> The main thread was waiting test_signals() lock (signalled_all) 
> while it is was interrupted by a signal. The signal handler calls
> Py_AddPendingCall() which blocks on acquiring "pending_lock".

Oh, the main thread receives SIGUSR1: the signal handler calls Py_AddPendingCall() to process it later. But while calling Py_AddPendingCall(), it receives a SIGUSR2: the same signal handler is called, and it calls Py_AddPendingCall().

And we have a deadlock because we try to acquire the lock twice, or try to acquire the lock before the lock was released.