commit cc4f9d5679278ce41cd5480fab3f5e71dba163ed
Author: Matthew Booth <mbooth@redhat.com>
Date:   Fri Mar 16 16:29:00 2012 +0100

    transport_send: Finish in-progress key exchange before sending data
    
    _libssh2_channel_write() first reads outstanding packets before writing
    new data. If it reads a key exchange request, it will immediately start
    key re-exchange, which will require sending a response. If the output
    socket is full, this will result in a return from
    _libssh2_transport_read() of LIBSSH2_ERROR_EAGAIN. In order not to block
    a write because there is no data to read, this error is explicitly
    ignored and the code continues marshalling a packet for sending. When it
    is sent, the remote end immediately drops the connection because it was
    expecting a continuation of the key exchange, but got a data packet.
    
    This change adds the same check for key exchange to
    _libssh2_transport_send() that is in _libssh2_transport_read(). This
    ensures that key exchange is completed before any data packet is sent.

diff --git a/src/transport.c b/src/transport.c
index 057dcf5..95b9a3a 100644
--- a/src/transport.c
+++ b/src/transport.c
@@ -296,7 +296,7 @@ int _libssh2_transport_read(LIBSSH2_SESSION * session)
          * is done!
          */
         _libssh2_debug(session, LIBSSH2_TRACE_TRANS, "Redirecting into the"
-                       " key re-exchange");
+                       " key re-exchange from _libssh2_transport_read");
         rc = _libssh2_kex_exchange(session, 1, &session->startup_key_state);
         if (rc)
             return rc;
@@ -687,6 +687,24 @@ int _libssh2_transport_send(LIBSSH2_SESSION *session,
     const unsigned char *orgdata = data;
     size_t orgdata_len = data_len;
 
+    /*
+     * If the last read operation was interrupted in the middle of a key
+     * exchange, we must complete that key exchange before continuing to write
+     * further data.
+     *
+     * See the similar block in _libssh2_transport_read for more details.
+     */
+    if (session->state & LIBSSH2_STATE_EXCHANGING_KEYS &&
+        !(session->state & LIBSSH2_STATE_KEX_ACTIVE)) {
+        /* Don't write any new packets if we're still in the middle of a key
+         * exchange. */
+        _libssh2_debug(session, LIBSSH2_TRACE_TRANS, "Redirecting into the"
+                       " key re-exchange from _libssh2_transport_send");
+        rc = _libssh2_kex_exchange(session, 1, &session->startup_key_state);
+        if (rc)
+            return rc;
+    }
+
     debugdump(session, "libssh2_transport_write plain", data, data_len);
     if(data2)
         debugdump(session, "libssh2_transport_write plain2", data2, data2_len);