Repository: census-instrumentation/opencensus-python (python)

Issue:
OpenCensusServerInterceptor reports incorrect timing
There is a bug in OpenCensusServerInterceptor that causes server spans to be finished immediately. You can reproduce it by running the helloworld grpc example in examples/trace/grpc. You should see a server span with a very short execution time, for example when I run it locally with a print exporter I get:

`{'traceId': 'a8fdeb7813144b178cb4b3d3d9b8fee4', 'spans': [{'childSpanCount': 0, 'displayName': {'truncated_byte_count': 0, 'value': 'grpc_server'}, 'parentSpanId': 4417962580138865, 'startTime': '2018-01-23T16:01:17.826761Z', 'attributes': {'attributeMap': {'/component': {'string_value': {'truncated_byte_count': 0, 'value': 'grpc'}}}}, 'spanId': 7098108131070600, 'endTime': '2018-01-23T16:01:17.826771Z'}]}`

The problem is that [this line](https://github.com/census-instrumentation/opencensus-python/blob/master/opencensus/trace/ext/grpc/server_interceptor.py#L62) is being traced, but it isn't actually executing the server handler code. It just returns an RPCMethodHandler, which defers execution. 

I hacked up a [quick fix](https://gist.github.com/wkiser/d086a3f623f135265914e2b51bb722e8) that wraps the RPCMethodHandler and instruments the span when the handler is evaluated. A better solution might be to have the RpcMethodHandlerWrapper accept arbitrary callbacks, or to update grpc to have RPCMethodHandler accept callbacks natively.

Happy to submit a PR if you have a preferred approach. Thanks!

Pull request description:
Update grpc server_interceptor to report accurate timing
fixes #107. Updated my example to accept pre handler and post handler callbacks as requested. I also updated the code to set the tracer in the interceptor as the current tracer in the execution context.

Patch:
diff --git a/opencensus/trace/ext/grpc/server_interceptor.py b/opencensus/trace/ext/grpc/server_interceptor.py
index 86e633e..6e07099 100644
--- a/opencensus/trace/ext/grpc/server_interceptor.py
+++ b/opencensus/trace/ext/grpc/server_interceptor.py
@@ -11,12 +11,14 @@
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
+import collections
+import logging
 
 import grpc
-import logging
 
 from opencensus.trace import attributes_helper
 from opencensus.trace import tracer as tracer_module
+from opencensus.trace import execution_context
 from opencensus.trace.ext import grpc as oc_grpc
 from opencensus.trace.propagation import binary_format
 
@@ -24,24 +26,72 @@ ATTRIBUTE_COMPONENT = 'COMPONENT'
 ATTRIBUTE_ERROR_NAME = 'ERROR_NAME'
 ATTRIBUTE_ERROR_MESSAGE = 'ERROR_MESSAGE'
 
+RpcRequestInfo = collections.namedtuple(
+    'RPCRequestInfo', ('request', 'context')
+)
+RpcResponseInfo = collections.namedtuple(
+    'RPCCallbackInfo', ('request', 'context', 'response', 'exc')
+)
+
+
+class RpcMethodHandlerWrapper(object):
+    """Wraps a grpc RPCMethodHandler and records the variables about the
+     execution context and response
+    """
+
+    def __init__(
+        self, handler, pre_handler_callbacks=None, post_handler_callbacks=None
+    ):
+        """
+        :param handler: instance of RpcMethodHandler
+
+        :param pre_handler_callbacks: iterable of callbacks that accept an
+         instance of RpcRequestInfo that are called before the server handler
+
+        :param post_handler_callbacks: iterable of callbacks that accept an
+         instance of RpcResponseInfo that are called after the server
+         handler finishes execution
+        """
+        self.handler = handler
+        self._pre_handler_callbacks = pre_handler_callbacks or []
+        self._post_handler_callbacks = post_handler_callbacks or []
+
+    def proxy(self, prop_name):
+        def _wrapper(request, context, *args, **kwargs):
+            for callback in self._pre_handler_callbacks:
+                callback(RpcRequestInfo(request, context))
+            exc = None
+            response = None
+            try:
+                response = getattr(
+                    self.handler, prop_name
+                )(request, context, *args, **kwargs)
+            except Exception as e:
+                logging.error(e)
+                exc = e
+                raise
+            finally:
+                for callback in self._post_handler_callbacks:
+                    callback(RpcResponseInfo(request, context, response, exc))
+            return response
+
+        return _wrapper
 
