Re: C-g crash redux

[Chong Yidong]

>> Typing C-g, which runs interrupt_signal, runs quit_throw_to_read_char
>> instead of Fsignal during when waiting_for_input is set (as it is
>> during wait_reading_process_output).  If I understand correctly, the
>> purpose is to enable read-char to return a `C-g' character; and that's
>> why it's both undesirable and unnecesssary to use a
>> record_unwind_protect inside wait_reading_process_output.
>
> Ok, I see.
>
> But how do we ensure that waiting_for_user_input_p is restored
> correctly on C-g?
>
> Or isn't that necessary?

That's not necessary, since quit_throw_to_read_char calls
clear_waiting_for_input, which resets waiting_for_input to 0.

I guess someone worked all this out at some time in the past.  I added
a comment to the code to clarify this.