ssh2bpp: try to report DISCONNECT before connection closure.

There was a race between toplevel callbacks: if we read enough data to
receive an SSH_MSG_DISCONNECT, and then returned, then whether we
reported the DISCONNECT message or the followup EOF would depend on
whether the BPP or the master PPL got called back first. Now the BPP
politely waits its turn (i.e. waits to see if it even gets called back
at all) before reporting EOF.

ssh2bpp.c

@@ -645,6 +645,13 @@ static void ssh2_bpp_handle_input(BinaryPacketProtocol *bpp)
     }
 
   eof:
+    /*
+     * We've seen EOF. But we might have pushed stuff on the outgoing
+     * packet queue first, and that stuff _might_ include a DISCONNECT
+     * message, in which case we'd like to use that as the diagnostic.
+     * So first wait for the queue to have been processed.
+     */
+    crMaybeWaitUntilV(!pq_peek(&s->bpp.in_pq));
     if (!s->bpp.expect_close) {
         ssh_remote_error(s->bpp.ssh,
                          "Remote side unexpectedly closed network connection");