Physical replication always ships WAL segment files to replicas once they are complete. This is a problem if one WAL record is split across a segment boundary and the primary server crashes before writing down the segment with the next portion of the WAL record: WAL writing after crash recovery would happily resume at the point where the broken record started, overwriting that record ... but any standby or backup may have already received a copy of that segment, and they are not rewinding. This causes standbys to stop following the primary after the latter crashes: LOG: invalid contrecord length 7262 at A8/D9FFFBC8 because the standby is still trying to read the continuation record (contrecord) for the original long WAL record, but it is not there and it will never be. A workaround is to stop the replica, delete the WAL file, and restart it -- at which point a fresh copy is brought over from the primary. But that's pretty labor intensive, and I bet many users would just give up and re-clone the standby instead.
A fix for this problem was already attempted in commit 515e3d84a0b5, but it only addressed the case for the scenario of WAL archiving, so streaming replication would still be a problem (as well as other things such as taking a filesystem-level backup while the server is down after having crashed), and it had performance scalability problems too; so it had to be reverted.
This commit fixes the problem using an approach suggested by Andres Freund, whereby the initial portion(s) of the split-up WAL record are kept, and a special type of WAL record is written where the contrecord was lost, so that WAL replay in the replica knows to skip the broken parts. With this approach, we can continue to stream/archive segment files as soon as they are complete, and replay of the broken records will proceed across the crash point without a hitch.
Because a new type of WAL record is added, users should be careful to upgrade standbys first, primaries later. Otherwise they risk the standby being unable to start if the primary happens to write such a record.
A new TAP test that exercises this is added, but the portability of it is yet to be seen.
This has been wrong since the introduction of physical replication, so backpatch all the way back. In stable branches, keep the new XLogReaderState members at the end of the struct, to avoid an ABI break.
Author: Álvaro Herrera
ff9f111bce Fix WAL replay in presence of an incomplete record
src/backend/access/rmgrdesc/xlogdesc.c | 12 ++
src/backend/access/transam/xlog.c | 154 +++++++++++++++++-
src/backend/access/transam/xlogreader.c | 40 ++++-
src/include/access/xlog_internal.h | 11 +-
src/include/access/xlogreader.h | 10 ++
src/include/catalog/pg_control.h | 2 +
src/test/recovery/t/026_overwrite_contrecord.pl | 207 ++++++++++++++++++++++++
src/test/recovery/t/idiosyncratic_copy | 20 +++
src/tools/pgindent/typedefs.list | 1 +
9 files changed, 450 insertions(+), 7 deletions(-)