Keep a reference on the parent/original context to allow accessing it from Rest Client interceptors

Author: cescoffier

independent-projects/resteasy-reactive/client/runtime/src/main/java/org/jboss/resteasy/reactive/client/impl/ClientRequestContextImpl.java

@@ -66,6 +66,18 @@ public ClientRequestContextImpl(RestClientRequestContext restClientRequestContex
         // A separate context allows integrations like OTel to create a separate Span for each invocation (expected)
         Context current = client.vertx.getOrCreateContext();
         this.context = VertxContext.createNewDuplicatedContext(current);
+        // If an interceptor wants to access the parent context, it can do so by using the "__PARENT_CONTEXT__" key.
+        // This approach is compatible with the next-to-be nested context support (from SmallRye Common).
+        // The "__PARENT_CONTEXT__" key will continue to reference the _parent_ context.
+        // So, currently, an interceptor needing to read from the original context will need to use:
+        // Context parent = ContextLocals.get("__PARENT_CONTEXT__");
+        // V v = parent.getLocal("some-key");
+        // To write to the original context, it will need to use:
+        // Context parent = ContextLocals.get("__PARENT_CONTEXT__");
+        // parent.putLocal("some-key", someValue);
+        // Note that unlike with nested contexts, multiple child contexts can write to the same parent key, and may
+        // lead to a race condition.
+        this.context.putLocal("__PARENT_CONTEXT__", current);
         restClientRequestContext.properties.put(VERTX_CONTEXT_PROPERTY, context);
     }