A single dispatch goroutine delivered every reply in the process — every
channel, every user — each Send blocking the next. One media upload of a
few seconds froze all outbound traffic for that long, and sustained
throughput was capped at one Send round-trip regardless of lane capacity.
The ordering that loop actually protected is per-conversation: a run emits
block replies, retry notices and a final answer to one ChatID, and those
must arrive in that order. A global order was never required.
Dispatch now shards on channel+ChatID. A conversation always maps to the
same shard and is delivered serially there, so the ordering guarantee is
unchanged, while unrelated conversations proceed in parallel.
GOCLAW_OUTBOUND_SHARDS tunes the worker count (default 8).
Temp media needed a real fix to go with it. Serial dispatch deduped it
implicitly — the first delivery removed the file, and a later message
carrying the same path found os.Stat failing and skipped it. Across
concurrent shards that check alone is a TOCTOU race, so claims are now
taken atomically and released after the send.
Also makes the pool limits that bound this path configurable, defaults
unchanged: GOCLAW_PG_MAX_OPEN_CONNS / GOCLAW_PG_MAX_IDLE_CONNS, and
GOCLAW_HTTP_MAX_IDLE_CONNS / GOCLAW_HTTP_MAX_IDLE_CONNS_PER_HOST. The
per-host limit binds when one provider takes nearly all traffic: every
request past it pays a fresh TCP+TLS handshake, which a deployment
raising LaneMain needs to be able to lift.
