nix-buffer support improvements.

Use inherit-local, add per-package elisp hooks.

pkgs/build-support/emacs/buffer.nix

@@ -1,23 +1,47 @@
 # Functions to build elisp files to locally configure emcas buffers.
 # See https://github.com/shlevy/nix-buffer
 
-{ lib, writeText }:
+{ lib, writeText, inherit-local }:
 
 {
   withPackages = pkgs: let
-      coqs = builtins.filter (x: (builtins.parseDrvName x.name).name == "coq") pkgs;
-      coq = builtins.head coqs;
-      pg-setup = if builtins.length coqs == 0 then "" else ''
-        (setq-local coq-prog-name "${coq}/bin/coqtop")
-        (setq-local coq-dependency-analyzer "${coq}/bin/coqdep")
-        (setq-local coq-compiler "${coq}/bin/coqc")
-	(setq-local coq-library-directory (get-coq-library-directory))
-	(coq-prog-args)
-      '';
+      extras = map (x: x.emacsBufferSetup pkgs) (builtins.filter (builtins.hasAttr "emacsBufferSetup") pkgs);
     in writeText "dir-locals.el" ''
+      (require 'inherit-local "${inherit-local}/share/emacs/site-lisp/elpa/inherit-local-${inherit-local.version}/inherit-local.elc")
+
+      ; Only set up nixpkgs buffer handling when we have some buffers active
+      (defvar nixpkgs--buffer-count 0)
+      (when (eq nixpkgs--buffer-count 0)
+        ; When generating a new temporary buffer (one whose name starts with a space), do inherit-local inheritance and make it a nixpkgs buffer
+        (defun nixpkgs--around-generate (orig name)
+          (if (eq (aref name 0) ?\s)
+              (let ((buf (funcall orig name)))
+                (when (inherit-local-inherit-child buf)
+                  (with-current-buffer buf
+                    (make-local-variable 'kill-buffer-hook)
+                    (setq nixpkgs--buffer-count (1+ nixpkgs--buffer-count))
+                    (add-hook 'kill-buffer-hook 'nixpkgs--decrement-buffer-count)))
+                buf)
+            (funcall orig name)))
+        (advice-add 'generate-new-buffer :around #'nixpkgs--around-generate)
+        ; When we have no more nixpkgs buffers, tear down the buffer handling
+        (defun nixpkgs--decrement-buffer-count ()
+          (setq nixpkgs--buffer-count (1- nixpkgs--buffer-count))
+          (when (eq nixpkgs--buffer-count 0)
+            (advice-remove 'generate-new-buffer #'nixpkgs--around-generate)
+            (fmakunbound 'nixpkgs--around-generate)
+            (fmakunbound 'nixpkgs--decrement-buffer-count))))
+      (setq nixpkgs--buffer-count (1+ nixpkgs--buffer-count))
+      (make-local-variable 'kill-buffer-hook)
+      (add-hook 'kill-buffer-hook 'nixpkgs--decrement-buffer-count)
+
+      ; Add packages to PATH and exec-path
       (make-local-variable 'process-environment)
+      (put 'process-environment 'permanent-local t)
+      (inherit-local 'process-environment)
       (setenv "PATH" (concat "${lib.makeSearchPath "bin" pkgs}:" (getenv "PATH")))
-      (setq-local exec-path (append '(${builtins.concatStringsSep " " (map (p: "\"${p}/bin\"") pkgs)}) exec-path))
-      ${pg-setup}
+      (inherit-local-permanent exec-path (append '(${builtins.concatStringsSep " " (map (p: "\"${p}/bin\"") pkgs)}) exec-path))
+
+      ${lib.concatStringsSep "\n" extras}
     '';
 }