Fix a race condition in updating procArray->replication_slot_xmin.HEADmaster

Previously, ReplicationSlotsComputeRequiredXmin() computed the oldest
xmin across all slots without holding ProcArrayLock (when
already_locked is false), acquiring the lock just before updating the
replication slot xmin.

This could lead to a race condition: if a backend created a new slot
and updates the global replication slot xmin, another backend
concurrently running ReplicationSlotsComputeRequiredXmin() could
overwrite that update with an invalid or stale value. This happens
because the concurrent backend might have computed the aggregate xmin
before the new slot was accounted for, but applied the update after
the new slot had already updated the global value.

In the reported failure, a walsender for an apply worker computed
InvalidTransactionId as the oldest xmin and overwrote a valid
replication slot xmin value computed by a walsender for a tablesync
worker. Consequently, the tablesync worker computed a transaction ID
via GetOldestSafeDecodingTransactionId() effectively without
considering the replication slot xmin. This led to the error "cannot
build an initial slot snapshot as oldest safe xid %u follows
snapshot's xmin %u", which was an assertion failure prior to commit
240e0dbacd3.

To fix this, we acquire ReplicationSlotControlLock in exclusive mode
during slot creation to perform the initial update of the slot
xmin. In ReplicationSlotsComputeRequiredXmin(), we hold
ReplicationSlotControlLock in shared mode until the global slot xmin
is updated in ProcArraySetReplicationSlotXmin(). This prevents
concurrent computations and updates of the global xmin by other
backends during the initial slot xmin update process, while still
permitting concurrent calls to ReplicationSlotsComputeRequiredXmin().

Backpatch to all supported versions.

Author: Zhijie Hou <houzj.fnst@fujitsu.com>
Reviewed-by: Masahiko Sawada <sawada.mshk@gmail.com>
Reviewed-by: Amit Kapila <amit.kapila16@gmail.com>
Reviewed-by: Pradeep Kumar <spradeepkumar29@gmail.com>
Reviewed-by: Hayato Kuroda (Fujitsu) <kuroda.hayato@fujitsu.com>
Reviewed-by: Robert Haas <robertmhaas@gmail.com>
Reviewed-by: Andres Freund <andres@anarazel.de>
Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Discussion: https://postgr.es/m/CAA4eK1L8wYcyTPxNzPGkhuO52WBGoOZbT0A73Le=ZUWYAYmdfw@mail.gmail.com
Backpatch-through: 14

0 files changed, 0 insertions, 0 deletions