Fix @tag_args being off-by-one (ahead)

changelog.d/13452.misc

@@ -0,0 +1 @@
+Fix `@tag_args` being off-by-one with the arguments when tagging a span (tracing).

synapse/logging/opentracing.py

@@ -901,6 +901,11 @@ def trace(func: Callable[P, R]) -> Callable[P, R]:
 def tag_args(func: Callable[P, R]) -> Callable[P, R]:
     """
     Tags all of the args to the active span.
+
+    Args:
+        func: `func` is assumed to be a method taking a `self` parameter, or a
+            `classmethod` taking a `cls` parameter. In either case, a tag is not
+            created for this parameter.
     """
 
     if not opentracing:
@@ -909,8 +914,14 @@ def tag_args(func: Callable[P, R]) -> Callable[P, R]:
     @wraps(func)
     def _tag_args_inner(*args: P.args, **kwargs: P.kwargs) -> R:
         argspec = inspect.getfullargspec(func)
-        for i, arg in enumerate(argspec.args[1:]):
-            set_tag("ARG_" + arg, str(args[i]))  # type: ignore[index]
+        # We use `[1:]` to skip the `self` object reference and `start=1` to
+        # make the index line up with `argspec.args`.
+        #
+        # FIXME: We could update this handle any type of function by ignoring the
+        #   first argument only if it's named `self` or `cls`. This isn't fool-proof
+        #   but handles the idiomatic cases.
+        for i, arg in enumerate(args[1:], start=1):  # type: ignore[index]
+            set_tag("ARG_" + argspec.args[i], str(arg))
         set_tag("args", str(args[len(argspec.args) :]))  # type: ignore[index]
         set_tag("kwargs", str(kwargs))
         return func(*args, **kwargs)