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While a new backend nominally participates in sinval signaling starting from the SharedInvalBackendInit call near the top of InitPostgres, it cannot recognize sinval messages for unshared catalogs of its database until it has set up MyDatabaseId. This is not problematic for the catcache or relcache, which by definition won't have loaded any data from or about such catalogs before that point. However, commit 568d4138c646cd7c introduced a mechanism for re-using MVCC snapshots for catalog scans, and made invalidation of those depend on recognizing relevant sinval messages. So it's possible to establish a catalog snapshot to read pg_authid and pg_database, then before we set MyDatabaseId, receive sinval messages that should result in invalidating that snapshot --- but do not, because we don't realize they are for our database. This mechanism explains the intermittent buildfarm failures we've seen since commit 31eae6028eca4365. That commit was not itself at fault, but it introduced a new regression test that does reconnections concurrently with the "vacuum full pg_am" command in vacuum.sql. This allowed the pre-existing error to be exposed, given just the right timing, because we'd fail to update our information about how to access pg_am. In principle any VACUUM FULL on a system catalog could have created a similar hazard for concurrent incoming connections. Perhaps there are more subtle failure cases as well. To fix, force invalidation of the catalog snapshot as soon as we've set MyDatabaseId. Back-patch to 9.4 where the error was introduced.
PostgreSQL Database Management System ===================================== This directory contains the source code distribution of the PostgreSQL database management system. PostgreSQL is an advanced object-relational database management system that supports an extended subset of the SQL standard, including transactions, foreign keys, subqueries, triggers, user-defined types and functions. This distribution also contains C language bindings. PostgreSQL has many language interfaces, many of which are listed here: http://www.postgresql.org/download See the file INSTALL for instructions on how to build and install PostgreSQL. That file also lists supported operating systems and hardware platforms and contains information regarding any other software packages that are required to build or run the PostgreSQL system. Copyright and license information can be found in the file COPYRIGHT. A comprehensive documentation set is included in this distribution; it can be read as described in the installation instructions. The latest version of this software may be obtained at http://www.postgresql.org/download/. For more information look at our web site located at http://www.postgresql.org/.