nixos/home-assistant: reload the daemon when configuration changed

Reload the service when configuration changes. This means that we don't
have a potentially slow startup for every small configuration change.

nixos/modules/services/home-automation/home-assistant.nix

@@ -369,6 +369,17 @@ in {
 
     networking.firewall.allowedTCPPorts = mkIf cfg.openFirewall [ cfg.config.http.server_port ];
 
+    # symlink the configuration to /etc/home-assistant
+    environment.etc = lib.mkMerge [
+      (lib.mkIf (cfg.config != null && !cfg.configWritable) {
+        "home-assistant/configuration.yaml".source = configFile;
+      })
+
+      (lib.mkIf (cfg.lovelaceConfig != null && !cfg.lovelaceConfigWritable) {
+        "home-assistant/ui-lovelace.yaml".source = lovelaceConfigFile;
+      })
+    ];
+
     systemd.services.home-assistant = {
       description = "Home Assistant";
       after = [
@@ -378,18 +389,22 @@ in {
         "mysql.service"
         "postgresql.service"
       ];
+      reloadTriggers = [
+        configFile
+        lovelaceConfigFile
+      ];
       preStart = let
         copyConfig = if cfg.configWritable then ''
           cp --no-preserve=mode ${configFile} "${cfg.configDir}/configuration.yaml"
         '' else ''
           rm -f "${cfg.configDir}/configuration.yaml"
-          ln -s ${configFile} "${cfg.configDir}/configuration.yaml"
+          ln -s /etc/home-assistant/configuration.yaml "${cfg.configDir}/configuration.yaml"
         '';
         copyLovelaceConfig = if cfg.lovelaceConfigWritable then ''
           cp --no-preserve=mode ${lovelaceConfigFile} "${cfg.configDir}/ui-lovelace.yaml"
         '' else ''
           rm -f "${cfg.configDir}/ui-lovelace.yaml"
-          ln -s ${lovelaceConfigFile} "${cfg.configDir}/ui-lovelace.yaml"
+          ln -s /etc/home-assistant/ui-lovelace.yaml "${cfg.configDir}/ui-lovelace.yaml"
         '';
       in
         (optionalString (cfg.config != null) copyConfig) +

nixos/tests/home-assistant.nix

@@ -98,9 +98,26 @@ in {
       };
       lovelaceConfigWritable = true;
     };
+
+    # Cause a configuration change inside `configuration.yml` and verify that the process is being reloaded.
+    specialisation.differentName = {
+      inheritParentConfig = true;
+      configuration.services.home-assistant.config.homeassistant.name = lib.mkForce "Test Home";
+    };
+
+    # Cause a configuration change that requires a service restart as we added a new runtime dependency
+    specialisation.newFeature = {
+      inheritParentConfig = true;
+      configuration.services.home-assistant.config.device_tracker = [
+        { platform = "bluetooth_tracker"; }
+      ];
+    };
   };
 
-  testScript = ''
+  testScript = { nodes, ... }: let
+    system = nodes.hass.config.system.build.toplevel;
+  in
+  ''
     import re
 
     start_all()
@@ -142,6 +159,19 @@ in {
     with subtest("Check extra components are considered in systemd unit hardening"):
         hass.succeed("systemctl show -p DeviceAllow home-assistant.service | grep -q char-ttyUSB")
 
+    with subtest("Check service reloads when configuration changes"):
+      # store the old pid of the process
+      pid = hass.succeed("systemctl show --property=MainPID home-assistant.service")
+      hass.succeed("${system}/specialisation/differentName/bin/switch-to-configuration test")
+      new_pid = hass.succeed("systemctl show --property=MainPID home-assistant.service")
+      assert pid == new_pid, "The PID of the process should not change between process reloads"
+
+    with subtest("check service restarts when package changes"):
+      pid = new_pid
+      hass.succeed("${system}/specialisation/newFeature/bin/switch-to-configuration test")
+      new_pid = hass.succeed("systemctl show --property=MainPID home-assistant.service")
+      assert pid != new_pid, "The PID of the process shoudl change when the HA binary changes"
+
     with subtest("Print log to ease debugging"):
         output_log = hass.succeed("cat ${configDir}/home-assistant.log")
         print("\n### home-assistant.log ###\n")