websocket: fix message send corruption

- Fix a bug in EAGAIN handling when sending frames that led to a
  corrupted last byte of the frame sent.

- Restore sanity to curl_ws_send() behaviour:

  - Partial writes are reported as OK with the actual number of
    payload bytes sent.

  - CURLE_AGAIN is only returned when none of the payload bytes
    (or for 0-length frames, not all of the frame header bytes)
    could be sent.

  - curl_ws_send() now behaves like a common send() call.

- Change 'ws-data' test client to allow concurrent send/recv
  operations and vary frame sizes and repeat count.

- Add DEBUG env var CURL_WS_CHUNK_EAGAIN to simulate blocking
  after a chunk of an encoded websocket frame has been sent.

- Add tests.


Prior to this change data corruption may occur when sending websocket
messages due to two bugs:

1) 3e64569a (precedes 8.10.0) caused a data corruption bug in the last
   byte of frame of large messages.

2) curl_ws_send had non-traditional send behavior and could return
   CURLE_AGAIN with bytes sent and expect the caller to adjust buffer
   and buflen in a subsequent call. That behavior was not documented.


Reported-by: na-trium-144@users.noreply.github.com

Fixes https://github.com/curl/curl/issues/15865
Fixes https://github.com/curl/curl/issues/15865#issuecomment-2569870144
Closes https://github.com/curl/curl/pull/15901

lib/bufq.c

@@ -45,11 +45,6 @@ static size_t chunk_len(const struct buf_chunk *chunk)
   return chunk->w_offset - chunk->r_offset;
 }
 
-static size_t chunk_space(const struct buf_chunk *chunk)
-{
-  return chunk->dlen - chunk->w_offset;
-}
-
 static void chunk_reset(struct buf_chunk *chunk)
 {
   chunk->next = NULL;
@@ -287,24 +282,6 @@ size_t Curl_bufq_len(const struct bufq *q)
   return len;
 }
 
-size_t Curl_bufq_space(const struct bufq *q)
-{
-  size_t space = 0;
-  if(q->tail)
-    space += chunk_space(q->tail);
-  if(q->spare) {
-    struct buf_chunk *chunk = q->spare;
-    while(chunk) {
-      space += chunk->dlen;
-      chunk = chunk->next;
-    }
-  }
-  if(q->chunk_count < q->max_chunks) {
-    space += (q->max_chunks - q->chunk_count) * q->chunk_size;
-  }
-  return space;
-}
-
 bool Curl_bufq_is_empty(const struct bufq *q)
 {
   return !q->head || chunk_is_empty(q->head);