emacs: allow wrapped emacs to execute itself again

fixes #145302 #237855

emacsWithPackages wrapper script/`site-start.el` sanitize
EMACSLOADPATH, to make nested emacs invocations independent of the
package set specified in emacsWithPackages.

But there are valid use cases when one needs to call nested emacs with
the same package set. This includes built-in emacs functionality such
as async native compilations, and also external packages like
`emacs-async` and `esup`. In all of these cases
`invocation-directory`/`invocation-name` variables are being used to
launch nested emacs.

With this patch these variables will be populated to point to the
emacsWithPackages wrapper executable, so that executing
`(file-name-concat invocation-directory invocation-name)` will give a
fully functional emacs again.

`EMACSLOADPATH` sanitization was introduced by #106486, this behaviour
stays unchanged. The reasoning was to be able to run different emacs
executables without polluting their EMACSLOADPATH (as described here
https://github.com/NixOS/nixpkgs/pull/106486/commits/23d4bfb6661ca57a9e331a2cf4184232d38ac38b).
The only change is that invoking itself is again feasible (and that's
what emacs actually expects).

Co-authored-by: Lin Jian <[e-mail]>

pkgs/applications/editors/emacs/build-support/wrapper.nix

···
         --subst-var-by bash ${emacs.stdenv.shell} \
         --subst-var-by wrapperSiteLisp "$deps/share/emacs/site-lisp" \
         --subst-var-by wrapperSiteLispNative "$deps/share/emacs/native-lisp" \
+        --subst-var-by wrapperInvocationDirectory "$out/bin/" \
+        --subst-var-by wrapperInvocationName "$progname" \
         --subst-var prog
       chmod +x $out/bin/$progname
       # Create a “NOP” binary wrapper for the pure sake of it becoming a
···
         --subst-var-by bash ${emacs.stdenv.shell} \
         --subst-var-by wrapperSiteLisp "$deps/share/emacs/site-lisp" \
         --subst-var-by wrapperSiteLispNative "$deps/share/emacs/native-lisp" \
+        --subst-var-by wrapperInvocationDirectory "$out/Applications/Emacs.app/Contents/MacOS/" \
+        --subst-var-by wrapperInvocationName "Emacs" \
         --subst-var-by prog "$emacs/Applications/Emacs.app/Contents/MacOS/Emacs"
       chmod +x $out/Applications/Emacs.app/Contents/MacOS/Emacs
       wrapProgramBinary $out/Applications/Emacs.app/Contents/MacOS/Emacs

pkgs/applications/editors/emacs/build-support/wrapper.sh

···
 export EMACSNATIVELOADPATH="${newNativeLoadPath[*]}"
 export emacsWithPackages_siteLispNative=@wrapperSiteLispNative@
 
+export emacsWithPackages_invocationDirectory=@wrapperInvocationDirectory@
+export emacsWithPackages_invocationName=@wrapperInvocationName@
+
 exec @prog@ "$@"

pkgs/applications/editors/emacs/site-start.el

···
       (setenv "EMACSNATIVELOADPATH" (when new-env-list
                                 (mapconcat 'identity new-env-list ":"))))))
 
+(let ((wrapper-invocation-directory (getenv "emacsWithPackages_invocationDirectory")))
+  (when wrapper-invocation-directory
+    (setq invocation-directory (file-name-as-directory wrapper-invocation-directory))
+    (setenv "emacsWithPackages_invocationDirectory" nil)))
+
+(let ((wrapper-invocation-name (getenv "emacsWithPackages_invocationName")))
+  (when wrapper-invocation-name
+    (setq invocation-name wrapper-invocation-name)
+    (setenv "emacsWithPackages_invocationName" nil)))
+
 ;;; Set up native-comp load path.
 (when (featurep 'native-compile)
   ;; Append native-comp subdirectories from `NIX_PROFILES'.