pg_fix_truncation
=================

PostgreSQL versions up to 9.6.0, 9.5.4, 9.4.9 and 9.3.14 may be subject
to corruptions in the file-space map file associated to a relation on
a promoted standby, preventing the insertion of new data in a relation
in such a case.

Detecting corrupted FSM is possible by using the in-core extension module
pg_freespacemap() to see if there is any reference to blocks that have been
already truncated in a relation.  Removing those FSM files requires however
manual handling of the file deletion while the server is stopped.  This
extension provides a way to truncate the FSM to its right size, at the cost
of taking an exclusive lock on the relation while this is happening. This
is still less expensive than having to stop momentarily a server for the
duration of the FSM, as little as it may be.

See upstream commit 917dc7d for more details on the matter.