emacs: use PATH instead of exec-path to expose wrapped binaries

(info "(elisp) Subprocess Creation") says:

> Generally, you should not modify ‘exec-path’ directly.  Instead,
> ensure that your ‘PATH’ environment variable is set appropriately
> before starting Emacs.  Trying to modify ‘exec-path’ independently
> of ‘PATH’ can lead to confusing results.

One example of "confusing results" can be found in envrc[1].

This patch follows that recommendation of Emacs lisp manual.

In addition, we added tests for PATH and exec-path.

[1]: https://github.com/purcell/envrc/issues/9

pkgs/applications/editors/emacs/build-support/wrapper-test/with-packages.el

@@ -41,6 +41,13 @@ which is generated by nix when building this elisp package."
 (defun with-packages-unwrapped-site-start-is-loaded ()
   (fboundp 'nix--profile-paths))
 
+(defun with-packages-eval-in-sub-emacs (form)
+  "Evaluate FORM in a subprocess Emacs and return its result."
+  (pcase-exhaustive
+      (process-lines "emacs" "--batch"
+                     "--eval" (format "(prin1 %S)" form))
+    (`(,result) (read result))))
+
 ;;;; Tests that can be run in a batch Emacs
 
 (ert-deftest with-packages-requested-packages-are-available ()
@@ -71,8 +78,24 @@ which is generated by nix when building this elisp package."
 (ert-deftest with-packages-unwrapped-site-start-is-loaded ()
   (should (with-packages-unwrapped-site-start-is-loaded)))
 
-(ert-deftest with-packages-bin-dirs-of-requested-packages-are-added-to-exec-path ()
-  (should (executable-find "cowsay")))
+(ert-deftest with-packages-binaries-of-requested-packages-are-available ()
+  (ert-info ("find binary via PATH")
+    (should (equal (call-process-shell-command "cowsay") 0)))
+  (ert-info ("find binary via exec-path")
+    (should (executable-find "cowsay")))
+  (ert-info ("each item is unique")
+    (cl-flet ((item-should-be-unique (items)
+                (should (equal items
+                               (cl-remove-duplicates items :test #'string=)))))
+      (ert-info ("PATH of this Emacs")
+        (item-should-be-unique (parse-colon-path (getenv "PATH"))))
+      (ert-info ("PATH of sub-Emacs")
+        (item-should-be-unique
+         (parse-colon-path (with-packages-eval-in-sub-emacs '(getenv "PATH")))))
+      (ert-info ("exec-path of this Emacs")
+        (item-should-be-unique exec-path))
+      (ert-info ("exec-path of sub-Emacs")
+        (item-should-be-unique (with-packages-eval-in-sub-emacs 'exec-path))))))
 
 (ert-deftest with-packages-tree-sitter-dir-is-added-to-treesit-extra-load-path ()
   (skip-unless (treesit-available-p))

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

@@ -171,9 +171,6 @@ runCommand (lib.appendToName "with-packages" emacs).name
           cat >"$siteStart" <<EOF
           ;;; -*- lexical-binding: t -*-
           (load "$emacs/share/emacs/site-lisp/site-start" nil t)
-          ;; "$out/share/emacs/site-lisp" is added to load-path in wrapper.sh
-          ;; "$out/share/emacs/native-lisp" is added to native-comp-eln-load-path in wrapper.sh
-          (add-to-list 'exec-path "$out/bin")
           ${lib.optionalString withTreeSitter ''
             (add-to-list 'treesit-extra-load-path "$out/lib/")
           ''}
@@ -213,6 +210,7 @@ runCommand (lib.appendToName "with-packages" emacs).name
         --subst-var-by bash ${emacs.stdenv.shell} \
         --subst-var-by wrapperSiteLisp "$deps/share/emacs/site-lisp" \
         --subst-var-by wrapperSiteLispNative "$deps/share/emacs/native-lisp" \
+        --subst-var-by wrapperBinDir "$deps/bin" \
         --subst-var-by wrapperInvocationDirectory "$out/bin/" \
         --subst-var-by wrapperInvocationName "$progname" \
         --subst-var prog
@@ -239,6 +237,7 @@ runCommand (lib.appendToName "with-packages" emacs).name
         --subst-var-by bash ${emacs.stdenv.shell} \
         --subst-var-by wrapperSiteLisp "$deps/share/emacs/site-lisp" \
         --subst-var-by wrapperSiteLispNative "$deps/share/emacs/native-lisp" \
+        --subst-var-by wrapperBinDir "$deps/bin" \
         --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"

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

@@ -53,4 +53,9 @@ export emacsWithPackages_siteLispNative=@wrapperSiteLispNative@
 export emacsWithPackages_invocationDirectory=@wrapperInvocationDirectory@
 export emacsWithPackages_invocationName=@wrapperInvocationName@
 
+if [[ ${PATH-} != *"@wrapperBinDir@":* ]]
+then
+    export PATH="@wrapperBinDir@${PATH:+:$PATH}"
+fi
+
 exec @prog@ "$@"