log_destination=file

Attached is a very much VIP (AKA rough draft) for $subject.

Right now we have two parameters controlling logging destination, and they
work in interesting combinations:

log_destination=stderr, logging_collector=off -> log to stderr (makes sense)
log_destination=stderr, logging_collector=on -> log to file (*highly*
illogical for most users, to set stderr when they want file)
log_destination=csvlog, logging_collector=on -> log to csvfile (makes sense)
log_destination=csvlog, logging_collector=off -> fail (ugh)

(beyond that we also have syslog and eventlog, but those are different and
not touched by this patch)

My understanding is that the main reason for this is that we cannot change
logging_collector without restarting postmaster, whereas we can change
log_destination.

My suggestion is we work around this by just always starting the logging
collector, even if we're not planning to use it. Then we'd just have:

log_destination = stderr -> log to stderr
log_destination = file -> log to file
log_destination = csvlog -> log to csv file

The main difference here is that log_destination=stderr would also go
through the logging collector which would then assemble it and write it out
to it's own stderr.

Do people see an actual problem with that? I agree it's an extra round of
indirection there, but is that a problem? It would also cause one more
backgorund process to always be running, but again, is that really a
problem? The overhead is not exactly large.

It would make configuration a lot more logical for logging. It would also
make it possible to switch between all logging configurations without
restarting.

The attached WIP is mostly working for the simple cases. It fails
completely if the syslogger is restarted at this point, simply because I
haven't figured out how to pass the original stderr down to the syslogger.
I'm sure there are also many other smaller issues around it, but I wanted
to get the discussion done before I spend the time to go through those.

Thoughts?

--
Magnus Hagander
Me: https://www.hagander.net/ <http://www.hagander.net/>
Work: https://www.redpill-linpro.com/ <http://www.redpill-linpro.com/>