Bug#599007: apt-get can't be interrupted with Ctrl+C nor Ctrl-\ anymore

[Adam Borowski <kilobyte@angband.pl>]

On Wed, Oct 06, 2010 at 03:58:41PM +0200, Michael Vogt wrote:
> > It appears that since a recent upgrade, you can't interrupt apt runs
> > anymore.  
> [..]
> 
> This is a bug in the code, it looks like this is a regression caused
> by setting the termios rtt var via "rtt.c_lflag |= ISIG". This fixed
> making it possible to background dpkg, but apparently it broke
> ctrl-c. Which is odd given that the documentation says that is is used
> to enable signals.

Uhm, 1. it is set by default, 2. it indeed enables recognition of signals. 
I remember disabling it in a program of mine:
http://angband.pl/viewvc/kbtin/trunk/user_tty.c?view=markup
and after checking again, it does work as in the documentation.

Quoting glibc's docs:

# -- Macro: tcflag_t ISIG
#     This bit controls whether the INTR, QUIT, and SUSP characters are
#     recognized.  The functions associated with these characters are
#     performed if and only if this bit is set.  Being in canonical or
#     noncanonical input mode has no affect on the interpretation of
#     these characters.
#
#     You should use caution when disabling recognition of these
#     characters.  Programs that cannot be interrupted interactively are
#     very user-unfriendly.  If you clear this bit, your program should
#     provide some alternate interface that allows the user to
#     interactively send the signals associated with these characters,
#     or to escape from the program.  

With that bit clear, Ctrl-C produces a character 0x03.  With it set, it
sends SIGINT.

I didn't look at the apt code, but there must be more to it than merely
setting this flag.

-- 
1KB		// Microsoft corollary to Hanlon's razor:
		//	Never attribute to stupidity what can be
		//	adequately explained by malice.