Function TupleQueueReaderNext

src/backend/executor/tqueue.c:175–210

* Fetch a tuple from a tuple queue reader. * * The return value is NULL if there are no remaining tuples or if * nowait = true and no tuple is ready to return. *done, if not NULL, * is set to true when there are no remaining tuples and otherwise to false. * * The returned tuple, if any, is either in shared memory or a private buffer * and should not be freed. The pointer is invalid after

 * this with nowait = true even if nothing is returned.
 */
MinimalTuple
TupleQueueReaderNext(TupleQueueReader *reader, bool nowait, bool *done)
{
	MinimalTuple tuple;
	shm_mq_result result;
	Size		nbytes;
	void	   *data;

	if (done != NULL)
		*done = false;

	/* Attempt to read a message. */
	result = shm_mq_receive(reader->queue, &nbytes, &data, nowait);

	/* If queue is detached, set *done and return NULL. */
	if (result == SHM_MQ_DETACHED)
	{
		if (done != NULL)
			*done = true;
		return NULL;
	}

	/* In non-blocking mode, bail out if no message ready yet. */
	if (result == SHM_MQ_WOULD_BLOCK)
		return NULL;
	Assert(result == SHM_MQ_SUCCESS);

	/*
	 * Return a pointer to the queue memory directly (which had better be
	 * sufficiently aligned).
	 */
	tuple = (MinimalTuple) data;
	Assert(tuple->t_len == nbytes);

	return tuple;
}