cf-socket: improvements in socket I/O handling

- Curl_write_plain/Curl_read_plain have been eliminated. Last code use
  now uses Curl_conn_send/recv so that requests use conn->send/revc
  callbacks which defaults to cfilters use.
- Curl_recv_plain/Curl_send_plain have been internalized in cf-socket.c.
- USE_RECV_BEFORE_SEND_WORKAROUND (active on Windows) has been moved
  into cf-socket.c. The pre_recv buffer is held at the socket filter
  context.  `postponed_data` structures have been removed from
  `connectdata`.
- the hanger in HTTP/2 request handling was a result of read buffering
  on all sends and the multi handling is not prepared for this. The
  following happens:

   - multi preforms on a HTTP/2 easy handle
   - h2 reads and processes data
   - this leads to a send of h2 data
   - which receives and buffers before the send
   - h2 returns
   - multi selects on the socket, but no data arrives (its in the buffer already)
   the workaround now receives data in a loop as long as there is something in
   the buffer. The real fix would be for multi to change, so that `data_pending`
   is evaluated before deciding to wait on the socket.

io_buffer, optional, in cf-socket.c, http/2 sets state.drain if lower
filter have pending data.

This io_buffer is only available/used when the
-DUSE_RECV_BEFORE_SEND_WORKAROUND is active, e.g. on Windows
configurations. It also maintains the original checks on protocol
handler being HTTP and conn->send/recv not being replaced.

The HTTP/2 (nghttp2) cfilter now sets data->state.drain when it finds
out that the "lower" filter chain has still pending data at the end of
its IO operation. This prevents the processing from becoming stalled.

Closes #10280

1 files changed, 8 insertions, 2 deletions

diff --git a/lib/cfilters.c b/lib/cfilters.c
index 9582a951a..97ddbb9b6 100644
--- a/lib/cfilters.c
+++ b/lib/cfilters.c
@@ -323,6 +323,14 @@ int Curl_conn_cf_get_select_socks(struct Curl_cfilter *cf,
   return 0;
 }
 
+bool Curl_conn_cf_data_pending(struct Curl_cfilter *cf,
+                               const struct Curl_easy *data)
+{
+  if(cf)
+    return cf->cft->has_data_pending(cf, data);
+  return FALSE;
+}
+
 ssize_t Curl_conn_cf_send(struct Curl_cfilter *cf, struct Curl_easy *data,
                           const void *buf, size_t len, CURLcode *err)
 {
@@ -426,8 +434,6 @@ bool Curl_conn_data_pending(struct Curl_easy *data, int sockindex)
   (void)data;
   DEBUGASSERT(data);
   DEBUGASSERT(data->conn);
-  if(Curl_recv_has_postponed_data(data->conn, sockindex))
-    return TRUE;
 
   cf = data->conn->cfilter[sockindex];
   while(cf && !cf->connected) {