Thanks for your kind reply, Eli. Here are some more questions:

* If I found a problem in existing code that is unrelated to my new
  feature but is quite trivial to fix (2~3 lines), should I create a
  new branch and send it as an separate patch to bug tracker, or
  include it in my new feature?

* IIUC, I send the patch to the bug tracker and people will review it,
  is that correct?

* If the patch is a fix, should I first create a bug report and reply
  to that report?

I'm still not quite sure how everything works, hope I'm not too
garrulous.

Attached is the patch for the bug fix. Changes are:

1. gdb-mi uses a buffer to store and parse output from gdb process
   and it is cleared after each complete conversation, however
   the cleanup function is not protected against call to `error'
   in handlers who process the output's content and do various things to it.

2. gdb-mi uses `error' for error messages from gdb, I think it should
   use `user-error' instead because the gdb-mi code is running well,
   the error is from gdb and are normal errors like "symbol not
   found in current context". We shouldn't drop a user into a backtrace
   for errors like this
   
Yuan