bug#38406: 27.0.50; post-self-insert-hook does not hold its contract in cc-mode derived modes

[Alan Mackenzie <acm@muc.de>]

Hello, Eli.

On Sun, Dec 01, 2019 at 22:47:01 +0200, Eli Zaretskii wrote:
> > Date: Sun, 1 Dec 2019 19:27:09 +0000
> > Cc: yyoncho@gmail.com, 38406@debbugs.gnu.org
> > From: Alan Mackenzie <acm@muc.de>

> > post-self-insert-hook's functions, unusually amongs hooks, interfere
> > with its triggering event.  This contrasts with, say,
> > after-change-functions, where the functions don't insert into or
> > delete from the buffer, or pre-redisplay-functions, where the
> > functions don't try to prevent a particular window getting
> > displayed.

> You'd be surprised to know what some of those hooks do.  Everything
> you say they don't, and then some.

Any chance you could name one (or even two), thus letting me see for
myself?

> There's nothing we can do to prevent people from shooting themselves
> in the foot or hanging themselves with the rope we provided.

In the case of post-self-insert-hook, the damaging functions are part of
Emacs itself, not crazy user-written code.

> And if you think you are the only one who needs to harden your code to
> let people do the craziest things with these hooks, please don't think
> so: you are definitely not alone.

OK.

> But breaking a hook's contract as a means to teach people not to shoot
> themselves in the foot is not right.  If the uses are legitimate, they
> should be able to do them; if they aren't, let them cope with the
> consequences.

> > So to call this hook at the end of c-electric-brace would mean having to
> > filter the hook first (at the very least, to remove
> > electric-pair-post-self-insert-function), which just seems very hackish
> > and unsatisfactory.

> It doesn't seem too hackish to me, and as a nice bonus we will have
> post-self-insert-hook act as per its contract again.

> So could you please do that?  TIA.

OK, I'll do that.  It's not a nice thing to do, but we're kind of
lacking nice things in this situation.  Give me a few days, please - I'm
a touch busy in RL at the moment.

Additionally, how about reversing the encouragement in the Elisp manual
to put buffer changing functions onto post-self-insert-hook?

-- 
Alan Mackenzie (Nuremberg, Germany).