debugger_v2: start multiplexer lazily

docs/debugger_v2.md

@@ -118,8 +118,9 @@ tensorboard --logdir /tmp/tfdbg2_logdir
 ```
 
 In the web browser, navigate to TensorBoard’s page at http://localhost:6006. The
-“Debugger V2” plugin should be activated by default, displaying a page that
-looks like the following:
+“Debugger V2” plugin will be inactive by default, so select it from the
+“Inactive plugins” menu at top right. Once selected, it should look like the
+following:
 
 ![Debugger V2 full view screenshot](./images/debugger_v2_1_full_view.png)
 

tensorboard/plugins/debugger_v2/debugger_v2_plugin.py

@@ -15,6 +15,8 @@
 """The TensorBoard Debugger V2 plugin."""
 
 
+import threading
+
 from werkzeug import wrappers
 
 from tensorboard import errors
@@ -50,11 +52,23 @@ def __init__(self, context):
         """
         super(DebuggerV2Plugin, self).__init__(context)
         self._logdir = context.logdir
-        # TODO(cais): Implement factory for DataProvider that takes into account
-        # the settings.
-        self._data_provider = debug_data_provider.LocalDebuggerV2DataProvider(
-            self._logdir
-        )
+        self._underlying_data_provider = None
+        # Held while initializing `_underlying_data_provider` for the first
+        # time, to make sure that we only construct one.
+        self._data_provider_init_lock = threading.Lock()
+
+    @property
+    def _data_provider(self):
+        if self._underlying_data_provider is not None:
+            return self._underlying_data_provider
+        with self._data_provider_init_lock:
+            if self._underlying_data_provider is not None:
+                return self._underlying_data_provider
+            # TODO(cais): Implement factory for DataProvider that takes into account
+            # the settings.
+            dp = debug_data_provider.LocalDebuggerV2DataProvider(self._logdir)
+            self._underlying_data_provider = dp
+            return dp
 
     def get_plugin_apps(self):
         # TODO(cais): Add routes as they are implemented.
@@ -73,16 +87,8 @@ def get_plugin_apps(self):
         }
 
     def is_active(self):
-        """Check whether the Debugger V2 Plugin is always active.
-
-        When no data in the tfdbg v2 format is available, a custom information
-        screen is displayed to instruct the user on how to generate such data
-        to be able to use the plugin.
-
-        Returns:
-          `True` if and only if data in tfdbg v2's DebugEvent format is available.
-        """
-        return bool(self._data_provider.list_runs(experiment_id=""))
+        """The Debugger V2 plugin must be manually selected."""
+        return False
 
     def frontend_metadata(self):
         return base_plugin.FrontendMetadata(

tensorboard/plugins/debugger_v2/debugger_v2_plugin_test.py

@@ -150,7 +150,7 @@ def testPluginIsNotActiveByDefault(self):
 
     def testPluginIsActiveWithDataExists(self):
         _generate_tfdbg_v2_data(self.logdir)
-        self.assertTrue(self.plugin.is_active())
+        self.assertFalse(self.plugin.is_active())  # never explicitly active
 
     def testConcurrentCallsToPluginIsActiveWhenNotActive(self):
         results = collections.deque()
@@ -165,20 +165,6 @@ def query_is_active():
             thread.join()
         self.assertEqual(list(results), [False] * 4)
 
-    def testConcurrentCallsToPluginIsActiveWhenActive(self):
-        _generate_tfdbg_v2_data(self.logdir)
-        results = collections.deque()
-
-        def query_is_active():
-            results.append(self.plugin.is_active())
-
-        threads = [threading.Thread(target=query_is_active) for _ in range(4)]
-        for thread in threads:
-            thread.start()
-        for thread in threads:
-            thread.join()
-        self.assertEqual(list(results), [True] * 4)
-
     def testServeRunsWithoutExistingRuns(self):
         response = self.server.get(_ROUTE_PREFIX + "/runs")
         self.assertEqual(200, response.status_code)