_libssh2_channel_write: don't iterate over transport writes

When a call to _libssh2_transport_write() succeeds, we must return from
_libssh2_channel_write() to allow the caller to provide the next chunk
of data.

We cannot move on to send the next piece of data that may already have
been provided in this same function call, as we risk getting EAGAIN for
that and we can't return information both about sent data as well as
EAGAIN. So, by returning short now, the caller will call this function
again with new data to send.

src/channel.c

@@ -1918,9 +1918,8 @@ _libssh2_channel_packet_data_len(LIBSSH2_CHANNEL * channel, int stream_id)
 /*
  * _libssh2_channel_write
  *
- * Send data to a channel. Note that if this returns EAGAIN or simply didn't
+ * Send data to a channel. Note that if this returns EAGAIN, the caller must
- * send the entire packet, the caller must call this function again with the
+ * call this function again with the SAME input arguments.
- * SAME input arguments.
  *
  * Returns: number of bytes sent, or if it returns a negative number, that is
  * the error code!
@@ -2034,13 +2033,6 @@ _libssh2_channel_write(LIBSSH2_CHANNEL *channel, int stream_id,
                                           channel->write_s -
                                           channel->write_packet);
             if (rc == LIBSSH2_ERROR_EAGAIN) {
-                if(wrote) {
-                    /* some pieces of data was sent before the EAGAIN so we
-                       return that amount! As we ignore EAGAIN, we must drain
-                       the outgoing transport buffer. */
-                    _libssh2_transport_drain(session);
-                    goto _channel_write_done;
-                }
                 return _libssh2_error(session, rc,
                                       "Unable to send channel data");
             }
@@ -2054,17 +2046,20 @@ _libssh2_channel_write(LIBSSH2_CHANNEL *channel, int stream_id,
             /* Shrink local window size */
             channel->local.window_size -= channel->write_bufwrite;
 
-            /* Adjust buf for next iteration */
-            buflen -= channel->write_bufwrite;
-            buf += channel->write_bufwrite;
-            channel->write_bufwrote += channel->write_bufwrite;
             wrote += channel->write_bufwrite;
 
-            channel->write_state = libssh2_NB_state_allocated;
+            /* Since _libssh2_transport_write() succeeded, we must return
-        }
+               now to allow the caller to provide the next chunk of data.
-    }
 
-  _channel_write_done:
+               We cannot move on to send the next piece of data that may
+               already have been provided in this same function call, as we
+               risk getting EAGAIN for that and we can't return information
+               both about sent data as well as EAGAIN. So, by returning short
+               now, the caller will call this function again with new data to
+               send */
+            break;
+        }
+    }
 
     LIBSSH2_FREE(session, channel->write_packet);
     channel->write_packet = NULL;