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Avoid storing the result of PQsocket() in a pgsocket variable; it's declared as int, and the no-socket test is properly written as "x < 0" not "x == PGINVALID_SOCKET". This accidentally had no bad effect because we never got to init_slot() with a bad connection, but it's still wrong. Actually, it seems like we should avoid storing the result for a long period at all. The function's not so expensive that it's worth avoiding, and the existing coding technique here would fail if anyone tried to PQreset the connection during the life of the program. Hence, just re-call PQsocket every time we construct a select(2) mask. Speaking of select(), GetIdleSlot imagined that it could compute the select mask once and continue to use it over multiple calls to select_loop(), which is pretty bogus since that would stomp on the mask on return. This could only matter if the function's outer loop iterated more than once, which is unlikely (it'd take some connection receiving data, but not enough to complete its command). But if it did happen, we'd acquire "tunnel vision" and stop watching the other connections for query termination, with the effect of losing parallelism. Another way in which GetIdleSlot could lose parallelism is that once PQisBusy returns false, it would lock in on that connection and do PQgetResult until that returns NULL; in some cases that could result in blocking. (Perhaps this can never happen in vacuumdb due to the limited set of commands that it can issue, but I'm not quite sure of that, and even if true today it's not a future-proof assumption.) Refactor the code to do that properly, so that it risks blocking in PQgetResult only in cases where we need to wait anyway. Another loss-of-parallelism problem, which *is* easily demonstrable, is that any setup queries issued during prepare_vacuum_command() were always issued on the last-to-be-created connection, whether or not that was idle. Long-running operations on that connection thus prevented issuance of additional operations on the other ones, except in the limited cases where no preparatory query was needed. Instead, wait till we've identified a free connection and use that one. Also, avoid core dump due to undersized malloc request in the case that no tables are identified to be vacuumed. The bogus no-socket test was noted by CharSyam, the other problems identified in my own code review. Back-patch to 9.5 where parallel vacuumdb was introduced. Discussion: https://postgr.es/m/CAMrLSE6etb33-192DTEUGkV-TsvEcxtBDxGWG1tgNOMnQHwgDA@mail.gmail.com |
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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: https://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 https://www.postgresql.org/download/. For more information look at our web site located at https://www.postgresql.org/.