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Defer restoration of libraries in parallel workers.

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Several users of extensions complained of crashes in parallel workers
that turned out to be due to syscache access from their _PG_init()
functions.  Reorder the initialization of parallel workers so that
libraries are restored after the caches are initialized, and inside a
transaction.

This was reported in bug #15350 and elsewhere.  We don't consider it
to be a bug: extensions shouldn't do that, because then they can't be
used in shared_preload_libraries.  However, it's a fairly obscure
hazard and these extensions worked in practice before parallel query
came along.  So let's make it work.  Later commits might add a warning
message and eventually an error.

Back-patch to 9.6, where parallel query landed.

Author: Thomas Munro
Reviewed-by: Amit Kapila
Reported-by: Kieran McCusker, Jimmy
Discussion: https://postgr.es/m/153512195228.1489.8545997741965926448%40wrigleys.postgresql.org
This commit is contained in:
Thomas Munro 2018-09-20 14:02:40 +12:00
parent 40cfe86068
commit 6c3c9d4189
1 changed files with 9 additions and 9 deletions
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18
src/backend/access/transam/parallel.c
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@ -1311,14 +1311,6 @@ ParallelWorkerMain(Datum main_arg)
fps->parallel_master_pid)) fps->parallel_master_pid))
return; return;
/*
* Load libraries that were loaded by original backend. We want to do
* this before restoring GUCs, because the libraries might define custom
* variables.
*/
libraryspace = shm_toc_lookup(toc, PARALLEL_KEY_LIBRARY, false);
RestoreLibraryState(libraryspace);
/* /*
* Identify the entry point to be called. In theory this could result in * Identify the entry point to be called. In theory this could result in
* loading an additional library, though most likely the entry point is in * loading an additional library, though most likely the entry point is in
@ -1341,9 +1333,17 @@ ParallelWorkerMain(Datum main_arg)
*/ */
SetClientEncoding(GetDatabaseEncoding()); SetClientEncoding(GetDatabaseEncoding());
/*
* Load libraries that were loaded by original backend. We want to do
* this before restoring GUCs, because the libraries might define custom
* variables.
*/
libraryspace = shm_toc_lookup(toc, PARALLEL_KEY_LIBRARY, false);
StartTransactionCommand();
RestoreLibraryState(libraryspace);
/* Restore GUC values from launching backend. */ /* Restore GUC values from launching backend. */
gucspace = shm_toc_lookup(toc, PARALLEL_KEY_GUC, false); gucspace = shm_toc_lookup(toc, PARALLEL_KEY_GUC, false);
StartTransactionCommand();
RestoreGUCState(gucspace); RestoreGUCState(gucspace);
CommitTransactionCommand(); CommitTransactionCommand();