bug#50126: 28.0.50; "mode: conf" and "mode: tex" get unbounded recursion

[Bob Rogers]

   In "emacs -Q" in master rebuilt a few hours ago, when I open the file
shown below with "# -*- mode: conf; -*-" I get a "Lisp nesting exceeds
 max-lisp-eval-depth " error because conf-mode is trying to dispatch to
one of its submodes, but the submode then calls run-mode-hooks which
ends up back in conf-mode, ad inf.  Using this minimal broken.conf file:

        # -*- mode: conf; -*-

is sufficient; here's the top of the backtrace:

        Debugger entered--Lisp error: (error "Lisp nesting exceeds  
max-lisp-eval-depth ")
          apply(#f(compiled-function (orig-fun) "Redirect to one of the 
submodes when called directly." #<bytecode -0xe6df0765af3b07>) 
#f(compiled-function () (interactive nil) #<bytecode 0xdf70887525b4bad>) nil)
          conf-mode()
          conf-unix-mode()
          #f(compiled-function (orig-fun) "Redirect to one of the submodes when 
called directly." #<bytecode -0xe6df0765af3b07>)(#f(compiled-function () 
(interactive nil) #<bytecode 0xdf70887525b4bad>))
          apply(#f(compiled-function (orig-fun) "Redirect to one of the 
submodes when called directly." #<bytecode -0xe6df0765af3b07>) 
#f(compiled-function () (interactive nil) #<bytecode 0xdf70887525b4bad>) nil)
          conf-mode()
          hack-one-local-variable(mode conf)
          hack-local-variables-apply()
          hack-local-variables(no-mode)
          run-mode-hooks(conf-unix-mode-hook)
          conf-unix-mode()
          #f(compiled-function (orig-fun) "Redirect to one of the submodes when 
called directly." #<bytecode -0xe6df0765af3b07>)(#f(compiled-function () 
(interactive nil) #<bytecode 0xdf70887525b4bad>))
          apply(#f(compiled-function (orig-fun) "Redirect to one of the 
submodes when called directly." #<bytecode -0xe6df0765af3b07>) 
#f(compiled-function () (interactive nil) #<bytecode 0xdf70887525b4bad>) nil)
          conf-mode()
          hack-one-local-variable(mode conf)
          . . .

It fails with conf-unix-mode in the simple case of the configuration
file above; unsurprisingly, I've also seen it fail with conf-space-mode.
And "mode: tex;" has the same general problem, since it uses the same
dispatch logic.

   I would attempt a patch, but I have no clue what TRT would be here.
It would seem cleanest to keep conf-mode and tex-mode as user-visible
modes, and make new generic-conf-mode and generic-tex-mode parents for
their submodes, but that seems like it could be a fairly large change.
Using delay-mode-hooks might be simpler, but I have no idea how that
would work.

                                        -- Bob Rogers
                                           http://www.rgrjr.com/

------------------------------------------------------------------------
In GNU Emacs 28.0.50 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.20, cairo 
version 1.16.0)
 of 2021-08-19 built on orion
Repository revision: a44432a773f7e131692e3f5c9d765cffeef0739a
Repository branch: master
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