Optimise LoggingContext creation and copying

It turns out that the only thing we use the __dict__ of LoggingContext for is
`request`, and given we create lots of LoggingContexts and then copy them every
time we do a db transaction or log line, using the __dict__ seems a bit
redundant. Let's try to optimise things by making the request attribute
explicit.

synapse/util/logcontext.py

@@ -52,13 +52,16 @@ except Exception:
 class LoggingContext(object):
     """Additional context for log formatting. Contexts are scoped within a
     "with" block.
+
     Args:
         name (str): Name for the context for debugging.
     """
 
     __slots__ = [
-        "previous_context", "name", "usage_start", "usage_end", "main_thread",
+        "previous_context", "name", "ru_stime", "ru_utime",
-        "__dict__", "tag", "alive",
+        "db_txn_count", "db_txn_duration", "usage_start", "usage_end",
+        "main_thread", "alive",
+        "request", "tag",
     ]
 
     thread_local = threading.local()
@@ -96,7 +99,9 @@ class LoggingContext(object):
         self.db_txn_count = 0
         self.db_txn_duration = 0.
         self.usage_start = None
+        self.usage_end = None
         self.main_thread = threading.current_thread()
+        self.request = None
         self.tag = ""
         self.alive = True
 
@@ -105,7 +110,11 @@ class LoggingContext(object):
 
     @classmethod
     def current_context(cls):
-        """Get the current logging context from thread local storage"""
+        """Get the current logging context from thread local storage
+
+        Returns:
+            LoggingContext: the current logging context
+        """
         return getattr(cls.thread_local, "current_context", cls.sentinel)
 
     @classmethod
@@ -155,11 +164,13 @@ class LoggingContext(object):
         self.alive = False
 
     def copy_to(self, record):
-        """Copy fields from this context to the record"""
+        """Copy logging fields from this context to a log record or
-        for key, value in self.__dict__.items():
+        another LoggingContext
-            setattr(record, key, value)
+        """
 
-        record.ru_utime, record.ru_stime = self.get_resource_usage()
+        # 'request' is the only field we currently use in the logger, so that's
+        # all we need to copy
+        record.request = self.request
 
     def start(self):
         if threading.current_thread() is not self.main_thread:

tests/crypto/test_keyring.py

@@ -68,7 +68,7 @@ class KeyringTestCase(unittest.TestCase):
 
     def check_context(self, _, expected):
         self.assertEquals(
-            getattr(LoggingContext.current_context(), "test_key", None),
+            getattr(LoggingContext.current_context(), "request", None),
             expected
         )
 
@@ -82,7 +82,7 @@ class KeyringTestCase(unittest.TestCase):
         lookup_2_deferred = defer.Deferred()
 
         with LoggingContext("one") as context_one:
-            context_one.test_key = "one"
+            context_one.request = "one"
 
             wait_1_deferred = kr.wait_for_previous_lookups(
                 ["server1"],
@@ -96,7 +96,7 @@ class KeyringTestCase(unittest.TestCase):
             wait_1_deferred.addBoth(self.check_context, "one")
 
         with LoggingContext("two") as context_two:
-            context_two.test_key = "two"
+            context_two.request = "two"
 
             # set off another wait. It should block because the first lookup
             # hasn't yet completed.
@@ -137,7 +137,7 @@ class KeyringTestCase(unittest.TestCase):
         @defer.inlineCallbacks
         def get_perspectives(**kwargs):
             self.assertEquals(
-                LoggingContext.current_context().test_key, "11",
+                LoggingContext.current_context().request, "11",
             )
             with logcontext.PreserveLoggingContext():
                 yield persp_deferred
@@ -145,7 +145,7 @@ class KeyringTestCase(unittest.TestCase):
         self.http_client.post_json.side_effect = get_perspectives
 
         with LoggingContext("11") as context_11:
-            context_11.test_key = "11"
+            context_11.request = "11"
 
             # start off a first set of lookups
             res_deferreds = kr.verify_json_objects_for_server(
@@ -173,7 +173,7 @@ class KeyringTestCase(unittest.TestCase):
             self.assertIs(LoggingContext.current_context(), context_11)
 
             context_12 = LoggingContext("12")
-            context_12.test_key = "12"
+            context_12.request = "12"
             with logcontext.PreserveLoggingContext(context_12):
                 # a second request for a server with outstanding requests
                 # should block rather than start a second call
@@ -211,7 +211,7 @@ class KeyringTestCase(unittest.TestCase):
         sentinel_context = LoggingContext.current_context()
 
         with LoggingContext("one") as context_one:
-            context_one.test_key = "one"
+            context_one.request = "one"
 
             defer = kr.verify_json_for_server("server9", {})
             try:

tests/util/test_logcontext.py

@@ -12,12 +12,12 @@ class LoggingContextTestCase(unittest.TestCase):
 
     def _check_test_key(self, value):
         self.assertEquals(
-            LoggingContext.current_context().test_key, value
+            LoggingContext.current_context().request, value
         )
 
     def test_with_context(self):
         with LoggingContext() as context_one:
-            context_one.test_key = "test"
+            context_one.request = "test"
             self._check_test_key("test")
 
     @defer.inlineCallbacks
@@ -25,14 +25,14 @@ class LoggingContextTestCase(unittest.TestCase):
         @defer.inlineCallbacks
         def competing_callback():
             with LoggingContext() as competing_context:
-                competing_context.test_key = "competing"
+                competing_context.request = "competing"
                 yield sleep(0)
                 self._check_test_key("competing")
 
         reactor.callLater(0, competing_callback)
 
         with LoggingContext() as context_one:
-            context_one.test_key = "one"
+            context_one.request = "one"
             yield sleep(0)
             self._check_test_key("one")
 
@@ -43,14 +43,14 @@ class LoggingContextTestCase(unittest.TestCase):
 
         @defer.inlineCallbacks
         def cb():
-            context_one.test_key = "one"
+            context_one.request = "one"
             yield function()
             self._check_test_key("one")
 
             callback_completed[0] = True
 
         with LoggingContext() as context_one:
-            context_one.test_key = "one"
+            context_one.request = "one"
 
             # fire off function, but don't wait on it.
             logcontext.preserve_fn(cb)()
@@ -107,7 +107,7 @@ class LoggingContextTestCase(unittest.TestCase):
         sentinel_context = LoggingContext.current_context()
 
         with LoggingContext() as context_one:
-            context_one.test_key = "one"
+            context_one.request = "one"
 
             d1 = logcontext.make_deferred_yieldable(blocking_function())
             # make sure that the context was reset by make_deferred_yieldable
@@ -124,7 +124,7 @@ class LoggingContextTestCase(unittest.TestCase):
         argument isn't actually a deferred"""
 
         with LoggingContext() as context_one:
-            context_one.test_key = "one"
+            context_one.request = "one"
 
             d1 = logcontext.make_deferred_yieldable("bum")
             self._check_test_key("one")