I confirm the bug. But I don't think disabling Ctrl-C (SIGINT) handling by default is a good idea. Proper Ctrl-C support seems like a fundamental feature for a command line debugger. 

However, I think the bug is easily fixable w/o changing SIGINT handling. Basically, just put try/except around signal.signal, catch the ValueError and proceed. Would this approach solve your problem?

Patch attached.

PS. and here is a small program demonstrating the bug (just run it and execute "c" command at pdb prompt)

 import threading
 import pdb

 def start_pdb():
    pdb.Pdb().set_trace()
    x = 1
    y = 1

 t = threading.Thread( target=start_pdb)
 t.start()