Fix deadlock in heap_compute_xid_horizon_for_tuples().

We can't call code that uses syscache while we hold buffer locks
on a catalog relation.  If passed such a relation, just fall back
to the general effective_io_concurrency GUC rather than trying to
look up the containing tablespace's IO concurrency setting.

We might find a better way to control prefetching in follow-up
work, but for now this is enough to avoid the deadlock introduced
by commit 558a9165e0.

Reviewed-by: Andres Freund
Diagnosed-by: Peter Geoghegan
Discussion: https://postgr.es/m/CA%2BhUKGLCwPF0S4Mk7S8qw%2BDK0Bq65LueN9rofAA3HHSYikW-Zw%40mail.gmail.com
Discussion: https://postgr.es/m/962831d8-c18d-180d-75fb-8b842e3a2742%40chrullrich.net

diff --git a/src/backend/access/heap/heapam.c b/src/backend/access/heap/heapam.c
index de5bb9194e34d5f613fe8a694f29cb9bedd97207..05ceb6550d5e0718449cdb0d639a5460b749fead 100644 (file)
--- a/src/backend/access/heap/heapam.c
+++ b/src/backend/access/heap/heapam.c
@@ -6976,8 +6976,15 @@ heap_compute_xid_horizon_for_tuples(Relation rel,
         * more prefetching in this case, too. It may be that this formula is too
         * simplistic, but at the moment there is no evidence of that or any idea
         * about what would work better.
+        *
+        * Since the caller holds a buffer lock somewhere in rel, we'd better make
+        * sure that isn't a catalog relation before we call code that does
+        * syscache lookups, to avoid risk of deadlock.
         */
-       io_concurrency = get_tablespace_io_concurrency(rel->rd_rel->reltablespace);
+       if (IsCatalogRelation(rel))
+               io_concurrency = effective_io_concurrency;
+       else
+               io_concurrency = get_tablespace_io_concurrency(rel->rd_rel->reltablespace);
        prefetch_distance = Min((io_concurrency) + 10, MAX_IO_CONCURRENCY);
 
        /* Start prefetching. */