_libssh2_channel_read: Honour window_size_initial

_libssh2_channel_read was using an arbitrary hard-coded limit to trigger
the window adjusting code. The adjustment used was also hard-coded and
arbitrary, 15MB actually, which would limit the usability of libssh2 on
systems with little RAM.

This patch, uses the window_size parameter passed to
libssh2_channel_open_ex (stored as remote.window_size_initial) plus the
buflen as the base for the trigger and the adjustment calculation.

The memory usage when using the default window size is reduced from 22MB
to 256KB per channel (actually, if compression is used, these numbers
should be incremented by ~50% to account for the errors between the
decompressed packet sizes and the predicted sizes).

My tests indicate that this change does not impact the performance of
transfers across localhost or a LAN, being it on par with that of
OpenSSH. On the other hand, it will probably slow down transfers on
networks with high bandwidth*delay when the default window size
(LIBSSH2_CHANNEL_WINDOW_DEFAULT=256KB) is used.

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

src/channel.c

@@ -1761,14 +1761,17 @@ ssize_t _libssh2_channel_read(LIBSSH2_CHANNEL *channel, int stream_id,
                    stream_id);
 
     /* expand the receiving window first if it has become too narrow */
-    if((channel->read_state == libssh2_NB_state_jump1) ||
-       (channel->remote.window_size < (LIBSSH2_CHANNEL_WINDOW_DEFAULT*30))) {
+    if( (channel->read_state == libssh2_NB_state_jump1) ||
+        (channel->remote.window_size < channel->remote.window_size_initial / 4 * 3 + buflen) ) {
+
+        uint32_t adjustment = channel->remote.window_size_initial + buflen - channel->remote.window_size;
+        if (adjustment < LIBSSH2_CHANNEL_MINADJUST)
+            adjustment = LIBSSH2_CHANNEL_MINADJUST;
 
         /* the actual window adjusting may not finish so we need to deal with
            this special state here */
         channel->read_state = libssh2_NB_state_jump1;
-        rc = _libssh2_channel_receive_window_adjust(channel,
-                                                    (LIBSSH2_CHANNEL_WINDOW_DEFAULT*60),
+        rc = _libssh2_channel_receive_window_adjust(channel, adjustment,
                                                     0, NULL);
         if (rc)
             return rc;