-class OpenCensusServerInterceptor(grpc.ServerInterceptor):
+    def __getattr__(self, item):
+        if item in (
+            'unary_unary', 'unary_stream', 'stream_unary', 'stream_stream'
+        ):
+            return self.proxy(item)
+        return getattr(self.handler, item)
 
+
+class OpenCensusServerInterceptor(grpc.ServerInterceptor):
     def __init__(self, sampler=None, exporter=None):
         self.sampler = sampler
         self.exporter = exporter
 
-    def _start_server_span(self, tracer):
-        span = tracer.start_span(name='grpc_server')
-        tracer.add_attribute_to_current_span(
-            attribute_key=attributes_helper.COMMON_ATTRIBUTES.get(
-                ATTRIBUTE_COMPONENT),
-            attribute_value='grpc')
-
-        return span
-
-    def intercept_handler(self, continuation, handler_call_details):
-        metadata = handler_call_details.invocation_metadata
+    def _start_server_span(self, rpc_request_info):
+        metadata = rpc_request_info.context.invocation_metadata()
         span_context = None
 
         if metadata is not None:
@@ -55,21 +105,30 @@ class OpenCensusServerInterceptor(grpc.ServerInterceptor):
                                       sampler=self.sampler,
                                       exporter=self.exporter)
 
-        with self._start_server_span(tracer):
-            response = None
+        span = tracer.start_span(name='grpc_server')
+        tracer.add_attribute_to_current_span(
+            attribute_key=attributes_helper.COMMON_ATTRIBUTES.get(
+                ATTRIBUTE_COMPONENT),
+            attribute_value='grpc')
 
-            try:
-                response = continuation(handler_call_details)
-            except Exception as e:  # pragma: NO COVER
-                logging.error(e)
-                tracer.add_attribute_to_current_span(
-                    attributes_helper.COMMON_ATTRIBUTES.get(
-                        ATTRIBUTE_ERROR_MESSAGE),
-                    str(e))
-                tracer.end_span()
-                raise
+        execution_context.set_opencensus_tracer(tracer)
+        execution_context.set_current_span(span)
+
+    def _end_server_span(self, rpc_response_info):
+        tracer = execution_context.get_opencensus_tracer()
+        if rpc_response_info.exc is not None:
+            tracer.add_attribute_to_current_span(
+                attributes_helper.COMMON_ATTRIBUTES.get(
+                    ATTRIBUTE_ERROR_MESSAGE),
+                str(rpc_response_info.exc))
+        tracer.end_span()
 
-        return response
+    def intercept_handler(self, continuation, handler_call_details):
+        return RpcMethodHandlerWrapper(
+            continuation(handler_call_details),
+            pre_handler_callbacks=[self._start_server_span],
+            post_handler_callbacks=[self._end_server_span]
+        )
 
     def intercept_service(self, continuation, handler_call_details):
         return self.intercept_handler(continuation, handler_call_details)


Test patch:
diff --git a/tests/unit/trace/ext/grpc/test_server_interceptor.py b/tests/unit/trace/ext/grpc/test_server_interceptor.py
index dee3aaa..d7c4d29 100644
--- a/tests/unit/trace/ext/grpc/test_server_interceptor.py
+++ b/tests/unit/trace/ext/grpc/test_server_interceptor.py
@@ -16,8 +16,8 @@ import unittest
 
 import mock
 
-from opencensus.trace.ext.grpc import server_interceptor
 from opencensus.trace import execution_context
+from opencensus.trace.ext.grpc import server_interceptor
 
 
 class TestOpenCensusServerInterceptor(unittest.TestCase):
@@ -29,19 +29,28 @@ class TestOpenCensusServerInterceptor(unittest.TestCase):
         self.assertEqual(interceptor.sampler, sampler)
         self.assertEqual(interceptor.exporter, exporter)
 
