Re: exposing wait events for non-backends (was: Tracking wait event for latches)

On Wed, Mar 22, 2017 at 9:54 PM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> On Wed, Mar 22, 2017 at 12:20 PM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
>> On Wed, Mar 22, 2017 at 1:31 AM, Michael Paquier
>> <michael(dot)paquier(at)gmail(dot)com> wrote:
>>> Okay, switched as ready for committer. One note for the committer
>>> though: keeping the calls of pgstat_bestart() out of
>>> BackgroundWorkerInitializeConnection() and
>>> BackgroundWorkerInitializeConnectionByOid() keeps users the
>>> possibility to not have backends connected to the database show up in
>>> pg_stat_activity. This may matter for some users (cloud deployment for
>>> example). I am as well in favor in keeping the work of those routines
>>> minimal, without touching at pgstat.
>>
>> I think that's just inviting bugs of omission, in both core and
>> extension code. I think it'd be much better to do this in a
>> centralized place.
>
> I mean, your argument boils down to "somebody might want to
> deliberately hide things from pg_stat_activity". But that's not
> really a mode we support in general, and supporting it only for
> certain cases doesn't seem like something that this patch should be
> about. We could add an option to BackgroundWorkerInitializeConnection
> and BackgroundWorkerInitializeConnectionByOid to suppress it, if it's
> something that somebody wants, but actually I'd be more inclined to
> think that everybody (who has a shared memory connection) should go
> into the machinery and then security-filtering should be left to some
> higher-level facility that can make policy decisions rather than being
> hard-coded in the individual modules.
>
> But I'm slightly confused as to how this even arises. Background
> workers already show up in pg_stat_activity output, or at least I sure
> think they do. So why does this patch need to make any change to that
> case at all?
When we initialize a process through InitPostgres, the control returns
from the method at different locations based on the process's type(as
soon as its relevant information is initialized). Following is the
order of returning a process from InitPostgres:

IsAutoVacuumLauncherProcess
walsender not connected to a DB
bgworker not connected to a DB
Other processes using InitPostgres

Before the patch, not all the processes are shown in pg_stat_activity.
Hence, only at two locations in InitPostgres, we need to call
pgstat_bestart() to initialize the entry for the respective process.
But, since we're increasing the types of a process shown in
pg_stat_activity, it seems to be reasonable to move the
pgstat_bestart() call to a proc's main entry point. I've followed the
same approach for auxiliary processes as well.

AutovacuumLauncher - AutoVacLauncherMain()
bgwriter BackgroundWriterMain()
checkpointer CheckpointerMain()
startup StartupProcessMain()
walwriter WalWriterMain()
walreceiver WalReceiverMain()
walsender InitWalSender()

Hence, to be consistent with others, bgworker processes can be
initialized from BackgroundWorkerInitializeConnectionBy[Oid].

I've attached the updated patches which reflect the above change. PFA.

--
Thanks & Regards,
Kuntal Ghosh
EnterpriseDB: http://www.enterprisedb.com