Prevent idle in transaction session timeout from sometimes being ignored.

The previous coding in ProcessInterrupts() could lead to
idle_in_transaction_session_timeout being ignored, when
statement_timeout occurred earlier.

The problem was that ProcessInterrupts() would return before
processing the transaction timeout if QueryCancelPending was set while
QueryCancelHoldoffCount != 0 - which is the case when reading new
commands from the client. Ergo when the idle transaction timeout would
hit.

Fix that by removing the early return. Alternatively the transaction
timeout code could have been moved up, but that early return seems
like an issue that could hit other cases too.

Author: Lukas Fittl
Bug: #14821
Discussion:
    https://www.postgresql.org/message-id/20170921010956.17345.61461%40wrigleys.postgresql.org
    https://www.postgresql.org/message-id/CAP53PkxQnv3OWJpyNPGJYT62uY=n1=2CF_Lpc6gVOFnc0-gazw@mail.gmail.com
Backpatch: 9.6-, where idle_in_transaction_session_timeout was introduced.

src/backend/tcop/postgres.c

@@ -2941,26 +2941,24 @@ ProcessInterrupts(void)
 						 " database and repeat your command.")));
 	}
 
-	if (QueryCancelPending)
+	/*
+	 * Don't allow query cancel interrupts while reading input from the
+	 * client, because we might lose sync in the FE/BE protocol.  (Die
+	 * interrupts are OK, because we won't read any further messages from
+	 * the client in that case.)
+	 */
+	if (QueryCancelPending && QueryCancelHoldoffCount != 0)
 	{
-		bool		lock_timeout_occurred;
-		bool		stmt_timeout_occurred;
-
 		/*
-		 * Don't allow query cancel interrupts while reading input from the
-		 * client, because we might lose sync in the FE/BE protocol.  (Die
-		 * interrupts are OK, because we won't read any further messages from
-		 * the client in that case.)
+		 * Re-arm InterruptPending so that we process the cancel request
+		 * as soon as we're done reading the message.
 		 */
-		if (QueryCancelHoldoffCount != 0)
-		{
-			/*
-			 * Re-arm InterruptPending so that we process the cancel request
-			 * as soon as we're done reading the message.
-			 */
-			InterruptPending = true;
-			return;
-		}
+		InterruptPending = true;
+	}
+	else if (QueryCancelPending)
+	{
+		bool		lock_timeout_occurred;
+		bool		stmt_timeout_occurred;
 
 		QueryCancelPending = false;