walsender doesn't send keepalives when writes are pending

Hi,

In WalSndLoop() we do:

wakeEvents = WL_LATCH_SET | WL_POSTMASTER_DEATH | WL_TIMEOUT |
WL_SOCKET_READABLE;

if (pq_is_send_pending())
wakeEvents |= WL_SOCKET_WRITEABLE;
else if (wal_sender_timeout > 0 && !ping_sent)
{
...
if (GetCurrentTimestamp() >= timeout)
WalSndKeepalive(true);
...

I think those two if's should simply be separate. There's no reason not
to ask for a ping when we're writing. On a busy server that might be the
case most of the time.
The if (pq_is_send_pending()) should also be *after* sending the
keepalive, after all, it might not immediately have been flushed as
unlikely as that is.

The attached patch is unsurprisingly simple.

Greetings,

Andres Freund

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