Telemetry: Explicitly disconnect InspectorBackend.

Previously, the websocket was disconnected in __del__, whose timing was
non-deterministic. Now the websocket is disconnected as soon as its owner
(typically devtools_client_backend) no longer wants to keep track of it.

BUG=461153

Review URL: https://codereview.chromium.org/980043003

Cr-Commit-Position: refs/heads/master@{#319775}

tools/telemetry/telemetry/core/backends/chrome/ios_browser_finder.py

@@ -100,9 +100,12 @@ def FindAllAvailableBrowsers(finder_options, device):
   browser_types = set()
   for url in debug_urls:
     context = {'webSocketDebuggerUrl': url, 'id': 1}
-    inspector = inspector_backend.InspectorBackend(
-        backend.app, backend.devtools_client, context)
-    res = inspector.EvaluateJavaScript("navigator.userAgent")
+    try:
+      inspector = inspector_backend.InspectorBackend(
+          backend.app, backend.devtools_client, context)
+      res = inspector.EvaluateJavaScript("navigator.userAgent")
+    finally:
+      inspector.Disconnect()
     match_browsers = re.search(browser_pattern, res)
     if match_browsers:
       browser_types.add(match_browsers.group(1))

tools/telemetry/telemetry/core/backends/chrome_inspector/devtools_client_backend.py

@@ -30,6 +30,11 @@ def _IsDevToolsAgentAvailable(devtools_http_instance):
 
 
 class DevToolsClientBackend(object):
+  """An object that communicates with Chrome's devtools.
+
+  This class owns a map of InspectorBackends. It is responsible for creating
+  them and destroying them.
+  """
   def __init__(self, devtools_port, remote_devtools_port, app_backend):
     """Creates a new DevToolsClientBackend.
 
@@ -194,6 +199,8 @@ class _DevToolsContextMapBackend(object):
     context_ids = [context['id'] for context in contexts]
     for context_id in self._inspector_backends_dict.keys():
       if context_id not in context_ids:
+        backend = self._inspector_backends_dict[context_id]
+        backend.Disconnect()
         del self._inspector_backends_dict[context_id]
 
     valid_contexts = []

tools/telemetry/telemetry/core/backends/chrome_inspector/inspector_backend.py

@@ -34,13 +34,19 @@ def _HandleInspectorWebSocketExceptions(func):
   def inner(inspector_backend, *args, **kwargs):
     try:
       return func(inspector_backend, *args, **kwargs)
-    except (socket.error, websocket.WebSocketException) as e:
+    except (socket.error, websocket.WebSocketException,
+            inspector_websocket.WebSocketDisconnected) as e:
       inspector_backend._ConvertExceptionFromInspectorWebsocket(e)
 
   return inner
 
 
 class InspectorBackend(object):
+  """Class for communicating with a devtools client.
+
+  The owner of an instance of this class is responsible for calling
+  Disconnect() before disposing of the instance.
+  """
   def __init__(self, app, devtools_client, context, timeout=60):
     self._websocket = inspector_websocket.InspectorWebsocket()
     self._websocket.RegisterDomain(
@@ -68,8 +74,17 @@ class InspectorBackend(object):
     self._network = inspector_network.InspectorNetwork(self._websocket)
     self._timeline_model = None
 
+  def Disconnect(self):
+    """Disconnects the inspector websocket.
+
+    This method intentionally leaves the self._websocket object around, so that
+    future calls it to it will fail with a relevant error.
+    """
+    if self._websocket:
+      self._websocket.Disconnect()
+
   def __del__(self):
-    self._websocket.Disconnect()
+    self.Disconnect()
 
   @property
   def app(self):

tools/telemetry/telemetry/core/backends/chrome_inspector/inspector_websocket.py

@@ -8,8 +8,13 @@ import logging
 import socket
 import time
 
+from telemetry.core import exceptions
 from telemetry.core.backends.chrome_inspector import websocket
 
+class WebSocketDisconnected(exceptions.Error):
+  """An attempt was made to use a web socket after it had been disconnected."""
+  pass
+
 
 class InspectorWebsocket(object):
 
@@ -71,9 +76,12 @@ class InspectorWebsocket(object):
     """Sends a request without waiting for a response.
 
     Raises:
-      websocket.WebSocketException
-      socket.error
+      websocket.WebSocketException: Error from websocket library.
+      socket.error: Error from websocket library.
+      exceptions.WebSocketDisconnected: The socket was disconnected.
     """
+    if not self._socket:
+      raise WebSocketDisconnected()
     req['id'] = self._next_request_id
     self._next_request_id += 1
     data = json.dumps(req)
@@ -85,12 +93,13 @@ class InspectorWebsocket(object):
     """Sends a request and waits for a response.
 
     Raises:
-      websocket.WebSocketException
-      socket.error
+      websocket.WebSocketException: Error from websocket library.
+      socket.error: Error from websocket library.
+      exceptions.WebSocketDisconnected: The socket was disconnected.
     """
     self.SendAndIgnoreResponse(req)
 
-    while self._socket:
+    while True:
       res = self._Receive(timeout)
       if 'id' in res and res['id'] == req['id']:
         return res
@@ -99,8 +108,9 @@ class InspectorWebsocket(object):
     """Waits for responses from the websocket, dispatching them as necessary.
 
     Raises:
-      websocket.WebSocketException
-      socket.error
+      websocket.WebSocketException: Error from websocket library.
+      socket.error: Error from websocket library.
+      exceptions.WebSocketDisconnected: The socket was disconnected.
     """
     self._Receive(timeout)
 
@@ -110,9 +120,10 @@ class InspectorWebsocket(object):
       self._cur_socket_timeout = timeout
 
   def _Receive(self, timeout=10):
-    self._SetTimeout(timeout)
     if not self._socket:
-      return None
+      raise WebSocketDisconnected()
+
+    self._SetTimeout(timeout)
     data = self._socket.recv()
     result = json.loads(data)
     if logging.getLogger().isEnabledFor(logging.DEBUG):