Fix the trace function for async functions. (#7872)

Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>

changelog.d/7872.bugfix

@@ -0,0 +1 @@
+Fix a long standing bug where the tracing of async functions with opentracing was broken.

synapse/logging/opentracing.py

@@ -733,6 +733,23 @@ def trace(func=None, opname=None):
 
         _opname = opname if opname else func.__name__
 
+        if inspect.iscoroutinefunction(func):
+
+            @wraps(func)
+            async def _trace_inner(*args, **kwargs):
+                if opentracing is None:
+                    return await func(*args, **kwargs)
+
+                with start_active_span(_opname) as scope:
+                    try:
+                        return await func(*args, **kwargs)
+                    except Exception:
+                        scope.span.set_tag(tags.ERROR, True)
+                        raise
+
+        else:
+            # The other case here handles both sync functions and those
+            # decorated with inlineDeferred.
             @wraps(func)
             def _trace_inner(*args, **kwargs):
                 if opentracing is None: