[PGPRO-4074] Multimaster and temp tables.

1) Allow on request to prepare xacts who manipulated temp tables.

We don't have any special handling of related problems, but they don't seem to
be too troubling.

2) Disarm autovacuum from dropping mtm temp tables on receiver side.

3) Remove assert that temp nsp is not set during SetTempNamespaceState (also
   for pooler).

4) Export on_commits.

Cherry pick of

commit 48de7232ea2eb580c4c349c23245df86d0c08a08
Author: Stas Kelvich <stanconn@gmail.com>
Date:   Tue Jan 21 19:37:23 2020 +0300

    [MM-SUPPORT] rework temp relations handling

commit 25ba3642e66b8e814d500ab5c28854644ffbcdd0
Author: Alexey Kondratov <kondratov.aleksey@gmail.com>
Date:   Fri May 22 19:31:16 2020 +0300

    [MM-SUPPORT] Compatibility with d7684c38a5e: save temp tables in MM from
    autovacuum

commit d09da98271d0df71968fa0d8cacf574ad360a54a
Author: Stas Kelvich <stanconn@gmail.com>
Date:   Tue Mar 3 13:05:41 2020 +0300

    do not try to estimate relation size for mm temp table at receiver side

(cherry picked from commit d09da98271d0df71968fa0d8cacf574ad360a54a)

tags: multimaster, connpool
(cherry picked from commit 3586623c83aa50629a12131a999fc0eb7764ef04)

Author: arssher

src/backend/access/transam/xact.c

@@ -118,6 +118,9 @@ TransactionId *ParallelCurrentXids;
  */
 int			MyXactFlags;
 
+/* MTM-CRUTCH: allow to prepare transactions with temp objects */
+bool		AllowTempIn2PC;
+
 /*
  *	transaction states - transaction state from server perspective
  */
@@ -2390,7 +2393,14 @@ PrepareTransaction(void)
 	 * cases, such as a temp table created and dropped all within the
 	 * transaction.  That seems to require much more bookkeeping though.
 	 */
-	if ((MyXactFlags & XACT_FLAGS_ACCESSEDTEMPNAMESPACE))
+
+	/*
+	 * MTM-CRUTCH: allow to proceed if requested via AllowTempIn2PC.
+	 * Note that for now we just ignore the problems stated above, there is
+	 * no special handling for that. Backend who can't exit until xact is
+	 * resolved is ok in mm: we can't do much while it hangs around anyway.
+	 */
+	if (!AllowTempIn2PC && (MyXactFlags & XACT_FLAGS_ACCESSEDTEMPNAMESPACE))
 		ereport(ERROR,
 				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
 				 errmsg("cannot PREPARE a transaction that has operated on temporary objects")));

src/backend/catalog/namespace.c

@@ -3224,6 +3224,27 @@ isOtherTempNamespace(Oid namespaceId)
 	return isAnyTempNamespace(namespaceId);
 }
 
+/*
+ * MTM-CRUTCH for temp table.
+ *
+ * From avtovacuum point of view temp schemas in multimaster
+ * receivers are not used. Setting proc->tempNamespaceId would not be enough
+ * since 'mtm_temp_' schemas can be moved between pool workers.
+ *
+ * XXX: add cleanup for crashed backends.
+ */
+bool
+isMtmTemp(Oid namespaceId)
+{
+	char	   *nspname;
+
+	nspname = get_namespace_name(namespaceId);
+	if (!nspname)
+		return false;
+
+	return strncmp(nspname, "mtm_tmp_", 8) == 0;
+}
+
 /*
  * checkTempNamespaceStatus - is the given namespace owned and actively used
  * by a backend?
@@ -3241,6 +3262,9 @@ checkTempNamespaceStatus(Oid namespaceId)
 
 	Assert(OidIsValid(MyDatabaseId));
 
+	if (isMtmTemp(namespaceId))
+		return TEMP_NAMESPACE_IN_USE;
+
 	backendId = GetTempNamespaceBackendId(namespaceId);
 
 	/* No such namespace, or its name shows it's not temp? */
@@ -3328,9 +3352,13 @@ GetTempNamespaceState(Oid *tempNamespaceId, Oid *tempToastNamespaceId)
 void
 SetTempNamespaceState(Oid tempNamespaceId, Oid tempToastNamespaceId)
 {
-	/* Worker should not have created its own namespaces ... */
-	Assert(myTempNamespace == InvalidOid);
-	Assert(myTempToastNamespace == InvalidOid);
+	/*
+	 * However in case of multimaster or pooler that is legit.
+	 * MTM-CRUTCH.
+	 *
+	 * Assert(myTempNamespace == InvalidOid);
+	 * Assert(myTempToastNamespace == InvalidOid);
+	 */
 	Assert(myTempNamespaceSubID == InvalidSubTransactionId);
 
 	/* Assign same namespace OIDs that leader has */

src/backend/optimizer/util/plancat.c

@@ -28,6 +28,7 @@
 #include "catalog/catalog.h"
 #include "catalog/dependency.h"
 #include "catalog/heap.h"
+#include "catalog/namespace.h"
 #include "catalog/pg_am.h"
 #include "catalog/pg_proc.h"
 #include "catalog/pg_statistic_ext.h"
@@ -212,6 +213,16 @@ get_relation_info(PlannerInfo *root, Oid relationObjectId, bool inhparent,
 				continue;
 			}
 
+			/*
+			 * MTM-CRUTCH: Ignore empty index for multimaster temp table on
+			 * receiver.
+			 */
+			if (isMtmTemp(RelationGetNamespace(indexRelation)))
+			{
+				index_close(indexRelation, NoLock);
+				continue;
+			}
+
 			/*
 			 * Ignore partitioned indexes, since they are not usable for
 			 * queries.

src/include/access/xact.h

@@ -95,6 +95,7 @@ extern int	synchronous_commit;
  * recording flags.
  */
 extern int	MyXactFlags;
+extern PGDLLIMPORT bool	AllowTempIn2PC;
 
 /*
  * XACT_FLAGS_ACCESSEDTEMPNAMESPACE - set when a temporary object is accessed.

src/include/catalog/namespace.h

@@ -185,4 +185,6 @@ extern char *namespace_search_path;
 extern List *fetch_search_path(bool includeImplicit);
 extern int	fetch_search_path_array(Oid *sarray, int sarray_len);
 
+extern bool isMtmTemp(Oid namespaceId);
+
 #endif							/* NAMESPACE_H */