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transport_send: Finish in-progress key exchange before sending data

Backport of upstream commit cc4f9d5679278ce41cd5480fab3f5e71dba163ed

_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_write()
that is in _libssh2_transport_read(). This ensures that key exchange is
completed before any data packet is sent.

diff -up libssh2-1.2.7/src/transport.c.bz804155 libssh2-1.2.7/src/transport.c
--- libssh2-1.2.7/src/transport.c.bz804155
+++ libssh2-1.2.7/src/transport.c
@@ -312,7 +312,7 @@ int _libssh2_transport_read(LIBSSH2_SESS
          * 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;
@@ -718,6 +718,24 @@ _libssh2_transport_write(LIBSSH2_SESSION
     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_write");
+        rc = _libssh2_kex_exchange(session, 1, &session->startup_key_state);
+        if (rc)
+            return rc;
+    }
     debugdump(session, "libssh2_transport_write plain", data, data_len);
     /* FIRST, check if we have a pending write to complete */