Transaction chaining

Add command variants COMMIT AND CHAIN and ROLLBACK AND CHAIN, which
start new transactions with the same transaction characteristics as the
just finished one, per SQL standard.

Support for transaction chaining in PL/pgSQL is also added.  This
functionality is especially useful when running COMMIT in a loop in
PL/pgSQL.

Reviewed-by: Fabien COELHO <coelho@cri.ensmp.fr>
Discussion: https://www.postgresql.org/message-id/flat/28536681-324b-10dc-ade8-ab46f7645a5a@2ndquadrant.com

src/backend/access/transam/xact.c

@@ -190,6 +190,7 @@ typedef struct TransactionStateData
 	bool		startedInRecovery;	/* did we start in recovery? */
 	bool		didLogXid;		/* has xid been included in WAL record? */
 	int			parallelModeLevel;	/* Enter/ExitParallelMode counter */
+	bool		chain;			/* start a new block after this one */
 	struct TransactionStateData *parent;	/* back link to parent */
 } TransactionStateData;
 
@@ -2775,6 +2776,36 @@ StartTransactionCommand(void)
 	MemoryContextSwitchTo(CurTransactionContext);
 }
 
+
+/*
+ * Simple system for saving and restoring transaction characteristics
+ * (isolation level, read only, deferrable).  We need this for transaction
+ * chaining, so that we can set the characteristics of the new transaction to
+ * be the same as the previous one.  (We need something like this because the
+ * GUC system resets the characteristics at transaction end, so for example
+ * just skipping the reset in StartTransaction() won't work.)
+ */
+static int	save_XactIsoLevel;
+static bool	save_XactReadOnly;
+static bool	save_XactDeferrable;
+
+void
+SaveTransactionCharacteristics(void)
+{
+	save_XactIsoLevel = XactIsoLevel;
+	save_XactReadOnly = XactReadOnly;
+	save_XactDeferrable = XactDeferrable;
+}
+
+void
+RestoreTransactionCharacteristics(void)
+{
+	XactIsoLevel = save_XactIsoLevel;
+	XactReadOnly = save_XactReadOnly;
+	XactDeferrable = save_XactDeferrable;
+}
+
+
 /*
  *	CommitTransactionCommand
  */
@@ -2783,6 +2814,9 @@ CommitTransactionCommand(void)
 {
 	TransactionState s = CurrentTransactionState;
 
+	if (s->chain)
+		SaveTransactionCharacteristics();
+
 	switch (s->blockState)
 	{
 			/*
@@ -2834,6 +2868,13 @@ CommitTransactionCommand(void)
 		case TBLOCK_END:
 			CommitTransaction();
 			s->blockState = TBLOCK_DEFAULT;
+			if (s->chain)
+			{
+				StartTransaction();
+				s->blockState = TBLOCK_INPROGRESS;
+				s->chain = false;
+				RestoreTransactionCharacteristics();
+			}
 			break;
 
 			/*
@@ -2853,6 +2894,13 @@ CommitTransactionCommand(void)
 		case TBLOCK_ABORT_END:
 			CleanupTransaction();
 			s->blockState = TBLOCK_DEFAULT;
+			if (s->chain)
+			{
+				StartTransaction();
+				s->blockState = TBLOCK_INPROGRESS;
+				s->chain = false;
+				RestoreTransactionCharacteristics();
+			}
 			break;
 
 			/*
@@ -2864,6 +2912,13 @@ CommitTransactionCommand(void)
 			AbortTransaction();
 			CleanupTransaction();
 			s->blockState = TBLOCK_DEFAULT;
+			if (s->chain)
+			{
+				StartTransaction();
+				s->blockState = TBLOCK_INPROGRESS;
+				s->chain = false;
+				RestoreTransactionCharacteristics();
+			}
 			break;
 
 			/*
@@ -3521,7 +3576,7 @@ PrepareTransactionBlock(const char *gid)
 	bool		result;
 
 	/* Set up to commit the current transaction */
-	result = EndTransactionBlock();
+	result = EndTransactionBlock(false);
 
