Re: Improve monitoring of shared memory allocations

Hi,

Analysis of the Bug
<https://www.postgresql.org/message-id/3d36f787-8ba5-4aa2-abb1-7ade129a7b0e%40vondra.me>
in 0002 reported by David Rowley :
The 0001* patch allocates memory for the hash header, directory, segments,
and elements collectively for both shared and non-shared hash tables. While
this approach works well for shared hash tables, it presents an issue for
non-
shared hash tables. Specifically, during the expand_table() process,
non-shared hash tables may reallocate a new directory and free the old one.
Since the directory's memory is no longer allocated individually, it cannot
be freed
separately. This results in the following error:

ERROR: pfree called with invalid pointer 0x60a15edc44e0 (header
0x0000002000000008)

These allocations are done together to improve reporting of shared memory
allocations
for shared hash tables. Similar change is done for non-shared hash tables
only to maintain
consistency since hash_create code is shared between both types of hash
tables.

One solution could be separating allocation of directory from rest of the
allocations for
the non-shared hash tables, but this approach would undermine the purpose
of
doing the change for a non-shared hash table.

A better/safer solution would be to do this change only for shared hash
tables and
exclude the non-shared hash tables.

I believe it's acceptable to allocate everything in a single block provided
we are not trying
to individually free any of these, which we do only for the directory
pointer in dir_realloc.
Additional segment allocation goes through seg_alloc and element
allocations through
element_alloc, which do not free existing chunks but instead allocate new
ones with
pointers in existing directories and segments.
Thus, as long as we don't reallocate the directory, which we avoid in the
case of shared
hash tables, it should be safe to proceed with this change.

Please find attached the patch which removes the changes for non-shared
hash tables
and keeps them for shared hash tables.

I tested this by running make-check, make-check world and the reproducer
script shared
by David. I also ran pgbench to test creation and expansion of some of the
shared hash tables.

Thank you,
Rahila Syed