pgsql: Add CHECK_FOR_INTERRUPTS() in scram_SaltedPassword() for the bac

Add CHECK_FOR_INTERRUPTS() in scram_SaltedPassword() for the backend

scram_SaltedPassword() could take a long time to compute when the number
of iterations used is large enough, and this code uses a tight loop to
compute a salted password.

Note that the same issue exists in libpq when using \password and a
large iteration number, but this cannot be interrupted. A CFI in the
backend is useful for server-side computations, at least.

Backpatch down to 16, where the user-settable GUC scram_iterations has
been added.

Author: Bowen Shi
Reviewed-by: Aleksander Alekseev, Daniel Gustafsson
Discussion: https://postgr.es/m/CAM_vCueV6xfr08KczfaCEk5J_qeTZtgqN7+orkNLx=g+phE82Q@mail.gmail.com
Backpatch-through: 16

Branch
------
REL_16_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/07cb7bc1c7a88299e6bce281320397f07ca045c2

Modified Files
--------------
src/common/scram-common.c | 11 +++++++++++
1 file changed, 11 insertions(+)