Scope manager using LogContexts

We piggy-back our tracer scopes by using log context.
The current log context gives us the current scope. If new scope is
created we create a stack of scopes in the context.

synapse/util/logcontext.py

@@ -164,6 +164,7 @@ class LoggingContext(object):
         "usage_start",
         "main_thread", "alive",
         "request", "tag",
+        "active_scope"
     ]
 
     thread_local = threading.local()
@@ -215,6 +216,7 @@ class LoggingContext(object):
         self.request = None
         self.tag = ""
         self.alive = True
+        self.active_scope = None
 
         self.parent_context = parent_context
 
@@ -299,10 +301,12 @@ class LoggingContext(object):
         another LoggingContext
         """
 
-        # 'request' is the only field we currently use in the logger, so that's
-        # all we need to copy
+        # we track the current request
         record.request = self.request
 
+        # we also track the current active_scope:
+        record.active_scope = self.active_scope
+
     def start(self):
         if threading.current_thread() is not self.main_thread:
             logger.warning("Started logcontext %s on different thread", self)

synapse/util/scopecontextmanager.py

@@ -0,0 +1,87 @@
+from .logcontext import LoggingContext
+from opentracing import ScopeManager, Scope
+import logging
+
+logger = logging.getLogger(__name__)
+
+class LogContextScopeManager(ScopeManager):
+
+    _homeserver_whitelist = ["*"]
+    _user_whitelist = ["*"]
+
+    def __init__(self, config):
+        # Set the whitelists
+        logger.info(config.tracer_config)
+        _homeserver_whitelist = config.tracer_config["homeserver_whitelist"]
+        _user_whitelist = config.tracer_config["user_whitelist"]
+
+    @property
+    def active(self):
+        """
+        Returns the currently active Scope which can be used to access the
+        currently active Scope.span.
+        If there is a non-null Scope, its wrapped Span
+        becomes an implicit parent of any newly-created Span at
+        Tracer.start_active_span() time.
+
+        Return: 
+            (Scope) : the Scope that is active, or None if not
+            available.
+        """
+        ctx = LoggingContext.current_context()
+        if ctx is LoggingContext.sentinel or ctx.active_scope is None:
+            return None
+        else:
+            return ctx.active_scope
+
+    def activate(self, span, finish_on_close):
+        """
+        Makes a Span active.
+        Args
+            span (Span): the span that should become active.
+            finish_on_close (Boolean): whether Span should be automatically
+                finished when Scope.close() is called.
+        
+        Return: 
+            Scope to control the end of the active period for
+            *span*. It is a programming error to neglect to call
+            Scope.close() on the returned instance.
+        """
+        logger.info("activating scope")
+        ctx = LoggingContext.current_context()
+        if ctx is LoggingContext.sentinel:
+            # We don't want this scope to affect.
+            logger.warning("Tried to activate scope outside of loggingcontext")
+            return Scope(None, span)
+
+        scope = _LogContextScope(self, span, finish_on_close)
+        self._set_logcontext_scope(scope, ctx)
+        return scope
+
+    def _set_logcontext_scope(self, scope, ctx=None):
+        if ctx is None:
+            ctx = LoggingContext.current_context()
+
+        ctx.active_scope = scope
+
+    def request_from_whitelisted_homeserver(self, request):
+        pass
+
+    def user_whitelisted(self, request):
+        pass
+
+class _LogContextScope(Scope):
+    def __init__(self, manager, span, finish_on_close):
+        super(_LogContextScope, self).__init__(manager, span)
+        self._finish_on_close = finish_on_close
+        self._to_restore = manager.active
+
+    def close(self):
+        if self.manager.active is not self:
+            logger.warning("Tried to close a none active scope!")
+            return
+
+        self.manager._set_logcontext_scope(self._to_restore)
+
+        if self._finish_on_close:
+            self.span.finish()