Commit4fd05bb

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Fix deadlock in heap_compute_xid_horizon_for_tuples().

We can't call code that uses syscache while we hold buffer lockson a catalog relation. If passed such a relation, just fall backto the general effective_io_concurrency GUC rather than trying tolook up the containing tablespace's IO concurrency setting.We might find a better way to control prefetching in follow-upwork, but for now this is enough to avoid the deadlock introducedby commit558a916.Reviewed-by: Andres FreundDiagnosed-by: Peter GeogheganDiscussion:https://postgr.es/m/CA%2BhUKGLCwPF0S4Mk7S8qw%2BDK0Bq65LueN9rofAA3HHSYikW-Zw%40mail.gmail.comDiscussion:https://postgr.es/m/962831d8-c18d-180d-75fb-8b842e3a2742%40chrullrich.net

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Original file line number	Diff line number	Diff line change
`@@ -6976,8 +6976,15 @@ heap_compute_xid_horizon_for_tuples(Relation rel,`
`6976`	`6976`	`* more prefetching in this case, too. It may be that this formula is too`
`6977`	`6977`	`* simplistic, but at the moment there is no evidence of that or any idea`
`6978`	`6978`	`* about what would work better.`
	`6979`	`+ *`
	`6980`	`+ * Since the caller holds a buffer lock somewhere in rel, we'd better make`
	`6981`	`+ * sure that isn't a catalog relation before we call code that does`
	`6982`	`+ * syscache lookups, to avoid risk of deadlock.`
`6979`	`6983`	`*/`
`6980`		`-io_concurrency=get_tablespace_io_concurrency(rel->rd_rel->reltablespace);`
	`6984`	`+if (IsCatalogRelation(rel))`
	`6985`	`+io_concurrency=effective_io_concurrency;`
	`6986`	`+else`
	`6987`	`+io_concurrency=get_tablespace_io_concurrency(rel->rd_rel->reltablespace);`
`6981`	`6988`	`prefetch_distance=Min((io_concurrency)+10,MAX_IO_CONCURRENCY);`
`6982`	`6989`
`6983`	`6990`	`/* Start prefetching. */`

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