Also reset transactions at connection shutdown (#6685)

In #6314 I refactored the connection cleanup to be simpler to
understand and use. However, by doing so I introduced a use-after-free
possibility (that valgrind luckily picked up):

In the `ShouldShutdownConnection` path of
`AfterXactHostConnectionHandling`
we free connections without removing the `transactionNode` from the
dlist that it might be part of. Before the refactoring this wasn't a
problem, because the dlist would be completely reset quickly after in
`ResetGlobalVariables` (without reading or writing the dlist entries).

The refactoring changed this by moving the `dlist_delete` call to
`ResetRemoteTransaction`, which in turn was called in the
`!ShouldShutdownConnection` path of `AfterXactHostConnectionHandling`.
Thus this `!ShouldShutdownConnection` path would now delete from the
`dlist`, but the `ShouldShutdownConnection` path would not. Thus to
remove itself the deleting path would sometimes update nodes in the list
that were freed right before.

There's two ways of fixing this:
1. Call `dlist_delete` from **both** of paths.
2. Call `dlist_delete` from **neither** of the paths.

This commit implements the second approach, and #6684 implements the
first. We need to choose which approach we prefer.

To make calling `dlist_delete` from both paths actually work, we also need
to use a slightly different check to determine if we need to call dlist_delete.
Various regression tests showed that there can be cases where the
`transactionState` is something else than `REMOTE_TRANS_NOT_STARTED`
but the connection was not added to the `InProgressTransactions` list
One example of such a case is when running `TransactionStateMachine`
without calling `StartRemoteTransactionBegin` beforehand. In those
cases the connection won't be added to `InProgressTransactions`, but
the `transactionState` is changed to `REMOTE_TRANS_SENT_COMMAND`. 

Sidenote: This bug already existed in 11.1, but valgrind didn't catch it
back then. My guess is that this happened because #6314 was merged after
the initial release branch was cut.

Fixes #6638

src/backend/distributed/connection/connection_management.c

@@ -1454,6 +1454,9 @@ AfterXactHostConnectionHandling(ConnectionHashEntry *entry, bool isCommit)
 		{
 			ShutdownConnection(connection);
 
+			/* remove from transactionlist before free-ing */
+			ResetRemoteTransaction(connection);
+
 			/* unlink from list */
 			dlist_delete(iter.cur);
 

src/backend/distributed/transaction/remote_transaction.c

@@ -80,6 +80,7 @@ StartRemoteTransactionBegin(struct MultiConnection *connection)
 
 	/* remember transaction as being in-progress */
 	dlist_push_tail(&InProgressTransactions, &connection->transactionNode);
+	connection->transactionInProgress = true;
 
 	transaction->transactionState = REMOTE_TRANS_STARTING;
 
@@ -865,11 +866,13 @@ ResetRemoteTransaction(struct MultiConnection *connection)
 	RemoteTransaction *transaction = &connection->remoteTransaction;
 
 	/* unlink from list of open transactions, if necessary */
-	if (transaction->transactionState != REMOTE_TRANS_NOT_STARTED)
+	if (connection->transactionInProgress)
 	{
 		/* XXX: Should we error out for a critical transaction? */
 
 		dlist_delete(&connection->transactionNode);
+		connection->transactionInProgress = false;
+		memset(&connection->transactionNode, 0, sizeof(connection->transactionNode));
 	}
 
 	/* just reset the entire state, relying on 0 being invalid/false */

src/include/distributed/connection_management.h

@@ -189,8 +189,12 @@ typedef struct MultiConnection
 	/* information about the associated remote transaction */
 	RemoteTransaction remoteTransaction;
 
-	/* membership in list of in-progress transactions */
+	/*
+	 * membership in list of in-progress transactions and a flag to indicate
+	 * that the connection was added to this list
+	 */
 	dlist_node transactionNode;
+	bool transactionInProgress;
 
 	/* list of all placements referenced by this connection */
 	dlist_head referencedPlacements;