Add support of dedicated backends

src/backend/access/transam/xact.c

@@ -84,6 +84,10 @@ bool		XactDeferrable;
 
 int			synchronous_commit = SYNCHRONOUS_COMMIT_ON;
 
+/* Backend needs to preserve session context */
+bool		is_dedicated_backend;
+bool		allow_dedicated_backends; /* GUC */
+
 /*
  * CheckXidAlive is a xid value pointing to a possibly ongoing (sub)
  * transaction.  Currently, it is used in logical decoding.  It's possible
@@ -4952,7 +4956,7 @@ TransactionBlockStatusCode(void)
 	{
 		case TBLOCK_DEFAULT:
 		case TBLOCK_STARTED:
-			return 'I';			/* idle --- not in transaction */
+			return is_dedicated_backend && allow_dedicated_backends ? 'T' : 'I';			/* idle --- not in transaction */
 		case TBLOCK_BEGIN:
 		case TBLOCK_SUBBEGIN:
 		case TBLOCK_INPROGRESS:

src/backend/commands/portalcmds.c

@@ -59,6 +59,9 @@ PerformCursorOpen(ParseState *pstate, DeclareCursorStmt *cstmt, ParamListInfo pa
 				(errcode(ERRCODE_INVALID_CURSOR_NAME),
 				 errmsg("invalid cursor name: must not be empty")));
 
+	if (cstmt->options & CURSOR_OPT_HOLD)
+		is_dedicated_backend = true; /* cursors are not compatible with builtin connection pooler */
+
 	/*
 	 * If this is a non-holdable cursor, we require that this statement has
 	 * been executed inside a transaction block (or else, it would have no

src/backend/commands/sequence.c

@@ -245,6 +245,19 @@ DefineSequence(ParseState *pstate, CreateSeqStmt *seq)
 	heap_freetuple(tuple);
 	table_close(rel, RowExclusiveLock);
 
+		/*
+	 * TODO:
+	 * Using currval() may cause incorrect behavior with connection pooler.
+	 * Unfortunately marking backend as tainted in currval() is too late.
+	 * This is why it is done in nextval(), although it is not strictly required, because
+	 * nextval() may be not followed by currval().
+	 * But currval() may be not preceded by nextval().
+	 * To make regression tests passed, backend is also marker as tainted when it creates
+	 * sequence. Certainly it is just temporary workaround, because sequence may be created
+	 * in one backend and accessed in another.
+	 */
+	is_dedicated_backend = true; /* in case of using currval() */
+
 	return address;
 }
 
@@ -611,6 +624,8 @@ nextval(PG_FUNCTION_ARGS)
 	 */
 	relid = RangeVarGetRelid(sequence, NoLock, false);
 
+	is_dedicated_backend = true; /* in case of using currval() */
+
 	PG_RETURN_INT64(nextval_internal(relid, true));
 }
 

src/backend/commands/tablecmds.c

@@ -733,6 +733,10 @@ DefineRelation(CreateStmt *stmt, char relkind, Oid ownerId,
 				(errcode(ERRCODE_INVALID_TABLE_DEFINITION),
 				 errmsg("ON COMMIT can only be used on temporary tables")));
 
+	if (stmt->relation->relpersistence == RELPERSISTENCE_TEMP
+		&& stmt->oncommit != ONCOMMIT_DROP)
+		is_dedicated_backend = true;
+
 	if (stmt->partspec != NULL)
 	{
 		if (relkind != RELKIND_RELATION)

src/backend/storage/lmgr/lock.c

@@ -822,7 +822,10 @@ LockAcquireExtended(const LOCKTAG *locktag,
 
 	/* Identify owner for lock */
 	if (sessionLock)
+	{
 		owner = NULL;
+		is_dedicated_backend = true;
+	}
 	else
 		owner = CurrentResourceOwner;
 

src/backend/utils/adt/lockfuncs.c

@@ -13,6 +13,7 @@
 #include "postgres.h"
 
 #include "access/htup_details.h"
+#include "access/xact.h"
 #include "funcapi.h"
 #include "miscadmin.h"
 #include "storage/predicate_internals.h"
@@ -611,12 +612,14 @@ pg_safe_snapshot_blocking_pids(PG_FUNCTION_ARGS)
  *	field4: 1 if using an int8 key, 2 if using 2 int4 keys
  */
 #define SET_LOCKTAG_INT64(tag, key64) \
+	is_dedicated_backend = true; \
 	SET_LOCKTAG_ADVISORY(tag, \
 						 MyDatabaseId, \
 						 (uint32) ((key64) >> 32), \
 						 (uint32) (key64), \
 						 1)
 #define SET_LOCKTAG_INT32(tag, key1, key2) \
+	is_dedicated_backend = true; \
 	SET_LOCKTAG_ADVISORY(tag, MyDatabaseId, key1, key2, 2)
 
 /*

src/backend/utils/init/postinit.c

@@ -1268,6 +1268,20 @@ InitPostgres(const char *in_dbname, Oid dboid,
 	/* close the transaction we started above */
 	if (!bootstrap)
 		CommitTransactionCommand();
+
+	/*
+	 * TODO: psql logic and prompt depends on status returned by ReadyForQuery message.
+	 * So the hack with enforcing dedicated backend by reporting in-transaction status doesn't
+	 * work in this case.
+	 * The solution can be to return some special status in ReadyForQuery message which
+	 * will be interpreted only by connection pooler but then mapped to idle ('I') when
+	 * forwarded to the client.
+	 */
+	if (application_name)
+	{
+		if (strcmp(application_name, "psql") == 0 || strncmp(application_name, "pg_regress", 10) == 0)
+			allow_dedicated_backends = false;
+	}
 }
 
 /*

src/backend/utils/misc/guc_funcs.c

@@ -53,6 +53,9 @@ ExecSetVariableStmt(VariableSetStmt *stmt, bool isTopLevel)
 				(errcode(ERRCODE_INVALID_TRANSACTION_STATE),
 				 errmsg("cannot set parameters during a parallel operation")));
 
+	if (!stmt->is_local)
+		is_dedicated_backend = true;
+
 	switch (stmt->kind)
 	{
 		case VAR_SET_VALUE:

src/backend/utils/misc/guc_tables.c

@@ -802,6 +802,18 @@ struct config_bool ConfigureNamesBool[] =
 		true,
 		NULL, NULL, NULL
 	},
+
+	{
+		{"allow_dedicated_backends", PGC_USERSET, CLIENT_CONN_OTHER,
+		 gettext_noop("Prevent connection pooler from reusing backends requiring session semantic."),
+			NULL,
+			GUC_EXPLAIN
+		},
+		&allow_dedicated_backends,
+		true,
+		NULL, NULL, NULL
+	},
+
 	{
 		{"enable_seqscan", PGC_USERSET, QUERY_TUNING_METHOD,
 			gettext_noop("Enables the planner's use of sequential-scan plans."),

src/include/access/xact.h

@@ -58,6 +58,9 @@ extern PGDLLIMPORT bool XactReadOnly;
 /* flag for logging statements in this transaction */
 extern PGDLLIMPORT bool xact_is_sampled;
 
+/* Backend needs to preserve session context */
+extern PGDLLIMPORT bool is_dedicated_backend;
+extern bool allow_dedicated_backends;
 /*
  * Xact is deferrable -- only meaningful (currently) for read only
  * SERIALIZABLE transactions