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Use of SnapshotNow is known to expose us to race conditions if the tuple(s) being sought could be updated by concurrently-committing transactions. CREATE DATABASE and DROP DATABASE are particularly exposed because they do heavyweight filesystem operations during their scans of pg_tablespace, so that the scans run for a very long time compared to most. Furthermore, the potential consequences of a missed or twice-visited row are nastier than average: * createdb() could fail with a bogus "file already exists" error, or silently fail to copy one or more tablespace's worth of files into the new database. * remove_dbtablespaces() could miss one or more tablespaces, thus failing to free filesystem space for the dropped database. * check_db_file_conflict() could likewise miss a tablespace, leading to an OID conflict that could result in data loss either immediately or in future operations. (This seems of very low probability, though, since a duplicate database OID would be unlikely to start with.) Hence, it seems worth fixing these three places to use MVCC snapshots, even though this will someday be superseded by a generic solution to SnapshotNow race conditions. Back-patch to all active branches. Stephen Frost and Tom Lane |
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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. Changes between all PostgreSQL releases are recorded in the file HISTORY. 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/.