fix: Avoid crashes when scope or hub is racy (#517)

* fix: Avoid crashes when scope or hub is racy

* fix: Fix None deref

sentry_sdk/hub.py

@@ -440,8 +440,9 @@ def start_span(
         kwargs.setdefault("hub", self)
 
         if span is None:
-            if scope.span is not None:
-                span = scope.span.new_span(**kwargs)
+            span = scope.span
+            if span is not None:
+                span = span.new_span(**kwargs)
             else:
                 span = Span(**kwargs)
 
@@ -570,17 +571,19 @@ def iter_trace_propagation_headers(self):
         # type: () -> Generator[Tuple[str, str], None, None]
         # TODO: Document
         client, scope = self._stack[-1]
-        if scope._span is None:
+        span = scope.span
+
+        if span is None:
             return
 
         propagate_traces = client and client.options["propagate_traces"]
         if not propagate_traces:
             return
 
         if client and client.options["traceparent_v2"]:
-            traceparent = scope._span.to_traceparent()
+            traceparent = span.to_traceparent()
         else:
-            traceparent = scope._span.to_legacy_traceparent()
+            traceparent = span.to_legacy_traceparent()
 
         yield "sentry-trace", traceparent
 

sentry_sdk/scope.py

@@ -66,6 +66,12 @@ class Scope(object):
     events that belong to it.
     """
 
+    # NOTE: Even though it should not happen, the scope needs to not crash when
+    # accessed by multiple threads. It's fine if it's full of races, but those
+    # races should never make the user application crash.
+    #
+    # The same needs to hold for any accesses of the scope the SDK makes.
+
     __slots__ = (
         "_level",
         "_name",
@@ -124,8 +130,9 @@ def transaction(self, value):
         # type: (Optional[str]) -> None
         """When set this forces a specific transaction name to be set."""
         self._transaction = value
-        if self._span:
-            self._span.transaction = value
+        span = self._span
+        if span:
+            span.transaction = value
 
     @_attr_setter
     def user(self, value):
@@ -143,8 +150,10 @@ def span(self):
     def span(self, span):
         # type: (Optional[Span]) -> None
         self._span = span
-        if span is not None and span.transaction:
-            self._transaction = span.transaction
+        if span is not None:
+            span_transaction = span.transaction
+            if span_transaction:
+                self._transaction = span_transaction
 
     def set_tag(
         self,