nixos/tracker: Define env var so it can find miners’ subcommands

tracker looks in its directory tree for executable files
to make available as subcommands. Users expect to find subcommands
from tracker-miners package but that fails as they are in different
tree. We also cannot change the lookup path since tracker-miners
also depends on a library from tracker package.

Until we can break the dependency cycle on package level:

   tracker -> tracker-miners -> tracker-sparql (tracker)

we need to work around it. I chose to set an environment
variable that overrides the subcommands lookup to a tree
symlinking files from both packages in GNOME NixOS module.

https://gitlab.gnome.org/GNOME/tracker/-/issues/341
Fixes: https://github.com/NixOS/nixpkgs/issues/153378

nixos/modules/services/desktops/gnome/tracker-miners.nix

@@ -47,6 +47,8 @@ with lib;
 
     systemd.packages = [ pkgs.tracker-miners ];
 
+    services.gnome.tracker.subcommandPackages = [ pkgs.tracker-miners ];
+
   };
 
 }

nixos/modules/services/desktops/gnome/tracker.nix

@@ -4,6 +4,9 @@
 
 with lib;
 
+let
+  cfg = config.services.gnome.tracker;
+in
 {
 
   meta = {
@@ -33,6 +36,15 @@ with lib;
         '';
       };
 
+      subcommandPackages = mkOption {
+        type = types.listOf types.package;
+        default = [ ];
+        internal = true;
+        description = ''
+          List of packages containing tracker3 subcommands.
+        '';
+      };
+
     };
 
   };
@@ -40,7 +52,7 @@ with lib;
 
   ###### implementation
 
-  config = mkIf config.services.gnome.tracker.enable {
+  config = mkIf cfg.enable {
 
     environment.systemPackages = [ pkgs.tracker ];
 
@@ -48,6 +60,17 @@ with lib;
 
     systemd.packages = [ pkgs.tracker ];
 
+    environment.variables = {
+      TRACKER_CLI_SUBCOMMANDS_DIR =
+        let
+          subcommandPackagesTree = pkgs.symlinkJoin {
+            name = "tracker-with-subcommands-${pkgs.tracker.version}";
+            paths = [ pkgs.tracker ] ++ cfg.subcommandPackages;
+          };
+        in
+        "${subcommandPackagesTree}/libexec/tracker3";
+    };
+
   };
 
 }