Fix out-of-memory error handling in ParameterDescription message processing.

If libpq ran out of memory while constructing the result set, it would hang,
waiting for more data from the server, which might never arrive. To fix,
distinguish between out-of-memory error and not-enough-data cases, and give
a proper error message back to the client on OOM.

There are still similar issues in handling COPY start messages, but let's
handle that as a separate patch.

Michael Paquier, Amit Kapila and me. Backpatch to all supported versions.

src/interfaces/libpq/fe-protocol3.c

@@ -45,7 +45,7 @@
 
 static void handleSyncLoss(PGconn *conn, char id, int msgLength);
 static int	getRowDescriptions(PGconn *conn, int msgLength);
-static int	getParamDescriptions(PGconn *conn);
+static int	getParamDescriptions(PGconn *conn, int msgLength);
 static int	getAnotherTuple(PGconn *conn, int msgLength);
 static int	getParameterStatus(PGconn *conn);
 static int	getNotify(PGconn *conn);
@@ -278,8 +278,17 @@ pqParseInput3(PGconn *conn)
 						return;
 					break;
 				case 'T':		/* Row Description */
-					if (conn->result == NULL ||
-						conn->queryclass == PGQUERY_DESCRIBE)
+					if (conn->result != NULL &&
+						conn->result->resultStatus == PGRES_FATAL_ERROR)
+					{
+						/*
+						 * We've already choked for some reason.  Just discard
+						 * the data till we get to the end of the query.
+						 */
+						conn->inCursor += msgLength;
+					}
+					else if (conn->result == NULL ||
+							 conn->queryclass == PGQUERY_DESCRIBE)
 					{
 						/* First 'T' in a query sequence */
 						if (getRowDescriptions(conn, msgLength))
@@ -329,9 +338,10 @@ pqParseInput3(PGconn *conn)
 					}
 					break;
 				case 't':		/* Parameter Description */
-					if (getParamDescriptions(conn))
+					if (getParamDescriptions(conn, msgLength))
 						return;
-					break;
+					/* getParamDescriptions() moves inStart itself */
+					continue;
 				case 'D':		/* Data Row */
 					if (conn->result != NULL &&
 						conn->result->resultStatus == PGRES_TUPLES_OK)
@@ -637,20 +647,21 @@ advance_and_error:
  * that shouldn't happen often, since 't' messages usually fit in a packet.
  */
 static int
-getParamDescriptions(PGconn *conn)
+getParamDescriptions(PGconn *conn, int msgLength)
 {
 	PGresult   *result;
 	int			nparams;
 	int			i;
+	const char *errmsg = NULL;
 
 	result = PQmakeEmptyPGresult(conn, PGRES_COMMAND_OK);
 	if (!result)
-		goto failure;
+		goto advance_and_error;
 
 	/* parseInput already read the 't' label and message length. */
 	/* the next two bytes are the number of parameters */
 	if (pqGetInt(&(result->numParameters), 2, conn))
-		goto failure;
+		goto not_enough_data;
 	nparams = result->numParameters;
 
 	/* allocate space for the parameter descriptors */
@@ -659,7 +670,7 @@ getParamDescriptions(PGconn *conn)
 		result->paramDescs = (PGresParamDesc *)
 			pqResultAlloc(result, nparams * sizeof(PGresParamDesc), TRUE);
 		if (!result->paramDescs)
-			goto failure;
+			goto advance_and_error;
 		MemSet(result->paramDescs, 0, nparams * sizeof(PGresParamDesc));
 	}
 
@@ -669,17 +680,48 @@ getParamDescriptions(PGconn *conn)
 		int			typid;
 
 		if (pqGetInt(&typid, 4, conn))
-			goto failure;
+			goto not_enough_data;
 		result->paramDescs[i].typid = typid;
 	}
 
 	/* Success! */
 	conn->result = result;
+
+	/* Advance inStart to show that the "t" message has been processed. */
+	conn->inStart = conn->inCursor;
+
 	return 0;
 
-failure:
+not_enough_data:
 	PQclear(result);
 	return EOF;
+
+advance_and_error:
+	/* Discard unsaved result, if any */
+	if (result && result != conn->result)
+		PQclear(result);
+
+	/* Discard the failed message by pretending we read it */
+	conn->inStart += 5 + msgLength;
+
+	/*
+	 * Replace partially constructed result with an error result. First
+	 * discard the old result to try to win back some memory.
+	 */
+	pqClearAsyncResult(conn);
+
+	/*
+	 * If preceding code didn't provide an error message, assume "out of
+	 * memory" was meant.  The advantage of having this special case is that
+	 * freeing the old result first greatly improves the odds that gettext()
+	 * will succeed in providing a translation.
+	 */
+	if (!errmsg)
+		errmsg = libpq_gettext("out of memory");
+	printfPQExpBuffer(&conn->errorMessage, "%s\n", errmsg);
+	pqSaveErrorResult(conn);
+
+	return 0;
 }
 
 /*