Add end-to-end conductor parity tests, and fix the divergence they found

[wbarnha]

Description

Closes the gap left by #762: the topic conductor — the per-message inner loop of a worker — exists twice and neither copy was covered. The existing conductor tests replace the handler with an AsyncMock and assert it was called, so the fan-out, event reuse, buffer-pressure callbacks, full-queue path and decode-error propagation were untested on both sides.

Both handlers take (conductor, tp, channels) and are awaited with a Message, so they can be driven over the same input and compared. tests/unit/transport/test_conductor_parity.py does that for each of those paths and diffs a full record of the outcome: which events reached which channels, refcount and acked state, decode counts, and every sensor and consumer callback.

Both implementations run against the same conductor and the same channels set, one after the other, rather than two separately-built environments. channels is a set of Topic objects hashed by identity, so two separate sets iterate in unrelated orders — anything order-sensitive (which channel decodes first, which ones a mid-fan-out decode error reaches) would then differ for reasons unrelated to the implementations. Sharing the set removes that variable; reset() clears queues and recorded callbacks between runs.

What it found

ConductorHandler never reused a decoded event. The conductor is supposed to deserialize once and reuse it for every channel whose (key_type, value_type) matches, but event_keyid was only ever assigned from _decode(), which returned it unchanged on the first pass. It stayed None forever, the reuse branch was dead, and every subscribed channel re-deserialized the payload.

That masked a second fault. Had the keyid ever been set, a mismatched pair fell off the end of _decode and returned a bare None — which unpacking into two names raises TypeError on. Fixing the reuse alone would have turned a silent inefficiency into a crash on any topic whose subscribers declare different key or value types. Confirmed by building that partial fix and watching the new heterogeneous-keyid test fail with TypeError: 'NoneType' object is not iterable at the unpack.

This is the same double-bug shape as _try_get_quick_value in streams.pyx (#762), arrived at independently: a dead optimization whose deadness concealed that it was also wrong.

It was not only a performance difference. A channel whose event is reused never calls decode, so a channel that would have failed to deserialize raised no error under the pure-Python conductor and raised one under the extension — changing which channels received the message and how many acks it got.

The fix ports conductor.py's loop faithfully: event/event_keyid stay pinned to the first channel, and a channel with a different pair gets its own dest_event without displacing the pinned one. _decode is gone; keyid and dest_event were already declared in __call__ and unused, which suggests this is what it was meant to be.

Verification

16 parity tests: fan-out over 1/2/3 channels, no subscribers, batches, event reuse for matching keyids, per-channel decode for differing keyids, decode errors (whole fan-out and single channel), the full-queue path, and the pressure callbacks.

Before the fix 6 failed, all tracing to that one root cause; after it, all pass. Full suite green in every configuration:

configuration	result
extensions built (FAUST_REQUIRE_CYTHON=1)	2270 passed
extensions absent	2207 passed, parity skipped
NO_CYTHON=1	passed
free-threaded 3.14t, PYTHON_GIL=0	2274 passed

flake8 / black / isort clean; docs build clean.

Note on the base branch

Based on claude/faust-free-threaded-support-tpmqh2 (#762), not master, because it depends on that branch's in-place extension build and FAUST_REQUIRE_CYTHON guard. Without those, CI never imports the compiled conductor and every comparison here would run the pure-Python implementation against itself — the same reason the parity tests in #751 note they otherwise "just run the pure-Python one twice". Retarget to master once #762 lands.

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Generated by Claude Code

[wbarnha]

Folded into #762 — the whole series is now one branch, so there is no stack to land in order. The commit (6e83c12, conductor parity suite + the event-reuse fix) is unchanged there.

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Generated by Claude Code