pgsql: Pause recovery for insufficient parameter settings

Pause recovery for insufficient parameter settings

When certain parameters are changed on a physical replication primary,
this is communicated to standbys using the XLOG_PARAMETER_CHANGE WAL
record. The standby then checks whether its own settings are at least
as big as the ones on the primary. If not, the standby shuts down
with a fatal error.

This patch changes this behavior for hot standbys to pause recovery at
that point instead. That allows read traffic on the standby to
continue while database administrators figure out next steps. When
recovery is unpaused, the server shuts down (as before). The idea is
to fix the parameters while recovery is paused and then restart when
there is a maintenance window.

Reviewed-by: Sergei Kornilov <sk(at)zsrv(dot)org>
Discussion: https://www.postgresql.org/message-id/flat/4ad69a4c-cc9b-0dfe-0352-8b1b0cd36c7b(at)2ndquadrant(dot)com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/15251c0a60be76eedee74ac0e94b433f9acca5af

Modified Files
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doc/src/sgml/high-availability.sgml | 51 ++++++++++++++++++++++++--------
src/backend/access/transam/xlog.c | 59 +++++++++++++++++++++++++++++++++----
2 files changed, 93 insertions(+), 17 deletions(-)