impr: Slightly speed up serializing scope (#4661)

Use a double checked lock for getting the span when building the
trace context.

CHANGELOG.md

@@ -6,6 +6,7 @@
 
 - Improve compiler error message for missing Swift declarations due to APPLICATION_EXTENSION_API_ONLY (#4603)
 - Mask screenshots for errors (#4623)
+- Slightly speed up serializing scope (#4661)
 
 ### Features
 

Sources/Sentry/SentryScope.m

@@ -465,9 +465,14 @@ - (void)clearAttachments
     }
 
     NSDictionary *traceContext = nil;
-    @synchronized(_spanLock) {
-        traceContext = [self buildTraceContext:_span];
+    id<SentrySpan> span = nil;
+
+    if (self.span != nil) {
+        @synchronized(_spanLock) {
+            span = self.span;
+        }
     }
+    traceContext = [self buildTraceContext:span];
     serializedData[@"traceContext"] = traceContext;
 
     NSDictionary *context = [self context];
@@ -606,8 +611,6 @@ - (SentryEvent *__nullable)applyToEvent:(SentryEvent *)event
         }
     }
 
-    // We don't need call synchronized(_spanLock) here because we get a copy of the span in the
-    // _spanLock above.
     newContext[@"trace"] = [self buildTraceContext:span];
 
     event.context = newContext;
@@ -619,9 +622,6 @@ - (void)addObserver:(id<SentryScopeObserver>)observer
     [self.observers addObject:observer];
 }
 
-/**
- * Make sure to call this inside @c  synchronized(_spanLock) caus this method isn't thread safe.
- */
 - (NSDictionary *)buildTraceContext:(nullable id<SentrySpan>)span
 {
     if (span != nil) {