Do not wait by default, only executor waits

src/backend/distributed/commands/multi_copy.c

@@ -3425,15 +3425,6 @@ CopyGetPlacementConnection(ShardPlacement *placement, bool stopOnFailure)
 		MultiShardConnectionType != SEQUENTIAL_CONNECTION)
 	{
 		connectionFlags |= CONNECTION_PER_PLACEMENT;
-
-		/*
-		 * Via connection throttling, the connection establishments may be suspended
-		 * until a connection slot is empty to the remote host. When forced to use
-		 * one connection per placement, do not enforce this restriction as it could
-		 * deadlock against concurrent operation where each operation is blocked on
-		 * waiting for others.
-		 */
-		connectionFlags |= NEVER_WAIT_FOR_CONNECTION;
 	}
 
 	connection = GetPlacementConnection(connectionFlags, placement, nodeUser);

src/backend/distributed/connection/connection_management.c

@@ -319,18 +319,9 @@ StartNodeUserDatabaseConnection(uint32 flags, const char *hostname, int32 port,
 	}
 
 
-	if (flags & NEVER_WAIT_FOR_CONNECTION)
+	if (flags & WAIT_FOR_CONNECTION)
 	{
-		/*
+		WaitLoopForSharedConnection(hostname, port);
-		 * The caller doesn't want the connection manager to wait
-		 * until a connection slot is available on the remote node.
-		 * In the end, we might fail to establish connection to the
-		 * remote node as it might not have any space in
-		 * max_connections for this connection establishment.
-		 *
-		 * Still, we keep track of the connnection counter.
-		 */
-		IncrementSharedConnectionCounter(hostname, port);
 	}
 	else if (flags & OPTIONAL_CONNECTION)
 	{
@@ -347,7 +338,16 @@ StartNodeUserDatabaseConnection(uint32 flags, const char *hostname, int32 port,
 	}
 	else
 	{
-		WaitOrErrorForSharedConnection(hostname, port);
+		/*
+		 * The caller doesn't want the connection manager to wait
+		 * until a connection slot is available on the remote node.
+		 * In the end, we might fail to establish connection to the
+		 * remote node as it might not have any space in
+		 * max_connections for this connection establishment.
+		 *
+		 * Still, we keep track of the connnection counter.
+		 */
+		IncrementSharedConnectionCounter(hostname, port);
 	}
 
 	/*

src/backend/distributed/connection/shared_connection_stats.c

@@ -263,15 +263,14 @@ GetMaxSharedPoolSize(void)
 
 
 /*
- * WaitOrErrorForSharedConnection tries to increment the shared connection
+ * WaitLoopForSharedConnection tries to increment the shared connection
  * counter for the given hostname/port and the current database in
  * SharedConnStatsHash.
  *
- * The function implements a retry mechanism. If the function cannot increment
+ * The function implements a retry mechanism via a condition variable.
- * the counter withing the specificed amount of the time, it throws an error.
  */
 void
-WaitOrErrorForSharedConnection(const char *hostname, int port)
+WaitLoopForSharedConnection(const char *hostname, int port)
 {
 	while (!TryToIncrementSharedConnectionCounter(hostname, port))
 	{

src/backend/distributed/executor/adaptive_executor.c

@@ -2395,16 +2395,15 @@ ManageWorkerPool(WorkerPool *workerPool)
 			 */
 			connectionFlags |= OPTIONAL_CONNECTION;
 		}
-		else if (UseConnectionPerPlacement())
+		else if (!UseConnectionPerPlacement())
 		{
 			/*
 			 * Via connection throttling, the connection establishments may be suspended
-			 * until a connection slot is empty to the remote host. When forced to use
+			 * until a connection slot is empty to the remote host. Adaptive executor can
-			 * one connection per placement, do not enforce this restriction as it could
+			 * always finish the execution with a single connection, so wait until we get
-			 * deadlock against concurrent operation where each operation is blocked on
+			 * one connection.
-			 * waiting for others.
 			 */
-			connectionFlags |= NEVER_WAIT_FOR_CONNECTION;
+			connectionFlags |= WAIT_FOR_CONNECTION;
 		}
 
 		/* open a new connection to the worker */

src/include/distributed/connection_management.h

@@ -67,7 +67,7 @@ enum MultiConnectionMode
 	 * until a connection slot is empty to the remote host. When this flag is passed,
 	 * the connection manager skips waiting.
 	 */
-	NEVER_WAIT_FOR_CONNECTION = 1 << 6
+	WAIT_FOR_CONNECTION = 1 << 6
 };
 
 

src/include/distributed/shared_connection_stats.h

@@ -21,7 +21,7 @@ extern void WakeupWaiterBackendsForSharedConnection(void);
 extern void RemoveInactiveNodesFromSharedConnections(void);
 extern int GetMaxSharedPoolSize(void);
 extern bool TryToIncrementSharedConnectionCounter(const char *hostname, int port);
-extern void WaitOrErrorForSharedConnection(const char *hostname, int port);
+extern void WaitLoopForSharedConnection(const char *hostname, int port);
 extern void DecrementSharedConnectionCounter(const char *hostname, int port);
 extern void IncrementSharedConnectionCounter(const char *hostname, int port);