Lazy allocation of pages required for verifying FPI consistency

Hi,

Postgres verifies consistency of FPI from WAL record with the replayed
page during recovery in verifyBackupPageConsistency() when either
wal_consistency_checking for the resource manager is enabled or a WAL
record with XLR_CHECK_CONSISTENCY flag is inserted. While doing so, it
uses two intermediate pages primary_image_masked (FPI from WAL record)
and replay_image_masked (replayed page) which are dynamically
allocated (palloc'd) before the recovery starts, however, they're not
used unless verifyBackupPageConsistency() is called. And these
variables are palloc'd here for getting MAXALIGNed memory as opposed
to static char arrays. Since verifyBackupPageConsistency() gets called
conditionally only when the WAL record has the XLR_CHECK_CONSISTENCY
flag set, it's a waste of memory for these two page variables.

I propose to statically allocate these two pages using PGAlignedBlock
structure lazily in verifyBackupPageConsistency() to not waste dynamic
memory worth 2*BLCKSZ bytes. I'm attaching a small patch herewith.

Thoughts?

--
Bharath Rupireddy
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com