio that went missing, it
was silence PulseAudio invented -- which is why PULSE_LATENCY_MSEC gave a
non-monotonic curve and never won. The instrument was wrong, not mistuned.
ALSA s file plugin has no clock; it tees exactly what the client writes, so a
slow producer yields a shorter file rather than a gap-riddled one. Control with
six distinct tones: 12.000 s against a 12.000 s source, 0.00 percent silence,
zero gaps, no duplicate channels. Channel order comes out as ALSA s
FL FR BL BR FC LFE rather than WAV s FL FR FC LFE BL BR -- deterministic and
invertible, not data loss.
Three configuration traps recorded in the order they bite: ALSA_CONFIG_PATH
replaces the whole config so the stock one must be included; but WITH that
include a pcm.!default override silently does not take, in either the inline or
the alias form, so the slave must be declared with an inline plugin type and no
include; and a pipe to head SIGPIPEs the producer before it writes, which looks
exactly like a broken config.
And the limit the proposer honestly flagged, now measured: a bare file tee is not
enough for Xenia, because its ALSA writer thread pads silence whenever the ring
buffer is empty (alsa_audio_driver.cc:359). Against a device that never blocks it
free-ran at about 250x real time -- 7.34 GB, 12746 s of nominal audio, in 50 s of
wall clock, nearly all driver-generated silence. Killed and deleted; it would
have filled the disk.
The configuration that satisfies both constraints is a tee in FRONT of a paced
slave: type file with slave.pcm { type pulse }. The file plugin captures what the
client writes and the slave supplies the clock, so the wall-clock silence
insertion happens downstream of the capture point. Control through that exact
config: 12.000 s, 0.00 percent silence, zero gaps.
Consequences for verification: short file becomes the failure mode, so a capture
check needs an expected-duration test alongside silence and gap rate, and a
runaway guard is not optional.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01QsEPXWVaEpyfudtR6re1Pd
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