window_size: redid window handling for flow control reasons

Until now, the window size (channel->remote.window_size) was being
updated just after receiving the packet from the transport layer.

That behaviour is wrong because the channel queue may grow uncontrolled
when data arrives from the network faster that the upper layer consumes
it.

This patch adds a new counter, read_avail, which keeps a count of the
bytes available from the packet queue for reading. Also, now the window
size is adjusted when the data is actually read by an upper layer.

That way, if the upper layer stops reading data, the window will
eventually fill and the remote host will stop sending data. When the
upper layers reads enough data, a window adjust packet is delivered and
the transfer resumes.

The read_avail counter is used to detect the situation when the remote
server tries to send data surpassing the window size. In that case, the
extra data is discarded.

Signed-off-by: Salvador <sfandino@yahoo.com>

src/libssh2_priv.h

@@ -356,6 +356,8 @@ struct _LIBSSH2_CHANNEL
     libssh2_channel_data local, remote;
     /* Amount of bytes to be refunded to receive window (but not yet sent) */
     uint32_t adjust_queue;
+    /* Data immediately available for reading */
+    uint32_t read_avail;
 
     LIBSSH2_SESSION *session;