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A cascading standby could fail to reconnect to its upstream standby with
"requested starting point ... is ahead of the WAL flush position" after
falling back to archive recovery. This happened because archive
recovery processes whole segment files, so after replaying a segment the
cascade's next read position lands at the start of the following
segment, which is ahead of the upstream's flush position reported by
GetStandbyFlushRecPtr() (still inside the just-replayed segment).
Fix by having the walreceiver check the upstream's current WAL flush
position via IDENTIFY_SYSTEM before issuing START_REPLICATION.
IDENTIFY_SYSTEM already returns this position (as xlogpos), but
walrcv_identify_system() previously discarded it; now we have a use for
it. If the requested start point exceeds the upstream's flush position
on the same timeline, the walreceiver waits for
wal_retrieve_retry_interval and retries.
The wait is limited to gaps of at most one WAL segment, which is the
expected case from the segment-granularity of archive recovery. Larger
gaps indicate the upstream is genuinely behind, so START_REPLICATION is
allowed to proceed (and fail) normally, letting the startup process fall
back to other WAL sources. The first wait is logged at LOG level;
subsequent waits are demoted to DEBUG1 to avoid log noise. The
walreceiver honors wal_receiver_timeout during the wait, so it will exit
if the upstream doesn't catch up in time.
To preserve ABI compatibility on back branches, the flush position from
IDENTIFY_SYSTEM is communicated via a new global variable
(WalRcvIdentifySystemLsn) rather than changing the signature of
walrcv_identify_system().
The bug was introduced in Postgres 9.3 by commit abfd192b1b, which
added a flush-position check in StartReplication() that rejects requests
ahead of the upstream server's WAL flush position.
Author: Marco Nenciarini <marco.nenciarini@enterprisedb.com>
Reviewed-by: Xuneng Zhou <xunengzhou@gmail.com>
Backpatch-through: 14
Discussion: https://postgr.es/m/CA+nrD2cTuTkkX5WXVZengTYYZbAO6zV8K+Tri-R0fbLFuoyMBA@mail.gmail.com
PostgreSQL Database Management System ===================================== This directory contains the source code distribution of the PostgreSQL database management system. PostgreSQL is an advanced object-relational database management system that supports an extended subset of the SQL standard, including transactions, foreign keys, subqueries, triggers, user-defined types and functions. This distribution also contains C language bindings. PostgreSQL has many language interfaces, many of which are listed here: https://www.postgresql.org/download/ See the file INSTALL for instructions on how to build and install PostgreSQL. That file also lists supported operating systems and hardware platforms and contains information regarding any other software packages that are required to build or run the PostgreSQL system. Copyright and license information can be found in the file COPYRIGHT. A comprehensive documentation set is included in this distribution; it can be read as described in the installation instructions. The latest version of this software may be obtained at https://www.postgresql.org/download/. For more information look at our web site located at https://www.postgresql.org/.