svc: hud: Protect window switches with a lock

Any time we need to switch Firefox windows, we need to use a lock to
prevent multiple simultaneous requests.  If we do not, interleaved
Marionette commands may result in performing operations on the wrong
window.  For example, making two simultaneous requests for screenshots
is liable to return the wrong window for one of them.

svc/src/hudctrl/hud.py

@@ -35,10 +35,12 @@ class HUDService:
         self.windows: Dict[str, str] = {}
 
         self.urls_file = Path('urls.json')
+        self.lock = asyncio.Lock()
 
     async def get_screens(self) -> Dict[str, HUDScreen]:
         assert self.marionette
         screens = {}
+        async with self.lock:
             for name, handle in self.windows.items():
                 await self.marionette.switch_to_window(handle)
                 title = await self.marionette.get_title()
@@ -56,6 +58,7 @@ class HUDService:
             raise NoMonitorConfig(
                 'Cannot initialize display: No monitor config supplied'
             )
+        async with self.lock:
             log.info(
                 'Initializing display for %d monitors',
                 len(self.monitor_config.monitors),
@@ -82,7 +85,9 @@ class HUDService:
                 )
                 await self.marionette.fullscreen()
                 log.info('Screen %s: Opening URL %s', monitor.name, url)
-            tasks.append(asyncio.create_task(self.marionette.navigate(url)))
+                tasks.append(
+                    asyncio.create_task(self.marionette.navigate(url))
+                )
                 window = None
             if tasks:
                 await asyncio.wait(tasks)
@@ -113,6 +118,7 @@ class HUDService:
 
     async def refresh_screen(self, name: str) -> None:
         assert self.marionette
+        async with self.lock:
             await self.marionette.switch_to_window(self.windows[name])
             await self.marionette.refresh()
 
@@ -138,6 +144,7 @@ class HUDService:
     async def take_screenshot(self, screen: str) -> bytes:
         assert self.marionette
         handle = self.windows[screen]
+        async with self.lock:
             await self.marionette.switch_to_window(handle)
             data = await self.marionette.take_screenshot()
         return base64.b64decode(data)