Re: [HACKERS] CLUSTER command progress monitor

On Tue, Sep 3, 2019 at 1:02 AM Masahiko Sawada <sawada.mshk@gmail.com> wrote:
> After more thought, even if we don't start a new command progress when
> there is another one already started the progress update functions
> could be called and these functions don't specify the command type.
> Therefore, the progress information might be changed with wrong value
> by different command. Probably we can change the caller of progress
> updating function so that it doesn't call all of them if the command
> could not start a new progress report, but it might be a big change.
>
> As an alternative idea, we can make pgstat_progress_end_command() have
> one argument that is command the caller wants to end. That is, we
> don't end the command progress when the specified command type doesn't
> match to the command type of current running command progress. We
> unconditionally clear the progress information at CommitTransaction()
> and AbortTransaction() but we can do that by passing
> PROGRESS_COMMAND_INVALID to pgstat_progress_end_command().

I think this is all going in the wrong direction.  It's the
responsibility of the people who are calling the pgstat_progress_*
functions to only do so when it's appropriate.  Having the
pgstat_progress_* functions try to untangle whether or not they ought
to ignore calls made to them is backwards.

It seems to me that the problem here can be summarized in this way:
there's a bunch of code reuse in the relevant paths here, and somebody
wasn't careful enough when adding progress reporting for one of the
new commands, and so now things are broken, because e.g. CLUSTER
progress reporting gets ended by a pgstat_progress_end_command() call
that was intended for some other utility command but is reached in the
CLUSTER case anyway.  The solution to that problem in my book is to
figure out which commit broke it, and then the person who made that
commit either needs to fix it or revert it.

It's quite possible here that we need a bigger redesign to make adding
progress reporting for new command easier and less prone to bugs, but
I don't think it's at all clear what such a redesign should look like
or even that we definitely need one, and September is not the right
time to be redesigning features for the pending release.

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company