Re: Document that server will start even if it's unable to open some TCP/IP ports

On Mon, Jun 19, 2023 at 5:48 PM Bruce Momjian <bruce(at)momjian(dot)us> wrote:
>
> On Tue, Jun 13, 2023 at 11:11:04PM -0400, Tom Lane wrote:
> >
> > There is certainly an argument that such a condition indicates that
> > something's very broken in our configuration and we should complain.
> > But I'm not sure how exciting the case is in practice. The systemd
> > guys would really like us to be willing to come up before any network
> > interfaces are up, and then auto-listen to those interfaces when they
> > do come up.

That sounds like a reasonable expectation, as the network conditions
can change without any explicit changes made by someone.

> > On the other hand, the situation with Unix sockets is
> > much more static: if you can't make a socket in /tmp or /var/run at
> > the instant of postmaster start, it's unlikely you will be able to do
> > so later.

I think you're describing a setup where Postgres startup is automatic,
as part of server/OS startup. That is the most common case.

In cases where someone is performing a Postgres startup manually, they
are very likely to have the permissions to fix the problem preventing
the startup.

> > Maybe we need different rules for TCP versus Unix-domain sockets?
> > I'm not sure what exactly, but lumping those cases together for
> > a discussion like this feels wrong.

+1.

> If we are going to retry for network configuration changes, it seems we
> would also retry Unix domain sockets for cases like when the permissions
> are wrong, and then fixed.

The network managers (systemd, etc.) are expected to respond to
dynamic conditions, and hence they may perform network config changes
in response to things like network outages, and hardware failures,
time of day, etc.

On the other hand, the permissions required to create files for Unix
domain sockets are only expected to change if someone decides to make
that change. I wouldn't expect these permissions to be changed
dynamically.

On those grounds, keeping the treatment of Unix domain sockets out of
this discussion for this patch seems reasonable.

> However, it seem hard to figure out exactly what _is_ working if we take
> the approach of dynamically retrying listen methods. Do we report
> anything helpful in the server logs when we start and can't listen on
> anything?

Yes. For every host listed in listen_addresses, if Postgres fails to
open the port on that address, we get a WARNING message in the server
log. After the end of processing of a non-empty listen_addresses, if
there are zero open TCP/IP connections, the server exits (with a FATAL
message, IIRC).

Best regards,
Gurjeet
http://Gurje.et