关于skynet_context_message_dispatch函数问题 #1677

lichangjian · 2022-11-15T11:27:53Z

lichangjian
Nov 15, 2022

在看skynet_context_message_dispatch有个疑问。
把消息队列取出来以后，如果在执行dispatch_message的过程中，callback 调用了 skynet_send，skynet_send是有可能会执行 skynet_context_push 把取出来的消息队列再塞回去的。一旦塞回去了就可能会被其他线程取出来执行。那么就有可能会有两个线程在执行同一个callback导致线程冲突。
有可能会这样吗？

skynet_context_message_dispatch(struct skynet_monitor *sm, struct message_queue *q, int weight) {
	if (q == NULL) {
		q = skynet_globalmq_pop();
		if (q==NULL)
			return NULL;
	}

	uint32_t handle = skynet_mq_handle(q);

	struct skynet_context * ctx = skynet_handle_grab(handle);
	if (ctx == NULL) {
		struct drop_t d = { handle };
		skynet_mq_release(q, drop_message, &d);
		return skynet_globalmq_pop();
	}

	int i,n=1;
	struct skynet_message msg;

	for (i=0;i<n;i++) {
		if (skynet_mq_pop(q,&msg)) {
			skynet_context_release(ctx);
			return skynet_globalmq_pop();
		} else if (i==0 && weight >= 0) {
			n = skynet_mq_length(q);
			n >>= weight;
		}
		int overload = skynet_mq_overload(q);
		if (overload) {
			skynet_error(ctx, "May overload, message queue length = %d", overload);
		}

		skynet_monitor_trigger(sm, msg.source , handle);

		if (ctx->cb == NULL) {
			skynet_free(msg.data);
		} else {
			dispatch_message(ctx, &msg);
		}

		skynet_monitor_trigger(sm, 0,0);
	}

	assert(q == ctx->queue);
	struct message_queue *nq = skynet_globalmq_pop();
	if (nq) {
		// If global mq is not empty , push q back, and return next queue (nq)
		// Else (global mq is empty or block, don't push q back, and return q again (for next dispatch)
		skynet_globalmq_push(q);
		q = nq;
	} 
	skynet_context_release(ctx);

	return q;
}
int
skynet_send(struct skynet_context * context, uint32_t source, uint32_t destination , int type, int session, void * data, size_t sz) {
	if ((sz & MESSAGE_TYPE_MASK) != sz) {
		skynet_error(context, "The message to %x is too large", destination);
		if (type & PTYPE_TAG_DONTCOPY) {
			skynet_free(data);
		}
		return -2;
	}
	_filter_args(context, type, &session, (void **)&data, &sz);

	if (source == 0) {
		source = context->handle;
	}

	if (destination == 0) {
		if (data) {
			skynet_error(context, "Destination address can't be 0");
			skynet_free(data);
			return -1;
		}

		return session;
	}
	if (skynet_harbor_message_isremote(destination)) {
		struct remote_message * rmsg = skynet_malloc(sizeof(*rmsg));
		rmsg->destination.handle = destination;
		rmsg->message = data;
		rmsg->sz = sz & MESSAGE_TYPE_MASK;
		rmsg->type = sz >> MESSAGE_TYPE_SHIFT;
		skynet_harbor_send(rmsg, source, session);
	} else {
		struct skynet_message smsg;
		smsg.source = source;
		smsg.session = session;
		smsg.data = data;
		smsg.sz = sz;

		if (skynet_context_push(destination, &smsg)) {
			skynet_free(data);
			return -1;
		}
	}
	return session;
}