Warn if root thread yields

Author: Quinn-With-Two-Ns

temporal-sdk/src/main/java/io/temporal/internal/sync/DeterministicRunnerImpl.java

@@ -449,7 +449,7 @@ private WorkflowThread newRootThread(Runnable runnable) {
           "newRootThread can be called only if there is no existing root workflow thread");
     }
     rootWorkflowThread =
-        new WorkflowThreadImpl(
+        new RootWorkflowThreadImpl(
             workflowThreadExecutor,
             workflowContext,
             this,

temporal-sdk/src/main/java/io/temporal/internal/sync/RootWorkflowThreadImpl.java

@@ -0,0 +1,49 @@
+package io.temporal.internal.sync;
+
+import io.temporal.common.context.ContextPropagator;
+import io.temporal.internal.worker.WorkflowExecutorCache;
+import java.util.List;
+import java.util.Map;
+import java.util.function.Supplier;
+import javax.annotation.Nonnull;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+public class RootWorkflowThreadImpl extends WorkflowThreadImpl {
+  private static final Logger log = LoggerFactory.getLogger(RootWorkflowThreadImpl.class);
+
+  RootWorkflowThreadImpl(
+      WorkflowThreadExecutor workflowThreadExecutor,
+      SyncWorkflowContext syncWorkflowContext,
+      DeterministicRunnerImpl runner,
+      @Nonnull String name,
+      int priority,
+      boolean detached,
+      CancellationScopeImpl parentCancellationScope,
+      Runnable runnable,
+      WorkflowExecutorCache cache,
+      List<ContextPropagator> contextPropagators,
+      Map<String, Object> propagatedContexts) {
+    super(
+        workflowThreadExecutor,
+        syncWorkflowContext,
+        runner,
+        name,
+        priority,
+        detached,
+        parentCancellationScope,
+        runnable,
+        cache,
+        contextPropagators,
+        propagatedContexts);
+  }
+
+  @Override
+  public void yield(String reason, Supplier<Boolean> unblockCondition)
+      throws DestroyWorkflowThreadError {
+    log.warn(
+        "Detected root workflow thread yielding {}. This can happen by making blocking calls during workflow instance creating, such as executing an activity inside a @WorkflowInit constructor. This can cause issues, like Queries or Updates rejection because the workflow instance creation is delayed",
+        getName());
+    super.yield(reason, unblockCondition);
+  }
+}

temporal-sdk/src/main/java/io/temporal/workflow/WorkflowInit.java

@@ -34,7 +34,9 @@
  * thrown by the constructor are treated the same as exceptions thrown by the workflow method.
  *
  * <p>Workflow initialization methods are called before the workflow method, signal handlers, update
- * handlers or query handlers.
+ * handlers or query handlers. Users should be careful not to block in constructors. Blocking in the
+ * constructor will delay handling of the workflow method and signal handlers, and cause rejection
+ * of the update and query handlers.
  *
  * <p>This annotation applies only to workflow implementation constructors.
  */