Change hashscan.c to keep its list of active hash index scans in

TopMemoryContext, rather than scattered through executor per-query contexts.
This poses no danger of memory leak since the ResourceOwner mechanism
guarantees release of no-longer-needed items.  It is needed because the
per-query context might already be released by the time we try to clean up
the hash scan list.  Report by ykhuang, diagnosis by Heikki.

Back-patch to 8.0, where the ResourceOwner-based cleanup was introduced.
The given test case does not fail before 8.2, probably because we rearranged
transaction abort processing somehow; but this coding is undoubtedly risky
so I'll patch 8.0 and 8.1 anyway.

src/backend/access/hash/hashscan.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/access/hash/hashscan.c,v 1.43 2008/01/01 19:45:46 momjian Exp $
+ *	  $PostgreSQL: pgsql/src/backend/access/hash/hashscan.c,v 1.44 2008/03/07 15:59:03 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -16,9 +16,20 @@
 #include "postgres.h"
 
 #include "access/hash.h"
+#include "utils/memutils.h"
 #include "utils/resowner.h"
 
 
+/*
+ * We track all of a backend's active scans on hash indexes using a list
+ * of HashScanListData structs, which are allocated in TopMemoryContext.
+ * It's okay to use a long-lived context because we rely on the ResourceOwner
+ * mechanism to clean up unused entries after transaction or subtransaction
+ * abort.  We can't safely keep the entries in the executor's per-query
+ * context, because that might be already freed before we get a chance to
+ * clean up the list.  (XXX seems like there should be a better way to
+ * manage this...)
+ */
 typedef struct HashScanListData
 {
 	IndexScanDesc hashsl_scan;
@@ -44,6 +55,11 @@ ReleaseResources_hash(void)
 	HashScanList next;
 
 	/*
+	 * Release all HashScanList items belonging to the current ResourceOwner.
+	 * Note that we do not release the underlying IndexScanDesc; that's in
+	 * executor memory and will go away on its own (in fact quite possibly
+	 * has gone away already, so we mustn't try to touch it here).
+	 *
 	 * Note: this should be a no-op during normal query shutdown. However, in
 	 * an abort situation ExecutorEnd is not called and so there may be open
 	 * index scans to clean up.
@@ -69,14 +85,15 @@ ReleaseResources_hash(void)
 }
 
 /*
- *	_Hash_regscan() -- register a new scan.
+ *	_hash_regscan() -- register a new scan.
  */
 void
 _hash_regscan(IndexScanDesc scan)
 {
 	HashScanList new_el;
 
-	new_el = (HashScanList) palloc(sizeof(HashScanListData));
+	new_el = (HashScanList) MemoryContextAlloc(TopMemoryContext,
+											   sizeof(HashScanListData));
 	new_el->hashsl_scan = scan;
 	new_el->hashsl_owner = CurrentResourceOwner;
 	new_el->hashsl_next = HashScans;