pg_backup_and_purge: Properly preserve needed base backups.

Without `FIND_FULL`, `wal_g delete before ...` will fail, rather than
delete a base backup which is needed by the delta backups after it.
By passing `FIND_FULL`[^1], we tell it explicitly that we're OK
preserving files before the specified one, as long as they are
necessary for the delta chain.

[^1]: https://github.com/wal-g/wal-g/blob/master/docs/README.md#delete

puppet/zulip/files/postgresql/pg_backup_and_purge

@@ -73,7 +73,14 @@ for line in lines[1:]:
 one_month_ago = now - timedelta(days=30)
 for date in sorted(backups.keys(), reverse=True):
     if date < one_month_ago:
-        subprocess.check_call(["env-wal-g", "delete", "--confirm", "before", backups[date]])
+        # We pass `FIND_FULL` such that if delta backups are being
+        # used, we keep the prior FULL backup and all of the deltas
+        # that we need.  In practice, this means that if we're doing
+        # weekly full backups (`backups_incremental = 6`), we only
+        # delete any data once a week.
+        subprocess.check_call(
+            ["env-wal-g", "delete", "--confirm", "before", "FIND_FULL", backups[date]]
+        )
         # Because we're going from most recent to least recent, we
         # only have to do one delete operation
         break