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Defer restoration of libraries in parallel workers.
2018年9月20日 02:02:40 +0000 (14:02 +1200)
2018年9月20日 02:21:18 +0000 (14:21 +1200)
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


diff --git a/src/backend/access/transam/parallel.c b/src/backend/access/transam/parallel.c
index c1681184670de99ccd81a552001a732d699d67a2..cdaa32e29a487281e91d6068dda4c750fdadb89a 100644 (file)
--- a/src/backend/access/transam/parallel.c
+++ b/src/backend/access/transam/parallel.c
@@ -1311,14 +1311,6 @@ ParallelWorkerMain(Datum main_arg)
fps->parallel_master_pid))
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
* loading an additional library, though most likely the entry point is in
@@ -1341,9 +1333,17 @@ ParallelWorkerMain(Datum main_arg)
*/
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. */
gucspace = shm_toc_lookup(toc, PARALLEL_KEY_GUC, false);
- StartTransactionCommand();
RestoreGUCState(gucspace);
CommitTransactionCommand();
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