Assertion failure in "curl_ws_recv"

[StanleyWucw]

I did this

I was trying to receive data from a remote WebSocket server, but I got a debug assertion error.
The code below is running in debug mode in Visual Studio 2022:

        CURL* curl = curl_easy_init();
	curl_easy_setopt(curl, CURLOPT_URL, "wss://ws.postman-echo.com/raw");
	curl_easy_setopt(curl, CURLOPT_CONNECT_ONLY, 2L);
	
	// Make a handshake
	bool bOK = CURLE_OK == curl_easy_perform(curl);
	ASSERT(bOK);

	// Send data
	size_t bytes_sent;
	std::string sOut = "{\"message\": \"Hello to libcurl!\"}";
	bOK = CURLE_OK == curl_ws_send(curl, sOut.c_str(), sOut.size(), &bytes_sent, 0, CURLWS_TEXT);
	ASSERT(bOK);

	// Receive data
	size_t rlen;
	struct curl_ws_frame* meta;
	char buffer[256];
	CURLcode result = curl_ws_recv(curl, buffer, sizeof(buffer), &rlen, &meta);

	curl_easy_cleanup(curl);

I expected the following

The free WebSocket server "wss://ws.postman-echo.com/raw" is an echo server. It sends back whatever it received.

In the above code, the content of the variable "buffer" should be "{"message": "Hello to libcurl!"}" which was returned successfully by the API call "curl_ws_recv." However, that happened only after I closed the pop-up warnings and clicked the "ignore" button. See the image below.

In the release mode of Visual Studio 2022, the above warning pop-up doesn't show.

curl/libcurl version

7.87

operating system

Windows 10 with the latest update

[dfandrich]

CURLcode result = curl_ws_recv(curl, buffer, sizeof(buffer), &rlen, &meta);

I don't think this is related to the problem, but the code should check the bytesleft field of the meta struct to see if all the data in the fragment has been received. If not, then curl_ws_recv has to be called again to receive more data. My gut feeling is that the assert in the code isn't right (this is experimental, after all) but I'm not really familiar with the WebSocket support. Does the problem go away if you increase the size of buffer? Could you enable debug logging and show what you get? I don't quite see how the existing logic in curl_ws_recv is supposed to work. If ws_decode decodes a full frame, oleft (and therefore wsp->frame.bytesleft) will be 0 and the decoded data length datalen will be >0, so the assertion DEBUGASSERT(wsp->frame.bytesleft >= datalen) will of course fail. At the point of the assert, wsp->frame.bytesleft already contains only the remaining bytes so the subtraction bytesleft -= datalen two lines down (as well as the assert that's protecting the subtraction) shouldn't be necessary. I'm probably missing something.

[StanleyWucw]

Increasing the buffer size doesn't make the assertion failure problem disappear.

I also did the opposite. The meta->bytesleft always returns 0, even if I set the buffer size to be a small number, such as 10. The existing logic of updating the "wsp->frame. bytesleft" inside 'curl_ws_recv' might be flawed.

[bagder]

See #10366 for a possible fix. It makes your test case run fine for me.