svc: Load URL list from file

Instead of hard-coding the list of URLs to open, we'll read it from a
JSON file on the disk.  The file contains a mapping of monitor names to
URLs, e.g.

```json
{
    "HDMI-1": "http://my.site.one",
    "HDMI-2": "http://my.site.two"
}
```

svc/src/hudctrl/api.py

@@ -86,3 +86,8 @@ async def refresh_screen(number: int):
 @app.on_event('shutdown')
 async def on_shutdown():
     await svc.shutdown()
+
+
+@app.on_event('startup')
+async def on_startup():
+    await svc.startup()

svc/src/hudctrl/hud.py

@@ -1,6 +1,8 @@
 import asyncio
+import json
 import logging
-from typing import List, Optional
+from pathlib import Path
+from typing import Dict, List, Optional
 
 import pydantic
 from aiomarionette import Marionette, WindowRect
@@ -10,11 +12,6 @@ from .xrandr import MonitorConfig
 
 log = logging.getLogger(__name__)
 
-URLS = [
-    'https://grafana.pyrocufflink.blue/d/p3CPbi87z/hud?kiosk',
-    'https://grafana.pyrocufflink.blue/d/lGT5FHOnz/home?kiosk',
-]
-
 
 class NoMonitorConfig(Exception):
     '''Raised when no monitor config has been provided yet'''
@@ -33,6 +30,9 @@ class HUDService:
         self.port: Optional[int] = None
         self.monitor_config: Optional[MonitorConfig] = None
         self.marionette: Optional[Marionette] = None
+        self.urls: Dict[str, str] = {}
+
+        self.urls_file = Path('urls.json')
 
     async def get_screens(self) -> List[HUDScreen]:
         assert self.marionette
@@ -57,30 +57,56 @@ class HUDService:
             'Initializing display for %d monitors',
             len(self.monitor_config.monitors),
         )
-        window = await self.marionette.new_window('window')
+        window: Optional[str] = await self.marionette.new_window('window')
         for handle in await self.marionette.get_window_handles():
             if handle == window:
                 continue
             await self.marionette.switch_to_window(handle)
             await self.marionette.close_window()
         tasks = []
-        for i, monitor in enumerate(self.monitor_config.monitors):
+        for monitor in self.monitor_config.monitors:
             try:
-                url = URLS[i]
-            except IndexError:
-                break
+                url = self.urls[monitor.name]
+            except KeyError:
+                continue
+            if window is None:
+                window = await self.marionette.new_window('window')
             await self.marionette.switch_to_window(window)
             await self.marionette.set_window_rect(
                 x=monitor.pos_x,
                 y=1,
             )
             await self.marionette.fullscreen()
-            log.info('Screen %d: Opening URL %s', i, url)
+            log.info('Screen %s: Opening URL %s', monitor.name, url)
             tasks.append(asyncio.create_task(self.marionette.navigate(url)))
-            if i + 1 < min(len(URLS), len(self.monitor_config.monitors)):
-                window = await self.marionette.new_window('window')
+            window = None
+        if tasks:
             await asyncio.wait(tasks)
 
+    def load_urls(self) -> None:
+        try:
+            f = self.urls_file.open(encoding='utf-8')
+        except FileNotFoundError:
+            return
+        except OSError as e:
+            log.error('Could not load URL list: %s', e)
+            return
+        log.info('Loading URL list from %s', self.urls_file)
+        with f:
+            try:
+                value = json.load(f)
+            except ValueError as e:
+                log.error('Failed to load URL list: %s', e)
+                return
+            if isinstance(value, dict):
+                self.urls = value
+            else:
+                log.error(
+                    'Invalid URL list: found %r, expected %r',
+                    type(value),
+                    dict,
+                )
+
     async def refresh_screen(self, number: int) -> None:
         assert self.marionette
         windows = await self.marionette.get_window_handles()
@@ -101,3 +127,7 @@ class HUDService:
         if self.marionette is not None:
             await self.marionette.close()
             self.marionette = None
+
+    async def startup(self) -> None:
+        loop = asyncio.get_running_loop()
+        await loop.run_in_executor(None, self.load_urls)