[core] Add import hook module

ddtrace/utils/hook.py

@@ -56,6 +56,11 @@ def register_post_import_hook(name, hook):
         sys.meta_path.insert(0, ImportHookFinder())
 
     hooks = _post_import_hooks.get(name, [])
+
+    if hook in hooks:
+        log.debug('hook "{}" already exists on module "{}"'.format(hook, name))
+        return
+
     module = sys.modules.get(name, None)
 
     # If the module has been imported already fire the hook and log a debug msg.

tests/subprocesstest.py

@@ -92,6 +92,7 @@ def _run_test_in_subprocess(self, result):
                 raise Exception('Subprocess Test "{}" Failed'.format(cmdf))
             except Exception:
                 exc_info = sys.exc_info()
+
             sys.stderr.write(stderr.decode())
             sys.stdout.write(stdout.decode())
             result.addFailure(self, exc_info)

tests/utils/test_hook.py

@@ -75,10 +75,20 @@ def test_register_post_import_hook_duplicate_register(self):
         Test that a function can be registered as a hook twice.
         """
         test_hook = mock.MagicMock()
-        register_post_import_hook('tests.utils.test_module', test_hook)
-        register_post_import_hook('tests.utils.test_module', test_hook)
-        import tests.utils.test_module  # noqa
-        self.assertEqual(test_hook.call_count, 2)
+        with mock.patch('ddtrace.utils.hook.log') as log_mock:
+            register_post_import_hook('tests.utils.test_module', test_hook)
+            register_post_import_hook('tests.utils.test_module', test_hook)
+            import tests.utils.test_module  # noqa
+
+            class Matcher(object):
+                def __eq__(self, other):
+                    return 'already exists on module "tests.utils.test_module"'
+
+            calls = [
+                mock.call(Matcher())
+            ]
+            self.assertEqual(test_hook.call_count, 1)
+            log_mock.debug.assert_has_calls(calls)
 
     def test_deregister_post_import_hook_no_register(self):
         """
@@ -128,8 +138,9 @@ def test_deregister_post_import_hook_after_register_multiple_one_match(self):
         """
         Test that only specified import hooks can be deregistered after being registered.
         """
-        test_hook = mock.MagicMock()
-        test_hook2 = mock.MagicMock()
+        # Enforce a spec so that hasattr doesn't vacuously return True.
+        test_hook = mock.MagicMock(spec=[])
+        test_hook2 = mock.MagicMock(spec=[])
         setattr(test_hook, '_test', True)
         setattr(test_hook2, '_test2', True)
         register_post_import_hook('tests.utils.test_module', test_hook)
@@ -141,7 +152,7 @@ def matcher(hook):
         deregister_post_import_hook('tests.utils.test_module', matcher)
         import tests.utils.test_module  # noqa
         self.assertEqual(test_hook.call_count, 0, 'hook has been deregistered and should be removed')
-        self.assertEqual(test_hook2.call_count, 0, 'hook has been deregistered and should be removed')
+        self.assertEqual(test_hook2.call_count, 1, 'hook should have been called')
 
     def test_deregister_post_import_hook_after_import(self):
         """