installer: only uninstall packages managed by this module (#23)

Allow managed and unmanaged packages to coexist.

This PR introduces a state file that keeps track of the module's modifications, so they can be managed separately from imperatively introduced modifications.

A new uninstallUnmanagedPackages option has been added. When set to true (default), unmanaged flatpaks won't be uninstalled on activation. When set to false, all flatpacks will be overridden on module activation.

modules/home-manager.nix

@@ -43,7 +43,7 @@ in
     };
 
     home.activation = {
-      start-service = lib.hm.dag.entryAfter [ "writeBoundary" ] ''
+      flatpak-managed-install = lib.hm.dag.entryAfter [ "reloadSystemd" ] ''
         export PATH=${lib.makeBinPath (with pkgs; [ systemd ])}:$PATH
 
         $DRY_RUN_CMD systemctl is-system-running -q && \

modules/installer.nix

@@ -1,20 +1,42 @@
 { cfg, pkgs, lib, installation ? "system", ... }:
 
 let
+  # Put the state file in the `gcroots` folder of the respective installation,
+  # which prevents it from being garbage collected. This could probably be
+  # improved in the future if there are better conventions for how this should
+  # be handled. Right now it introduces a small issue of the state file derivation
+  # not being garbage collected even when this module is removed. You can find
+  # more details on this design drawback in PR#23
+  gcroots =
+    if (installation == "system")
+    then "/nix/var/nix/gcroots/"
+    else "\${XDG_STATE_HOME:-$HOME/.local/state}/home-manager/gcroots";
+  stateFile = pkgs.writeText "flatpak-state.json" (builtins.toJSON {
+    packages = map (builtins.getAttr "appId") cfg.packages;
+  });
+  statePath = "${gcroots}/${stateFile.name}";
+
   updateApplications = cfg.update.onActivation || cfg.update.auto.enable;
-  applicationsToKeep = lib.strings.concatStringsSep " " (map (builtins.getAttr "appId") cfg.packages);
-  flatpakUninstallCmd = installation: {}: ''
-    APPS_TO_KEEP=("${applicationsToKeep}")
-    # Get a list of currently installed Flatpak application IDs
-    INSTALLED_APPS=$(${pkgs.flatpak}/bin/flatpak  --${installation} list --app --columns=application | ${pkgs.gawk}/bin/awk '{print ''$1}')
 
-    # Iterate through the installed apps and uninstall those not present in the to keep list
-    for APP_ID in $INSTALLED_APPS; do
-        if [[ ! " ''${APPS_TO_KEEP[@]} " =~ " ''${APP_ID} " ]]; then
-            ${pkgs.flatpak}/bin/flatpak uninstall --${installation} -y ''$APP_ID
-        fi
-    done
+  handleUnmanagedPackagesCmd = installation: uninstallUnmanagedPackages:
+    lib.optionalString uninstallUnmanagedPackages ''
+      # Add all installed Flatpak packages to the old state, so only the managed ones (new state) will be kept
+      INSTALLED_PACKAGES=$(${pkgs.flatpak}/bin/flatpak --${installation} list --app --columns=application)
+      OLD_STATE=$(${pkgs.jq}/bin/jq -r -n \
+        --argjson old "$OLD_STATE" \
+        --arg installed_packages "$INSTALLED_PACKAGES" \
+        '$old + { "packages" : $installed_packages | split("\n") }')
+    '';
 
+  flatpakUninstallCmd = installation: {}: ''
+    # Uninstall all packages that are present in the old state but not the new one
+    ${pkgs.jq}/bin/jq -r -n \
+      --argjson old "$OLD_STATE" \
+      --argjson new "$NEW_STATE" \
+      '($old.packages - $new.packages)[]' \
+      | while read -r APP_ID; do
+          ${pkgs.flatpak}/bin/flatpak uninstall --${installation} -y $APP_ID
+        done
   '';
 
   flatpakInstallCmd = installation: update: { appId, origin ? "flathub", commit ? null, ... }: ''
@@ -40,6 +62,17 @@ pkgs.writeShellScript "flatpak-managed-install" ''
   # This script is triggered at build time by a transient systemd unit.
   set -eu
 
+  # Setup state variables
+  NEW_STATE=$(cat ${stateFile})
+  if [[ -f ${statePath} ]]; then
+    OLD_STATE=$(cat ${statePath})
+  else
+    OLD_STATE={}
+  fi
+
+  # Handle unmanaged packages
+  ${handleUnmanagedPackagesCmd installation cfg.uninstallUnmanagedPackages}
+
   # Configure remotes
   ${mkFlatpakAddRemoteCmd installation cfg.remotes}
 
@@ -49,4 +82,7 @@ pkgs.writeShellScript "flatpak-managed-install" ''
 
   # Install packages
   ${mkFlatpakInstallCmd installation updateApplications cfg.packages}
+
+  # Save state
+  ln -sf ${stateFile} ${statePath}
 ''

modules/options.nix

@@ -144,4 +144,14 @@ in
       };
     '';
   };
+
+  uninstallUnmanagedPackages = mkOption {
+    type = with types; bool;
+    default = false;
+    description = lib.mdDoc ''
+      If enabled, uninstall packages not managed by this module on activation.
+      I.e. if packages were installed via Flatpak directly instead of this module,
+      they would get uninstalled on the next activation
+    '';
+  };
 }