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@gmail.com>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Reviewed-by: Michael Paquier <michael@paquier.xyz>
Reviewed-by: Xuneng Zhou <xunengzhou@gmail.com>
Discussion: https://postgr.es/m/19006-80fcaaf69000377e%40postgresql.org

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/b421223172a28db2e724d5e35304097fe68a1e38

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