Обсуждение: Clean up command argument assembly

This is a small code cleanup patch.

Several commands internally assemble command lines to call other 
commands.  This includes initdb, pg_dumpall, and pg_regress.  (Also 
pg_ctl, but that is different enough that I didn't consider it here.) 
This has all evolved a bit organically, with fixed-size buffers, and 
various optional command-line arguments being injected with 
confusing-looking code, and the spacing between options handled in 
inconsistent ways.  This patch cleans all this up a bit to look clearer 
and be more easily extensible with new arguments and options.  We start 
each command with printfPQExpBuffer(), and then append arguments as 
necessary with appendPQExpBuffer().  Also standardize on using 
initPQExpBuffer() over createPQExpBuffer() where possible.  pg_regress 
uses StringInfo instead of PQExpBuffer, but many of the same ideas apply.

On 26/06/2023 12:33, Peter Eisentraut wrote:
> This is a small code cleanup patch.
> 
> Several commands internally assemble command lines to call other
> commands.  This includes initdb, pg_dumpall, and pg_regress.  (Also
> pg_ctl, but that is different enough that I didn't consider it here.)
> This has all evolved a bit organically, with fixed-size buffers, and
> various optional command-line arguments being injected with
> confusing-looking code, and the spacing between options handled in
> inconsistent ways.  This patch cleans all this up a bit to look clearer
> and be more easily extensible with new arguments and options.

+1

> We start each command with printfPQExpBuffer(), and then append
> arguments as necessary with appendPQExpBuffer().  Also standardize on
> using initPQExpBuffer() over createPQExpBuffer() where possible.
> pg_regress uses StringInfo instead of PQExpBuffer, but many of the
> same ideas apply.

It's a bit bogus to use PQExpBuffer for these. If you run out of memory, 
you silently get an empty string instead. StringInfo, which exits the 
process on OOM, would be more appropriate. We have tons of such 
inappropriate uses of PQExpBuffer in all our client programs, though, so 
I don't insist on fixing this particular case right now.

-- 
Heikki Linnakangas
Neon (https://neon.tech)

On 04.07.23 14:14, Heikki Linnakangas wrote:
> On 26/06/2023 12:33, Peter Eisentraut wrote:
>> This is a small code cleanup patch.
>>
>> Several commands internally assemble command lines to call other
>> commands.  This includes initdb, pg_dumpall, and pg_regress.  (Also
>> pg_ctl, but that is different enough that I didn't consider it here.)
>> This has all evolved a bit organically, with fixed-size buffers, and
>> various optional command-line arguments being injected with
>> confusing-looking code, and the spacing between options handled in
>> inconsistent ways.  This patch cleans all this up a bit to look clearer
>> and be more easily extensible with new arguments and options.
> 
> +1

committed

>> We start each command with printfPQExpBuffer(), and then append
>> arguments as necessary with appendPQExpBuffer().  Also standardize on
>> using initPQExpBuffer() over createPQExpBuffer() where possible.
>> pg_regress uses StringInfo instead of PQExpBuffer, but many of the
>> same ideas apply.
> 
> It's a bit bogus to use PQExpBuffer for these. If you run out of memory, 
> you silently get an empty string instead. StringInfo, which exits the 
> process on OOM, would be more appropriate. We have tons of such 
> inappropriate uses of PQExpBuffer in all our client programs, though, so 
> I don't insist on fixing this particular case right now.

Interesting point.  But as you say better dealt with as a separate problem.

On 05.07.23 07:22, Peter Eisentraut wrote:
>> It's a bit bogus to use PQExpBuffer for these. If you run out of 
>> memory, you silently get an empty string instead. StringInfo, which 
>> exits the process on OOM, would be more appropriate. We have tons of 
>> such inappropriate uses of PQExpBuffer in all our client programs, 
>> though, so I don't insist on fixing this particular case right now.
> 
> Interesting point.  But as you say better dealt with as a separate problem.

I was inspired by a33e17f210 (for pg_rewind) to clean up some more 
fixed-buffer command assembly and replace it with extensible buffers and 
some more elegant code.  And then I remembered this thread, and it's 
really a continuation of this.

The first patch deals with pg_regress and pg_isolation_regress.  It is 
pretty straightforward.

The second patch deals with pg_upgrade.  It would require exporting 
appendPQExpBufferVA() from libpq, which might be overkill.  But this 
gets to your point earlier:  Should pg_upgrade rather be using 
StringInfo instead of PQExpBuffer?  (Then we'd use appendStringInfoVA(), 
which already exists, but even if not we wouldn't need to change libpq 
to get it.)  Should anything outside of libpq be using PQExpBuffer?

Peter Eisentraut <peter@eisentraut.org> writes:
> Should anything outside of libpq be using PQExpBuffer?

Perhaps not.  PQExpBuffer's behavior for OOM cases is designed
specifically for libpq, where exit-on-OOM is not okay and we
can hope to include failure checks wherever needed.  For most
of our application code, we'd much rather just exit-on-OOM
and not have to think about failure checks at the call sites.

Having said that, converting stuff like pg_dump would be quite awful
in terms of code churn and creating a back-patching nightmare.

Would it make any sense to think about having two sets of
routines with identical call APIs, but different failure
behavior, so that we don't have to touch the callers?

            regards, tom lane