The error message incorrect/deceptive. It was written to account for BagOStuff implementations that lack an deleteObjectsExpiringBefore implementation, thus returning false from the method's base class stub. This would eg. been seen by developers locally or third-parties if they set ParserCache to something that already has its own expiry mechanism (not Sql-based).

However, the current SqlBagOStuff::deleteObjectsExpiringBefore also returns false if it any of the db connection or db write/delete operations failed.

	public function deleteObjectsExpiringBefore(…) {
		…

		$ok = true;

		$keysDeletedCount = 0;
		foreach ( $shardIndexes as $numServersDone => $shardIndex ) {
			$db = null; // …
			try {
				$db = $this->getConnection( $shardIndex );
				$this->deleteServerObjectsExpiringBefore(
					…
				);
			} catch ( DBError $e ) {
				$this->handleWriteError( $e, $db, $shardIndex );
				$ok = false;
			}
		}

		return $ok;
	}

So next steps:

• What are the errors encountered by this script? They should be in Logstash, to be correlated by hostname and rough timestamp, and then confirmed by cli_argv to confirm its from purgeParserCache.php. Probably one of the exception/error/DBConnection/DBQuery channels.
• Are they tolerable?
  • If so, update deleteObjectsExpiringBefore to not return false for those cases.
  • Do we need to monitor this by other means instead? (e.g something less boolean)
• Are they not tolerable, and e.g. warrant some kind of fix?
  • Yay for having been alerted to it, and let's fix it :)

@AMooney: Assuming that "Set projects" was accidentally used instead of "Add projects", hence restoring some previous project tags.