pgsql: Fix rare bug in read_stream.c's split IO handling.

Fix rare bug in read_stream.c's split IO handling.

The internal queue of buffers could become corrupted in a rare edge case
that failed to invalidate an entry, causing a stale buffer to be
"forwarded" to StartReadBuffers(). This is a simple fix for the
immediate problem.

A small API change might be able to remove this and related fragility
entirely, but that will have to wait a bit.

Defect in commit ed0b87ca.

Bug: 19006
Backpatch-through: 18
Reported-by: Alexander Lakhin <exclusion(at)gmail(dot)com>
Reviewed-by: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Reviewed-by: Michael Paquier <michael(at)paquier(dot)xyz>
Reviewed-by: Xuneng Zhou <xunengzhou(at)gmail(dot)com>
Discussion: https://postgr.es/m/19006-80fcaaf69000377e%40postgresql.org

Branch
------
REL_18_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/9110d81641696b95ea55f3a15dd0907ec0743529

Modified Files
--------------
src/backend/storage/aio/read_stream.c | 34 ++++++++++++++++++++++++++++++++++
1 file changed, 34 insertions(+)