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, 2 insertions, 2 deletions

diff --git a/lib/vtls/schannel.h b/lib/vtls/schannel.h
index 93c7238f3..7fae39fa0 100644
--- a/lib/vtls/schannel.h
+++ b/lib/vtls/schannel.h
@@ -179,8 +179,8 @@ struct ssl_backend_data {
   size_t encdata_offset, decdata_offset;
   unsigned char *encdata_buffer, *decdata_buffer;
   /* encdata_is_incomplete: if encdata contains only a partial record that
-     can't be decrypted without another Curl_read_plain (that is, status is
-     SEC_E_INCOMPLETE_MESSAGE) then set this true. after Curl_read_plain writes
+     can't be decrypted without another recv() (that is, status is
+     SEC_E_INCOMPLETE_MESSAGE) then set this true. after an recv() adds
      more bytes into encdata then set this back to false. */
   bool encdata_is_incomplete;
   unsigned long req_flags, ret_flags;