Add a citus_use_snapshot function for basic distributed snapshot isolation

src/backend/distributed/connection/connection_management.c

@@ -678,6 +678,20 @@ CloseNodeConnectionsAfterTransaction(char *nodeName, int nodePort)
 }
 
 
+/*
+ * CloseConnectionList closes all connections in the given list.
+ */
+void
+CloseConnectionList(List *connectionList)
+{
+	MultiConnection *connection = NULL;
+	foreach_ptr(connection, connectionList)
+	{
+		CloseConnection(connection);
+	}
+}
+
+
 /*
  * Close a previously established connection.
  */

src/backend/distributed/connection/remote_commands.c

@@ -425,24 +425,35 @@ ExecuteCriticalRemoteCommand(MultiConnection *connection, const char *command)
 
 
 /*
- * ExecuteRemoteCommandInConnectionList executes a remote command, on all connections
- * given in the list, that is critical to the transaction. If the command fails then
- * the transaction aborts.
+ * SendCommandToConnectionList sends a given command over all connections
+ * in the list in parallel.
  */
 void
-ExecuteRemoteCommandInConnectionList(List *nodeConnectionList, const char *command)
+SendRemoteCommandToConnectionList(List *connectionList, const char *command)
 {
 	MultiConnection *connection = NULL;
-
-	foreach_ptr(connection, nodeConnectionList)
+	foreach_ptr(connection, connectionList)
 	{
 		int querySent = SendRemoteCommand(connection, command);
-
 		if (querySent == 0)
 		{
 			ReportConnectionError(connection, ERROR);
 		}
 	}
+}
+
+
+/*
+ * ExecuteRemoteCommandInConnectionList executes a remote command, on all connections
+ * given in the list, that is critical to the transaction. If the command fails then
+ * the transaction aborts.
+ */
+void
+ExecuteRemoteCommandInConnectionList(List *nodeConnectionList, const char *command)
+{
+	MultiConnection *connection = NULL;
+
+	SendRemoteCommandToConnectionList(nodeConnectionList, command);
 
 	/* Process the result */
 	foreach_ptr(connection, nodeConnectionList)

src/backend/distributed/operations/citus_create_restore_point.c

@@ -111,7 +111,7 @@ citus_create_restore_point(PG_FUNCTION_ARGS)
  * OpenConnectionsToAllNodes opens connections to all nodes and returns the list
  * of connections.
  */
-static List *
+List *
 OpenConnectionsToAllWorkerNodes(LOCKMODE lockMode)
 {
 	List *connectionList = NIL;

src/backend/distributed/sql/citus--11.1-1--11.2-1.sql

@@ -9,3 +9,4 @@ DROP FUNCTION pg_catalog.worker_append_table_to_shard(text, text, text, integer)
 #include "udfs/citus_get_transaction_clock/11.2-1.sql"
 #include "udfs/citus_is_clock_after/11.2-1.sql"
 #include "udfs/citus_internal_adjust_local_clock_to_remote/11.2-1.sql"
+#include "udfs/citus_use_snapshot/11.1-1.sql"

src/backend/distributed/sql/downgrades/citus--11.2-1--11.1-1.sql

@@ -1,6 +1,7 @@
 -- citus--11.2-1--11.1-1
 #include "../udfs/get_rebalance_progress/11.1-1.sql"
 #include "../udfs/citus_isolation_test_session_is_blocked/11.1-1.sql"
+DROP FUNCTION pg_catalog.citus_use_snapshot();
 DROP FUNCTION pg_catalog.citus_get_node_clock();
 DROP FUNCTION pg_catalog.citus_get_transaction_clock();
 DROP FUNCTION pg_catalog.citus_internal_adjust_local_clock_to_remote(cluster_clock);

src/backend/distributed/sql/udfs/citus_use_snapshot/latest.sql

@@ -0,0 +1,9 @@
+CREATE OR REPLACE FUNCTION pg_catalog.citus_use_snapshot()
+ RETURNS void
+ LANGUAGE c
+ STRICT
+AS '$libdir/citus', $function$citus_use_snapshot$function$;
+COMMENT ON FUNCTION pg_catalog.citus_use_snapshot()
+    IS 'use a consistent a consistent distributed snapshot for the remainder of the transaction';
+
+GRANT EXECUTE ON FUNCTION pg_catalog.citus_use_snapshot() TO PUBLIC;

src/backend/distributed/transaction/remote_transaction.c

@@ -28,6 +28,7 @@
 #include "distributed/placement_connection.h"
 #include "distributed/remote_commands.h"
 #include "distributed/remote_transaction.h"
+#include "distributed/transaction/snapshot.h"
 #include "distributed/transaction_identifier.h"
 #include "distributed/transaction_management.h"
 #include "distributed/transaction_recovery.h"
@@ -85,14 +86,22 @@ StartRemoteTransactionBegin(struct MultiConnection *connection)
 
 	StringInfo beginAndSetDistributedTransactionId = makeStringInfo();
 
-	/*
-	 * Explicitly specify READ COMMITTED, the default on the remote
-	 * side might have been changed, and that would cause problematic
-	 * behaviour.
-	 */
+	/* append the BEGIN command */
 	appendStringInfoString(beginAndSetDistributedTransactionId,
 						   BeginTransactionCommand());
 
+	/* when using a distributed snapshot, append SET TRANSACTION SNAPSHOT .. */
+	char *exportedSnapshotName = GetSnapshotNameForNode(connection->hostname,
+														connection->port,
+														connection->user,
+														connection->database);
+	if (exportedSnapshotName != NULL)
+	{
+		appendStringInfo(beginAndSetDistributedTransactionId,
+						 "SET TRANSACTION SNAPSHOT %s;",
+						 quote_literal_cstr(exportedSnapshotName));
+	}
+
 	/* append context for in-progress SAVEPOINTs for this transaction */
 	List *activeSubXacts = ActiveSubXactContexts();
 	transaction->lastSuccessfulSubXact = TopSubTransactionId;