Less than ideal error reporting in pg_stat_statements

A client was getting some hard to diagnose out of memory errors. What
made this especially confusing was that there was no context reported at
all, other than the (enormous) statement that triggered the error.

At first I thought the lack of context indicated a palloc had failed
during ereport() (since we apparently just toss the previous error when
that happens), but it turns out there's some error reporting in
pg_stat_statements that's less than ideal. Attached patch fixes, though
I'm not sure if %lld is portable or not.

I'll also argue that this is a bug and should be backpatched, but I'm
not hell-bent on that.

At the same time I looked for other messages that don't explicitly
reference pg_stat_statements; the only others are in
pg_stat_statements_internal() complaining about being called in an
inappropriate function context. Presumably at that point there's a
reasonable error context stack so I didn't bother with them.

This still seems a bit fragile to me though. Anyone working in here has
to notice that most every errmsg mentions pg_stat_statements and decide
there's a good reason for that. ISTM it'd be better to push a new
ErrorContextCallback onto the stack any time we enter the module. If
folks think that's a good idea I'll pursue it as a separate patch.
--
Jim Nasby, Data Architect, Blue Treble Consulting, Austin TX
Experts in Analytics, Data Architecture and PostgreSQL
Data in Trouble? Get it in Treble! http://BlueTreble.com