Prevent long-term memory leakage in autovacuum launcher.

tglsfdc · tglsfdc · commit 1c1294be716d · 2022-08-31T16:23:35.000-04:00
get_database_list() failed to restore the caller's memory context, instead leaving current context set to TopMemoryContext which is how CommitTransactionCommand() leaves it. The callers both think they are using short-lived contexts, for the express purpose of not having to worry about cleaning up individual allocations. The net effect therefore is that supposedly short-lived allocations could accumulate indefinitely in the launcher's TopMemoryContext. Although this has been broken for a long time, it seems we didn't have any obvious memory leak here until v15's rearrangement of the stats logic. I (tgl) am not entirely convinced that there's no other leak at all, though, and we're surely at risk of adding one in future back-patched fixes. So back-patch to all supported branches, even though this may be only a latent bug in pre-v15. Reid Thompson Discussion: https://postgr.es/m/972a4e12b68b0f96db514777a150ceef7dcd2e0f.camel@crunchydata.com
diff --git a/src/backend/postmaster/autovacuum.c b/src/backend/postmaster/autovacuum.c
@@ -1941,6 +1941,9 @@ get_database_list(void)
 
 	CommitTransactionCommand();
 
+	/* Be sure to restore caller's memory context */
+	MemoryContextSwitchTo(resultcxt);
+
 	return dblist;
 }
 

Original file line number	Diff line number	Diff line change
`@@ -1941,6 +1941,9 @@ get_database_list(void)`
`1941`	`1941`
`1942`	`1942`	`CommitTransactionCommand();`
`1943`	`1943`
	`1944`	`+ /* Be sure to restore caller's memory context */`
	`1945`	`+ MemoryContextSwitchTo(resultcxt);`
	`1946`	`+`
`1944`	`1947`	`return dblist;`
`1945`	`1948`	`}`
`1946`	`1949`