Re: Signal handling by readline

[Chet Ramey]

On 9/29/24 2:36 PM, Elad Lahav wrote:
> Hello,
>
> I was chasing an issue with bash signal handling on QNX, when I
> noticed that, whenever you hit enter, the readline library does the
> following for a few signals:
>
> 1. Install a new signal handler via sigaction()
> 2. Check the previous handler, as returned by this call
> 3. If it is SIG_IGN, call sigaction() again to restore SIG_IGN
>
> Since bash ignores these signals to begin with, all three actions are
> taken for each signal every time. 

Bash isn't the only application using readline. Most applications using
it don't do anything special with the job control signals. Bash, being
a job control shell, is of course the exception.

> In addition to being redundant, the
> sequence above introduces a window in which a signal can come in, with
> the end result that bash gets the signal itself (which would be very
> unfortunate if the signal is SIGTSTP). This may be mostly a
> theoretical issue, but why have the window at all?

There is no window, at least in versions of readline released in the
last decade. rl_set_signals() blocks the signals it modifies before it
changes any of them, and unblocks them after changing all of the
dispositions.

> Is there any reason to allow readline to change signal handling at
> all?

Yes, readline needs to perform internal cleanup (e.g., reset state and
free memory) and restore the terminal settings when it receives a signal.
Most applications using readline don't want to do that, especially since
they aren't the ones modifying the terminal settings in the first place,
or have access to internal readline data structures, so it's better for
readline to do it.

Applications that don't want readline to do this can disable it, at a
cost.

> If there is, then perhaps consider the following change, which I
> currently have defined for QNX only.

This would double the number of system calls in the overwhelming majority
of cases. Since there's no bash-specific code in readline, and I'd like
not to have any, I think the minor impact is worth it.

Chet
--
``The lyf so short, the craft so long to lerne.'' - Chaucer
                 ``Ars longa, vita brevis'' - Hippocrates
Chet Ramey, UTech, CWRU    chet@case.edu    http://tiswww.cwru.edu/~chet/

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