Re: BUG #16329: Valgrind detects an invalid read when building a gistindex with buffering

Could you please let me know if the fix is incorrect (or not elaborated
enough to be included in the upcoming releases) or this issue is false
positive?

I took a look at this.  I see the hazard, but I'm not sure that
I like your proposed quick-fix.  Essentially, the problem is that
gistBuildCallback is supposing that the tuple it creates will survive
the execution of its subroutines, which in fact it won't always.
But that means that at some point the tuple pointer it's passed down to
those subroutines becomes a dangling pointer.  What guarantee would we
have that the subroutines don't access the tuple after that point?

Thanks for your review!
Yes, I'm not excited by this fix too, so I'll try to find another not so quick way to fix it.

Or, if we do just hack it as you suggest, there had better be a couple
of fat comments in gistBufferingBuildInsert warning about the hazards.

I was somewhat dismayed to realize from looking at the code coverage
report that we have exactly zero test coverage of the gist buffering
build code paths today.  That's just awful; how did the feature get
committed with no test coverage?  Your proposed test additions raise the
coverage in gistbuild.c and gistbuildbuffers.c to something respectable,
at least.  But it looks like there are still some potentially-delicate
paths that aren't tested, notably the "have to stop buffering" business.
Can we do better at reasonable cost, or are those paths just never
reached without huge data volume?  (Could we make them more reachable
by turning down maintenance_work_mem or some other setting?)

Please look at the improved test that makes the code coverage for gistbuildbuffers.c almost 100%.

Best regards,
Alexander