 	/* If successful, change outer tblock state to PREPARE */
 	if (result)
@@ -3567,7 +3622,7 @@ PrepareTransactionBlock(const char *gid)
  * resource owner, etc while executing inside a Portal.
  */
 bool
-EndTransactionBlock(void)
+EndTransactionBlock(bool chain)
 {
 	TransactionState s = CurrentTransactionState;
 	bool		result = false;
@@ -3693,6 +3748,13 @@ EndTransactionBlock(void)
 			break;
 	}
 
+	Assert(s->blockState == TBLOCK_STARTED ||
+		   s->blockState == TBLOCK_END ||
+		   s->blockState == TBLOCK_ABORT_END ||
+		   s->blockState == TBLOCK_ABORT_PENDING);
+
+	s->chain = chain;
+
 	return result;
 }
 
@@ -3703,7 +3765,7 @@ EndTransactionBlock(void)
  * As above, we don't actually do anything here except change blockState.
  */
 void
-UserAbortTransactionBlock(void)
+UserAbortTransactionBlock(bool chain)
 {
 	TransactionState s = CurrentTransactionState;
 
@@ -3801,6 +3863,11 @@ UserAbortTransactionBlock(void)
 				 BlockStateAsString(s->blockState));
 			break;
 	}
+
+	Assert(s->blockState == TBLOCK_ABORT_END ||
+		   s->blockState == TBLOCK_ABORT_PENDING);
+
+	s->chain = chain;
 }
 
 /*

src/backend/catalog/sql_features.txt

@@ -443,7 +443,7 @@ T213	INSTEAD OF triggers			YES
 T231	Sensitive cursors			YES	
 T241	START TRANSACTION statement			YES	
 T251	SET TRANSACTION statement: LOCAL option			NO	
-T261	Chained transactions			NO	
+T261	Chained transactions			YES	
 T271	Savepoints			YES	
 T272	Enhanced savepoint management			NO	
 T281	SELECT privilege with column granularity			YES	

src/backend/executor/spi.c

@@ -217,8 +217,8 @@ SPI_start_transaction(void)
 	MemoryContextSwitchTo(oldcontext);
 }
 
-void
-SPI_commit(void)
+static void
+_SPI_commit(bool chain)
 {
 	MemoryContext oldcontext = CurrentMemoryContext;
 
@@ -250,14 +250,36 @@ SPI_commit(void)
 	while (ActiveSnapshotSet())
 		PopActiveSnapshot();
 
+	if (chain)
+		SaveTransactionCharacteristics();
+
 	CommitTransactionCommand();
+
+	if (chain)
+	{
+		StartTransactionCommand();
+		RestoreTransactionCharacteristics();
+	}
+
 	MemoryContextSwitchTo(oldcontext);
 
 	_SPI_current->internal_xact = false;
 }
 
 void
-SPI_rollback(void)
+SPI_commit(void)
+{
+	_SPI_commit(false);
+}
+
+void
+SPI_commit_and_chain(void)
+{
+	_SPI_commit(true);
+}
+
+static void
+_SPI_rollback(bool chain)
 {
 	MemoryContext oldcontext = CurrentMemoryContext;
 
@@ -274,12 +296,34 @@ SPI_rollback(void)
 
 	_SPI_current->internal_xact = true;
 
+	if (chain)
+		SaveTransactionCharacteristics();
+
 	AbortCurrentTransaction();
+
+	if (chain)
+	{
+		StartTransactionCommand();
+		RestoreTransactionCharacteristics();
+	}
+
 	MemoryContextSwitchTo(oldcontext);
 
 	_SPI_current->internal_xact = false;
 }
 
+void
+SPI_rollback(void)
+{
+	_SPI_rollback(false);
+}
+
+void
+SPI_rollback_and_chain(void)
+{
+	_SPI_rollback(true);
+}
+
 /*
  * Clean up SPI state.  Called on transaction end (of non-SPI-internal
  * transactions) and when returning to the main loop on error.

