Implement pg_wal_replay_wait() stored procedure

pg_wal_replay_wait() is to be used on standby and specifies waiting for
the specific WAL location to be replayed.  This option is useful when
the user makes some data changes on primary and needs a guarantee to see
these changes are on standby.

The queue of waiters is stored in the shared memory as an LSN-ordered pairing
heap, where the waiter with the nearest LSN stays on the top.  During
the replay of WAL, waiters whose LSNs have already been replayed are deleted
from the shared memory pairing heap and woken up by setting their latches.

pg_wal_replay_wait() needs to wait without any snapshot held.  Otherwise,
the snapshot could prevent the replay of WAL records, implying a kind of
self-deadlock.  This is why it is only possible to implement
pg_wal_replay_wait() as a procedure working without an active snapshot,
not a function.

Catversion is bumped.

Discussion: https://postgr.es/m/eb12f9b03851bb2583adab5df9579b4b%40postgrespro.ru
Author: Kartyshov Ivan, Alexander Korotkov
Reviewed-by: Michael Paquier, Peter Eisentraut, Dilip Kumar, Amit Kapila
Reviewed-by: Alexander Lakhin, Bharath Rupireddy, Euler Taveira
Reviewed-by: Heikki Linnakangas, Kyotaro Horiguchi

1 files changed, 2 insertions, 0 deletions

diff --git a/src/backend/utils/activity/wait_event_names.txt b/src/backend/utils/activity/wait_event_names.txt
index db37beeaae6..d10ca723dc8 100644
--- a/src/backend/utils/activity/wait_event_names.txt
+++ b/src/backend/utils/activity/wait_event_names.txt
@@ -87,6 +87,7 @@ LIBPQWALRECEIVER_CONNECT	"Waiting in WAL receiver to establish connection to rem
 LIBPQWALRECEIVER_RECEIVE	"Waiting in WAL receiver to receive data from remote server."
 SSL_OPEN_SERVER	"Waiting for SSL while attempting connection."
 WAIT_FOR_STANDBY_CONFIRMATION	"Waiting for WAL to be received and flushed by the physical standby."
+WAIT_FOR_WAL_REPLAY	"Waiting for a replay of the particular WAL position on the physical standby."
 WAL_SENDER_WAIT_FOR_WAL	"Waiting for WAL to be flushed in WAL sender process."
 WAL_SENDER_WRITE_DATA	"Waiting for any activity when processing replies from WAL receiver in WAL sender process."
 
@@ -345,6 +346,7 @@ WALSummarizer	"Waiting to read or update WAL summarization state."
 DSMRegistry	"Waiting to read or update the dynamic shared memory registry."
 InjectionPoint	"Waiting to read or update information related to injection points."
 SerialControl	"Waiting to read or update shared <filename>pg_serial</filename> state."
+WaitLSN	"Waiting to read or update shared Wait-for-LSN state."
 
 #
 # END OF PREDEFINED LWLOCKS (DO NOT CHANGE THIS LINE)