fish: resolve NixOS-related initialization problems

nixos/modules/programs/fish.nix

@@ -27,6 +27,30 @@ in
         '';
         type = types.bool;
       };
+      
+      vendor.config.enable = mkOption {
+        type = types.bool;
+        default = true;
+        description = ''
+          Whether fish should source configuration snippets provided by other packages.
+        '';
+      };
+
+      vendor.completions.enable = mkOption {
+        type = types.bool;
+        default = true;
+        description = ''
+          Whether fish should use completion files provided by other packages.
+        '';
+      };
+      
+      vendor.functions.enable = mkOption {
+        type = types.bool;
+        default = true;
+        description = ''
+          Whether fish should autoload fish functions provided by other packages.
+        '';
+      };
 
       shellAliases = mkOption {
         default = config.environment.shellAliases;
@@ -79,31 +103,72 @@ in
     environment.etc."fish/foreign-env/loginShellInit".text = cfge.loginShellInit;
     environment.etc."fish/foreign-env/interactiveShellInit".text = cfge.interactiveShellInit;
 
+    environment.etc."fish/nixos-env-preinit.fish".text = ''
+      # avoid clobbering the environment if it's been set by a parent shell
+
+      # This happens before $__fish_datadir/config.fish sets fish_function_path, so it is currently
+      # unset. We set it and then completely erase it, leaving its configuration to $__fish_datadir/config.fish
+      set fish_function_path ${pkgs.fish-foreign-env}/share/fish-foreign-env/functions $__fish_datadir/functions
+      
+      # source the NixOS environment config
+      fenv source ${config.system.build.setEnvironment}
+
+      # clear fish_function_path so that it will be correctly set when we return to $__fish_datadir/config.fish
+      set -e fish_function_path
+    '';
+
     environment.etc."fish/config.fish".text = ''
       # /etc/fish/config.fish: DO NOT EDIT -- this file has been generated automatically.
 
-      set fish_function_path $fish_function_path ${pkgs.fish-foreign-env}/share/fish-foreign-env/functions
+      # if our parent shell didn't source the general config, do it
+      if not set -q __fish_nixos_general_config_sourced
+        set fish_function_path ${pkgs.fish-foreign-env}/share/fish-foreign-env/functions $fish_function_path
+        fenv source /etc/fish/foreign-env/shellInit > /dev/null
+        set -e fish_function_path[1]
+        
+        ${cfg.shellInit}
 
-      fenv source ${config.system.build.setEnvironment} > /dev/null ^&1
-      fenv source /etc/fish/foreign-env/shellInit > /dev/null
-
-      ${cfg.shellInit}
-
-      if status --is-login
-        fenv source /etc/fish/foreign-env/loginShellInit > /dev/null
-        ${cfg.loginShellInit}
+        # and leave a note to our children to spare them the same work
+        set -gx __fish_nixos_general_config_sourced 1
       end
 
-      if status --is-interactive
+      # if our parent shell didn't source the login config, do it
+      status --is-login; and not set -q __fish_nixos_login_config_sourced
+      and begin
+        set fish_function_path ${pkgs.fish-foreign-env}/share/fish-foreign-env/functions $fish_function_path
+        fenv source /etc/fish/foreign-env/loginShellInit > /dev/null
+        set -e fish_function_path[1]
+        
+        ${cfg.loginShellInit}
+
+        # and leave a note to our children to spare them the same work
+        set -gx __fish_nixos_login_config_sourced 1
+      end
+
+      # if our parent shell didn't source the interactive config, do it
+      status --is-interactive; and not set -q __fish_nixos_interactive_config_sourced
+      and begin
         ${fishAliases}
+        
+
+        set fish_function_path ${pkgs.fish-foreign-env}/share/fish-foreign-env/functions $fish_function_path
         fenv source /etc/fish/foreign-env/interactiveShellInit > /dev/null
+        set -e fish_function_path[1]
+        
+        ${cfg.promptInit}
         ${cfg.interactiveShellInit}
+
+        # and leave a note to our children to spare them the same work
+        set -gx __fish_nixos_interactive_config_sourced 1
       end
     '';
 
     # include programs that bring their own completions
-    environment.pathsToLink = [ "/share/fish/vendor_completions.d" ];
-
+    environment.pathsToLink = []
+      ++ optional cfg.vendor.config.enable "/share/fish/vendor_conf.d"
+      ++ optional cfg.vendor.completions.enable "/share/fish/vendor_completions.d"
+      ++ optional cfg.vendor.functions.enable "/share/fish/vendor_functions.d";
+    
     environment.systemPackages = [ pkgs.fish ];
 
     environment.shells = [

pkgs/shells/fish/default.nix

@@ -2,15 +2,91 @@
   nettools, kbd, bc, which, gnused, gnugrep,
   groff, man-db, glibc, libiconv, pcre2,
   gettext, ncurses, python
+
+  , writeText
+
+  , useOperatingSystemEtc ? true
+  
 }:
 
 with stdenv.lib;
 
