authentication_timeout ineffective for replication connections

Hi,

I just noticed that authentication_timeout is ineffective for
replication=true type connections. That's because walsender doesn't
register a SIGINT handler and authentication_timeout relies on having
one.

There's no problem with reading the initial startup packet
(ProcessStartupPacket/BackendInitialize) because we use a separate
handler there. But once that's done, before finishing authentication,
WalSndSignals() will have set SIGINT's handler to SIG_IGN.

Demo python program attached. You'll only see the problem if the
authentication method requires a password/addititional packets.

I think we could fix this by simply mapping SIGINT to die() instead
SIG_IGN.

Greetings,

Andres Freund

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 Andres Freund                       http://www.2ndQuadrant.com/
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