Sending a fragmented WebSocket message.

From: Paul Fotheringham via curl-library <curl-library_at_lists.haxx.se>
Date: Mon, 18 Sep 2023 23:56:58 +0100

Following on from my post about detecting the final frame of a
fragmented WebSocket message there is also an issue sending fragmented
messages.

I cannot figure out the flags that you are meant to set when calling
curl_ws_send with each fragment and I suspect there is no correct
combination.

Looking at the code ws_enc_write_head makes a call to
ws_frame_flags2op to convert the supplied flags to an opcode. Now if
flags contains CURLWS_CONT (4) then ws_frame_flags2op will return zero
regardless of any other flags that are ORed with it. This return value
is assigned to opcode but the very next bit of code returns
CURLE_SEND_ERROR if (!opcode).

Adding to the confusion is the fact that the 4-bit opcode in the first
byte of the WebSocket header is not a bit-field despite the CURLWS_*
defines suggesting it is.

I propose
- changing the CURLWS_TEXT thru CURLWS_PING defines to be their actual
numerical 4-bit values
- rename the curl_ws_send's flags argument to opcode
- add an argument to curl_ws_send to replace the CURLWS_OFFSET functionality
- add libtest cases to cover csl_ws_send for non-control frames
-- libtest 2303 claims to test curl_ws_send but the code is missing to
actually send a (unfragmneted) text frame
-- add a libtest for sending a fragmented text frame, not sure how to
verify the data server-size though
- add a libtest for sending a frame in pieces i.e. the CURLWS_OFFSET
functionality

Paul

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Received on 2023-09-19