pgsql: Avoid scribbling of VACUUM options

Avoid scribbling of VACUUM options

This fixes two issues with the handling of VacuumParams in vacuum_rel().
This code path has the idea to change the passed-in pointer of
VacuumParams for the "truncate" and "index_cleanup" options for the
relation worked on, impacting the two following scenarios where
incorrect options may be used because a VacuumParams pointer is shared
across multiple relations:
- Multiple relations in a single VACUUM command.
- TOAST relations vacuumed with their main relation.

The problem is avoided by providing to the two callers of vacuum_rel()
copies of VacuumParams, before the pointer is updated for the "truncate"
and "index_cleanup" options.

The refactoring of the VACUUM option and parameters done in 0d831389749a
did not introduce an issue, but it has encouraged the problem we are
dealing with in this commit, with b84dbc8eb80b for "truncate" and
a96c41feec6b for "index_cleanup" that have been added a couple of years
after the initial refactoring. HEAD will be improved with a different
patch that hardens the uses of VacuumParams across the tree. This
cannot be backpatched as it introduces an ABI breakage.

The backend portion of the patch has been authored by Nathan, while I
have implemented the tests. The tests rely on injection points to check
the option values, making them faster, more reliable than the tests
originally proposed by Shihao, and they also provide more coverage.
This part can only be backpatched down to v17.

Reported-by: Shihao Zhong <zhong950419(at)gmail(dot)com>
Author: Nathan Bossart <nathandbossart(at)gmail(dot)com>
Co-authored-by: Michael Paquier <michael(at)paquier(dot)xyz>
Discussion: https://postgr.es/m/CAGRkXqTo+aK=GTy5pSc-9cy8H2F2TJvcrZ-zXEiNJj93np1UUw@mail.gmail.com
Backpatch-through: 13

Branch
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REL_15_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/354944663df38a360f8519814658a10cb5c3d04a

Modified Files
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src/backend/commands/vacuum.c | 18 ++++++++++++++++--
1 file changed, 16 insertions(+), 2 deletions(-)