+let
+  etcConfigAppendixText = ''
+    ############### ↓ Nix hook for sourcing /etc/fish/config.fish ↓ ###############
+    #                                                                             #
+    # Origin:
+    #     This fish package was called with the attribute
+    #     "useOperatingSystemEtc = true;".
+    #
+    # Purpose:
+    #     Fish ordinarily sources /etc/fish/config.fish as
+    #        $__fish_sysconfdir/config.fish,
+    #     and $__fish_sysconfdir is defined at compile-time, baked into the C++
+    #     component of fish. By default, it is set to "/etc/fish". When building
+    #     through Nix, $__fish_sysconfdir gets set to $out/etc/fish. Here we may
+    #     have included a custom $out/etc/config.fish in the fish package,
+    #     as specified, but according to the value of useOperatingSystemEtc, we
+    #     may want to further source the real "/etc/fish/config.fish" file.
+    #
+    #     When this option is enabled, this segment should appear the very end of
+    #     "$out/etc/config.fish". This is to emulate the behavior of fish itself
+    #     with respect to /etc/fish/config.fish and ~/.config/fish/config.fish:
+    #     source both, but source the more global configuration files earlier
+    #     than the more local ones, so that more local configurations inherit
+    #     from but override the more global locations.
+    
+    if test -f /etc/fish/config.fish
+      source /etc/fish/config.fish
+    end
+    
+    #                                                                             #
+    ############### ↑ Nix hook for sourcing /etc/fish/config.fish ↑ ###############
+  '';
+
+  fishPreInitHooks = ''
+    # source nixos environment if we're a login shell
+    builtin status --is-login
+    and test -f /etc/fish/nixos-env-preinit.fish
+    and source /etc/fish/nixos-env-preinit.fish
+
+    test -n "$NIX_PROFILES"
+    and begin
+      # We ensure that __extra_* variables are read in $__fish_datadir/config.fish
+      # with a preference for user-configured data by making sure the package-specific
+      # data comes last. Files are loaded/sourced in encounter order, duplicate
+      # basenames get skipped, so we assure this by prepending Nix profile paths
+      # (ordered in reverse of the $NIX_PROFILE variable)
+      #
+      # Note that at this point in evaluation, there is nothing whatsoever on the
+      # fish_function_path. That means we don't have most fish builtins, e.g., `eval`.
+
+
+      # additional profiles are expected in order of precedence, which means the reverse of the
+      # NIX_PROFILES variable (same as config.environment.profiles)
+      set -l __nix_profile_paths (echo $NIX_PROFILES | ${coreutils}/bin/tr ' ' '\n')[-1..1]
+
+      set __extra_completionsdir \
+        $__nix_profile_paths"/etc/fish/completions" \
+        $__nix_profile_paths"/share/fish/vendor_completions.d" \
+        $__extra_completionsdir
+      set __extra_functionsdir \
+        $__nix_profile_paths"/etc/fish/functions" \
+        $__nix_profile_paths"/share/fish/vendor_functions.d" \
+        $__extra_functionsdir
+      set __extra_confdir \
+        $__nix_profile_paths"/etc/fish/conf.d" \
+        $__nix_profile_paths"/share/fish/vendor_conf.d" \
+        $__extra_confdir
+    end
+  '';
+in
+
 stdenv.mkDerivation rec {
   name = "fish-${version}";
   version = "2.5.0";
 
-  patches = [ ./etc_config.patch ];
+  etcConfigAppendix = builtins.toFile "etc-config.appendix.fish" etcConfigAppendixText;
 
   src = fetchurl {
     url = "http://fishshell.com/files/${version}/${name}.tar.gz";
@@ -69,15 +145,10 @@ stdenv.mkDerivation rec {
             "$out/share/fish/tools/create_manpage_completions.py"
     sed -i "s|command manpath|command ${man-db}/bin/manpath|"     \
             "$out/share/fish/functions/man.fish"
+  '' + optionalString useOperatingSystemEtc ''
+    tee -a $out/etc/fish/config.fish < ${(writeText "config.fish.appendix" etcConfigAppendixText)}
   '' + ''
-    tee -a $out/share/fish/config.fish << EOF
-
-    # make fish pick up completions from nix profile
-    if status --is-interactive
-      set -l profiles (echo \$NIX_PROFILES | ${coreutils}/bin/tr ' ' '\n')
-      set fish_complete_path \$profiles"/share/fish/vendor_completions.d" \$fish_complete_path
-    end
-    EOF
+    tee -a $out/share/fish/__fish_build_paths.fish < ${(writeText "__fish_build_paths_suffix.fish" fishPreInitHooks)}
   '';
 
   meta = with stdenv.lib; {

pkgs/shells/fish/etc_config.patch

@@ -1,12 +0,0 @@
-diff --git a/etc/config.fish b/etc/config.fish
-index 9be6f07..61c9ae2 100644
---- a/etc/config.fish
-+++ b/etc/config.fish
-@@ -12,3 +12,7 @@
- # if status --is-interactiv
- #   ...
- # end
-+
-+if test -f /etc/fish/config.fish
-+  source /etc/fish/config.fish
-+end