Re: Streaming replication and non-blocking I/O

Tom Lane wrote:
> Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com> writes:
>> Changing the finish_time argument to pqWaitTimed into timeout_ms changes
>> the behavior connect_timeout option to PQconnectdb. It should wait for
>> max connect_timeout seconds in total, but now it is waiting for
>> connect_timeout seconds at each step in the connection process: opening
>> a socket, authenticating etc.
>
> Refresh my memory as to why this patch is touching any of that code at
> all?

Walreceiver wants to wait for data to arrive from the master or a
signal. PQgetXLogData(), which is the libpq function to read a piece of
WAL, takes a timeout argument to support that. Walreceiver calls
PQgetXLogData() in an endless loop, checking for a received sighup or
death of postmaster at every iteration.

In the synchronous replication mode, I presume it's also going to listen
for a signal from the startup process, so that it can send a
acknowledgment to the master as soon as a COMMIT record has been
replayed that a backend on the master is waiting for.

To implement the timeout in PQgetXLogData(), pqWaitTimed() was changed
to take a timeout instead of finishing_time argument. Which is a mistake
because it breaks PQconnectdb, and as I said I don't think
PQgetXLogData(9 should have a timeout argument to begin with. Instead,
it should have a boolean 'async' argument to return immediately if
there's no data, and walreceiver main loop should call poll()/select()
to wait. Ie. just like PQgetCopyData() works.

--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com