Summary

Tx.Close always sends UNWATCH before releasing the connection:

func (c *Tx) Close(ctx context.Context) error {
	_ = c.Unwatch(ctx).Err()
	return c.baseClient.Close()
}

Because Client.Watch runs defer tx.Close(ctx), every transaction pays an extra UNWATCH round trip — even after EXEC, which already discards the watched keys server-side, and even for Watch(ctx, fn) calls with no keys where nothing was ever watched.

Change

Add an unexported watchArmed flag to Tx, set on a successful WATCH and cleared whenever the server discards the watched keys: on UNWATCH, or on EXEC (commit, TxFailedErr, or EXECABORT). Close issues UNWATCH only while a watch is still armed.

EXEC clears WATCH whether the transaction commits or aborts, so the flag is cleared on any server reply to the MULTI/EXEC block (including TxFailedErr). A transport error leaves the flag set, but that connection is removed from the pool rather than reused, so the skipped UNWATCH cannot leak a watch onto a pooled connection.

Why "armed" rather than "cleared"

Tracking "is a watch armed" keeps the zero value (false) on the safe, cheap path: a Tx that never watched anything never sends UNWATCH, and a re-WATCH after an EXEC re-arms naturally, so Close still releases it. A "was it cleared" flag would default to sending UNWATCH, miss the no-key case, and need an extra reset to avoid leaking on re-WATCH.

Compatibility

No exported API or struct field changes. The only wire-visible difference is that the redundant no-op UNWATCH is no longer sent on the paths above; UNWATCH is still sent whenever a watch may still be active.

Tests

tx_unwatch_test.go asserts, both on the wire (via a recording hook) and server-side (via INFO commandstats):

• no UNWATCH / no cmdstat_unwatch after a committed EXEC;
• no UNWATCH for a no-key Watch;
• UNWATCH / cmdstat_unwatch present when fn returns before EXEC;
• a pooled connection (PoolSize=1) is never reused with a leaked WATCH (WATCH, bail, mutate the key, then run an unrelated transaction and assert it is not spuriously aborted).

Behavior note (unsupported usage)

Issuing WATCH directly via Tx.Process/Tx.Do (instead of Tx.Watch) is not tracked by the flag, so Close will not send a covering UNWATCH for it — previously the unconditional Close UNWATCH masked this. Use the typed Tx.Watch/Tx.Unwatch. This is documented on the watchArmed field.

Related: #435 (the no-key Watch round-trip this also removes).