When restoring a swapped out scope-stack, take defensive copy if the current thread is not the original. (#9403)

This mitigates a potential issue if the swapped out scope-stack is ever restored more than once on different threads.

Author: mcculls

dd-trace-core/src/main/java/datadog/trace/core/scopemanager/ContinuableScopeManager.java

@@ -367,7 +367,7 @@ public Context swap(Context context) {
     ContinuableScope newScope;
     if (context instanceof ScopeContext) {
       // restore previously swapped context stack
-      newStack = ((ScopeContext) context).scopeStack;
+      newStack = ((ScopeContext) context).restore();
       newScope = newStack.top;
     } else if (context != Context.root()) {
       // start a new stack and record the new context as active

dd-trace-core/src/main/java/datadog/trace/core/scopemanager/ScopeContext.java

@@ -6,7 +6,8 @@
 
 /** Wraps a {@link ScopeStack} as a {@link Context} so it can be swapped back later. */
 final class ScopeContext implements Context {
-  final ScopeStack scopeStack;
+  private final Thread originalThread = Thread.currentThread();
+  private final ScopeStack scopeStack;
   private final Context context;
 
   ScopeContext(ScopeStack scopeStack) {
@@ -18,6 +19,11 @@ private ScopeContext(ScopeStack scopeStack, Context context) {
     this.context = context;
   }
 
+  ScopeStack restore() {
+    // take defensive copy of original scope stack when restoring on different thread
+    return originalThread == Thread.currentThread() ? scopeStack : scopeStack.copy();
+  }
+
   @Nullable
   @Override
   public <T> T get(ContextKey<T> key) {