Multiple status files were being created by asynchronous archiving if a high-level error occurred after one or more WAL segments had already been transferred successfully. Error files were being written for every file in the queue regardless of whether it had already succeeded. To fix this, add an option to skip writing error files when an ok file already exists.
There are other situations where both files might exist (various fsync and filesystem error scenarios) so it seems best to retry in the case that multiple status files are found rather than throwing a hard error (which then means that archiving is completely stuck). In the case of multiple status files, a warning will be logged to alert the user that something unusual is happening and the command will be retried.
Reported by fpa-postgres, Joe Ayers, Douglas J Hunley.
PostgreSQL 11 introduces configurable WAL segment sizes, from 1MB to 1GB.
There are two areas that needed to be updated to support this: building the archive-get queue and checking that WAL has been archived after a backup. Both operations require the WAL segment size to properly build a list.
Checking the archive after a backup is still implemented in Perl and has an active database connection, so just get the WAL segment size from the database.
The archive-get command does not have a connection to the database, so get the WAL segment size from pg_control instead. This requires a deeper inspection of pg_control than has been done in the past, so it seemed best to copy the relevant data structures from each version of PostgreSQL and build a generic interface layer to address them. While this approach is a bit verbose, it has the advantage of being relatively simple, and can easily be updated for new versions of PostgreSQL.
Since the integration tests generate pg_control files for testing, teach Perl how to generate files with the correct offsets for both 32-bit and 64-bit architectures.