src/backend/nodes/copyfuncs.c

@@ -3666,6 +3666,7 @@ _copyTransactionStmt(const TransactionStmt *from)
 	COPY_NODE_FIELD(options);
 	COPY_STRING_FIELD(savepoint_name);
 	COPY_STRING_FIELD(gid);
+	COPY_SCALAR_FIELD(chain);
 
 	return newnode;
 }

src/backend/nodes/equalfuncs.c

@@ -1510,6 +1510,7 @@ _equalTransactionStmt(const TransactionStmt *a, const TransactionStmt *b)
 	COMPARE_NODE_FIELD(options);
 	COMPARE_STRING_FIELD(savepoint_name);
 	COMPARE_STRING_FIELD(gid);
+	COMPARE_SCALAR_FIELD(chain);
 
 	return true;
 }

src/backend/parser/gram.y

@@ -312,6 +312,7 @@ static Node *makeRecursiveViewSelect(char *relname, List *aliases, Node *query);
 %type <boolean>	opt_or_replace
 				opt_grant_grant_option opt_grant_admin_option
 				opt_nowait opt_if_exists opt_with_data
+				opt_transaction_chain
 %type <ival>	opt_nowait_or_skip
 
 %type <list>	OptRoleList AlterOptRoleList
@@ -9792,11 +9793,12 @@ UnlistenStmt:
  *****************************************************************************/
 
 TransactionStmt:
-			ABORT_P opt_transaction
+			ABORT_P opt_transaction opt_transaction_chain
 				{
 					TransactionStmt *n = makeNode(TransactionStmt);
 					n->kind = TRANS_STMT_ROLLBACK;
 					n->options = NIL;
+					n->chain = $3;
 					$$ = (Node *)n;
 				}
 			| BEGIN_P opt_transaction transaction_mode_list_or_empty
@@ -9813,25 +9815,28 @@ TransactionStmt:
 					n->options = $3;
 					$$ = (Node *)n;
 				}
-			| COMMIT opt_transaction
+			| COMMIT opt_transaction opt_transaction_chain
 				{
 					TransactionStmt *n = makeNode(TransactionStmt);
 					n->kind = TRANS_STMT_COMMIT;
 					n->options = NIL;
+					n->chain = $3;
 					$$ = (Node *)n;
 				}
-			| END_P opt_transaction
+			| END_P opt_transaction opt_transaction_chain
 				{
 					TransactionStmt *n = makeNode(TransactionStmt);
 					n->kind = TRANS_STMT_COMMIT;
 					n->options = NIL;
+					n->chain = $3;
 					$$ = (Node *)n;
 				}
-			| ROLLBACK opt_transaction
+			| ROLLBACK opt_transaction opt_transaction_chain
 				{
 					TransactionStmt *n = makeNode(TransactionStmt);
 					n->kind = TRANS_STMT_ROLLBACK;
 					n->options = NIL;
+					n->chain = $3;
 					$$ = (Node *)n;
 				}
 			| SAVEPOINT ColId
@@ -9931,6 +9936,12 @@ transaction_mode_list_or_empty:
 					{ $$ = NIL; }
 		;
 
+opt_transaction_chain:
+			AND CHAIN		{ $$ = true; }
+			| AND NO CHAIN	{ $$ = false; }
+			| /* EMPTY */	{ $$ = false; }
+		;
+
 
 /*****************************************************************************
  *

src/backend/tcop/utility.c

@@ -440,7 +440,7 @@ standard_ProcessUtility(PlannedStmt *pstmt,
 						break;
 
 					case TRANS_STMT_COMMIT:
-						if (!EndTransactionBlock())
+						if (!EndTransactionBlock(stmt->chain))
 						{
 							/* report unsuccessful commit in completionTag */
 							if (completionTag)
@@ -471,7 +471,7 @@ standard_ProcessUtility(PlannedStmt *pstmt,
 						break;
 
 					case TRANS_STMT_ROLLBACK:
-						UserAbortTransactionBlock();
+						UserAbortTransactionBlock(stmt->chain);
 						break;
 
 					case TRANS_STMT_SAVEPOINT: