serveConn dispatched under context.Background(), so a dispatch in flight
during shutdown could not be told to stop and closeGrace could only
abandon it. The server now carries a context, Close cancels it, and each
conn derives its own so nothing outlives the connection.
A zero-value Server built outside Listen falls back to Background; two
wiring tests do that.
Client.Close read c.conn with no lock while roundtrip re-dialed and
dropped it, which -race caught on the new test. The conn field now has
its own mutex, held only across a read or an assignment, so Close and
the watchdog reach the connection without queueing behind the call they
are interrupting.
claude
cbd8077d2c