keyboard.c: Allow SIGINT to `quit` in batch mode, instead of exit

In terminal sessions, SIGINT is turned into a `quit` ELisp signal,
but in batch it has traditionally killed Emacs.  It can be very
useful to cause a `quit` from outside the process when running
in batch (e.g. for "batch" sessions that provide a REPL via stdin/out),
so add a new var 'kill-emacs-on-sigint' to control that behavior.
(bug#80942)

* src/keyboard.c (handle_interrupt_signal): Obey `kill_emacs_on_sigint`.
(init_keyboard): Use `deliver_interrupt_signal` for SIGINT also for
batch sessions.
(syms_of_keyboard): New variable `kill_emacs_on_sigint`.

* test/src/keyboard-tests.el (keyboard-sigint-to-quit): New test.

* doc/emacs/cmdargs.texi (Initial Options): Mention the effect of
`kill-emacs-on-sigint` in batch mode.

1 files changed, 6 insertions, 1 deletions

diff --git a/doc/emacs/cmdargs.texi b/doc/emacs/cmdargs.texi
index 5a3c1c14ab1..f9904d1f2e6 100644
--- a/doc/emacs/cmdargs.texi
+++ b/doc/emacs/cmdargs.texi
@@ -260,9 +260,14 @@ on.  To invoke a Lisp program, use the @samp{-batch} option in
 conjunction with one or more of @samp{-l}, @samp{-f} or @samp{--eval}
 (@pxref{Action Arguments}).  @xref{Command Example}, for an example.
 
+@vindex kill-emacs-on-sigint
 In batch mode, Emacs does not display the text being edited, and the
 standard terminal interrupt characters such as @kbd{C-z} and @kbd{C-c}
-have their usual effect.  Emacs functions that normally print a
+have their usual effect: for @kbd{C-c} that effect is either to
+exit Emacs or to signal @code{quit}, depending on the variable
+@code{kill-emacs-on-sigint}.
+
+Emacs functions that normally print a
 message in the echo area will print to either the standard output
 stream (@code{stdout}) or the standard error stream (@code{stderr})
 instead.  (To be precise, functions like @code{prin1}, @code{princ}