emacs: test and work around upstream bug#81105 (#528764)

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

@@ -15,6 +15,10 @@ runCommand "test-emacs-withPackages-wrapper"
       ))
       git # needed by magit
     ];
+    env = {
+      # emulate a default NixOS env where INFOPATH is set like this (not ending with a ":")
+      INFOPATH = "/fake-info-dir1:/fake-info-dir2";
+    };
   }
   ''
     emacs --batch --eval="(require 'magit)"
@@ -23,5 +27,9 @@ runCommand "test-emacs-withPackages-wrapper"
     # transitive dependencies should be made available
     # https://github.com/NixOS/nixpkgs/issues/388829
     emacs --batch --eval="(require 'flx)"
+
+    # test that https://debbugs.gnu.org/cgi/bugreport.cgi?bug=81105 is fixed or worked around
+    emacs --batch --eval='(progn (package-activate-all) (info "(magit)Top"))'
+
     touch $out
   ''

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

@@ -72,6 +72,14 @@ least specific (the system profile)"
                          (nix--profile-paths))
                  woman-manpath)))
 
+;;; Make info manuals of installed elisp pkgs available (work around Emacs bug#81105)
+(when-let* ((path (getenv "INFOPATH")))
+  ;; Normally, Emacs 31 extends `Info-default-directory-list' when activating elisp pkgs.
+  ;; We add a trailing path-separator ":" to INFOPATH when needed
+  ;; to ensure `Info-default-directory-list' is used to initialize `Info-directory-list'.
+  (unless (string-suffix-p path-separator path)
+    (setenv "INFOPATH" (concat path path-separator))))
+
 ;;; Make tramp work for remote NixOS machines
 (defvar tramp-remote-path)
 (eval-after-load 'tramp