+    def test_rpc_handler_wrapper(self):
+        """Ensure that RPCHandlerWrapper proxies to the unerlying handler"""
+        mock_handler = mock.Mock()
+        mock_handler.response_streaming = False
+        wrapper = server_interceptor.RpcMethodHandlerWrapper(mock_handler)
+        self.assertEqual(wrapper.response_streaming, False)
+
     def test_intercept_handler_no_metadata(self):
         current_span = mock.Mock()
         mock_tracer = MockTracer(None, None, None)
         patch = mock.patch(
             'opencensus.trace.ext.grpc.server_interceptor.tracer_module.Tracer',
             MockTracer)
-        mock_details = mock.Mock()
-        mock_details.invocation_metadata = None
+        mock_context = mock.Mock()
+        mock_context.invocation_metadata = mock.Mock(return_value=None)
         interceptor = server_interceptor.OpenCensusServerInterceptor(
             None, None)
 
         with patch:
-            interceptor.intercept_handler(mock.Mock(), mock_details)
+            interceptor.intercept_handler(
+                mock.Mock(), mock.Mock()
+            ).unary_unary(mock.Mock(), mock_context)
 
         expected_attributes = {
             '/component': 'grpc',
@@ -57,13 +66,17 @@ class TestOpenCensusServerInterceptor(unittest.TestCase):
         patch = mock.patch(
             'opencensus.trace.ext.grpc.server_interceptor.tracer_module.Tracer',
             MockTracer)
-        mock_details = mock.Mock()
-        mock_details.invocation_metadata = (('test_key', b'test_value'),)
+        mock_context = mock.Mock()
+        mock_context.invocation_metadata = mock.Mock(
+            return_value=(('test_key', b'test_value'),)
+        )
         interceptor = server_interceptor.OpenCensusServerInterceptor(
             None, None)
 
         with patch:
-            interceptor.intercept_handler(mock.Mock(), mock_details)
+            interceptor.intercept_handler(
+                mock.Mock(), mock.Mock()
+            ).unary_unary(mock.Mock(), mock_context)
 
         expected_attributes = {
             '/component': 'grpc',
@@ -81,6 +94,36 @@ class TestOpenCensusServerInterceptor(unittest.TestCase):
         interceptor.intercept_service(None, None)
         self.assertTrue(mock_handler.called)
 
+    def test_intercept_handler_exception(self):
+        current_span = mock.Mock()
+        mock_tracer = MockTracer(None, None, None)
+        patch = mock.patch(
+            'opencensus.trace.ext.grpc.server_interceptor.tracer_module.Tracer',
+            MockTracer)
+        interceptor = server_interceptor.OpenCensusServerInterceptor(
+            None, None)
+        mock_context = mock.Mock()
+        mock_context.invocation_metadata = mock.Mock(return_value=None)
+        mock_continuation = mock.Mock()
+        mock_continuation.unary_unary = mock.Mock(side_effect=Exception('Test'))
+        with patch:
+            # patch the wrapper's handler to return an exception
+            rpc_wrapper = interceptor.intercept_handler(
+                mock.Mock(), mock.Mock())
+            rpc_wrapper.handler.unary_unary = mock.Mock(
+                side_effect=Exception('Test'))
+            with self.assertRaises(Exception):
+                rpc_wrapper.unary_unary(mock.Mock(), mock_context)
+
+        expected_attributes = {
+            '/component': 'grpc',
+            '/error/message': 'Test'
+        }
+
+        self.assertEqual(
+            execution_context.get_opencensus_tracer().current_span.attributes,
+            expected_attributes)
+
 
 class MockTracer(object):
     def __init__(self, *args, **kwargs):