Force view via request headers instead of temporary flag

The flag approach is not thread safe.

src/octoprint/server/__init__.py

@@ -762,10 +762,14 @@ class Server(object):
 							self._logger.info("Preemptively caching {} (plugin {}) for {!r}".format(route, plugin, kwargs))
 						else:
 							self._logger.info("Preemptively caching {} for {!r}".format(route, kwargs))
+
+						headers = kwargs.get("headers", dict())
+						headers["X-Force-View"] = plugin if plugin else "_default"
+						kwargs["headers"] = headers
+
 						builder = EnvironBuilder(**kwargs)
-						with preemptive_cache.cache_environment(dict(plugin=plugin if plugin is not None else "_default")):
-							with preemptive_cache.disable_access_logging():
-								app(builder.get_environ(), lambda *a, **kw: None)
+						with preemptive_cache.disable_access_logging():
+							app(builder.get_environ(), lambda *a, **kw: None)
 					except:
 						self._logger.exception("Error while trying to preemptively cache {} for {!r}".format(route, kwargs))
 

src/octoprint/server/util/flask.py

@@ -448,13 +448,6 @@ class PreemptiveCache(object):
 			yield
 			self._log_access = True
 
-	@contextlib.contextmanager
-	def cache_environment(self, environment):
-		with self._environment_lock:
-			self.environment = environment
-			yield
-			self.environment = None
-
 	def clean_all_data(self, cleanup_function):
 		assert callable(cleanup_function)
 

src/octoprint/server/views.py

@@ -327,10 +327,7 @@ def index():
 		                                                   dict())
 		return preemptively_cached()
 
-	forced_view = None
-	preemptive_cache_environment = preemptiveCache.environment
-	if preemptive_cache_environment is not None and isinstance(preemptive_cache_environment, dict):
-		forced_view = preemptive_cache_environment.get("plugin", "_default")
+	forced_view = request.headers.get("X-Forced-View", None)
 
 	if forced_view:
 		# we have view forced by the preemptive cache
@@ -353,6 +350,8 @@ def index():
 				response = plugin_view(plugin)
 				if response is not None:
 					break
+				else:
+					_logger.warn("UiPlugin {} returned an empty response".format(plugin._identifier))
 		else:
 			response = default_view()