Add WorkflowStubOutboundInterceptor which provides an access to Headers

Partially addresses #373.

Author: Spikhalskiy

temporal-sdk/src/main/java/io/temporal/common/interceptors/WorkflowClientInterceptor.java

@@ -26,12 +26,23 @@
 import java.util.Optional;
 
 public interface WorkflowClientInterceptor {
-
+  /**
+   * Called when workflow stub is created during creation of new workflow.
+   *
+   * @return decorated stub
+   */
   WorkflowStub newUntypedWorkflowStub(
       String workflowType, WorkflowOptions options, WorkflowStub next);
 
+  /**
+   * Called when workflow stub is created for a known existing execution
+   *
+   * @return decorated stub
+   */
   WorkflowStub newUntypedWorkflowStub(
       WorkflowExecution execution, Optional<String> workflowType, WorkflowStub next);
 
   ActivityCompletionClient newActivityCompletionClient(ActivityCompletionClient next);
+
+  WorkflowStubOutboundCallsInterceptor interceptStub(WorkflowStubOutboundCallsInterceptor next);
 }

temporal-sdk/src/main/java/io/temporal/common/interceptors/WorkflowClientInterceptorBase.java

@@ -44,4 +44,10 @@ public WorkflowStub newUntypedWorkflowStub(
   public ActivityCompletionClient newActivityCompletionClient(ActivityCompletionClient next) {
     return next;
   }
+
+  @Override
+  public WorkflowStubOutboundCallsInterceptor interceptStub(
+      WorkflowStubOutboundCallsInterceptor next) {
+    return next;
+  }
 }

temporal-sdk/src/main/java/io/temporal/common/interceptors/WorkflowStubOutboundCallsInterceptor.java

@@ -0,0 +1,90 @@
+/*
+ *  Copyright (C) 2020 Temporal Technologies, Inc. All Rights Reserved.
+ *
+ *  Copyright 2012-2016 Amazon.com, Inc. or its affiliates. All Rights Reserved.
+ *
+ *  Modifications copyright (C) 2017 Uber Technologies, Inc.
+ *
+ *  Licensed under the Apache License, Version 2.0 (the "License"). You may not
+ *  use this file except in compliance with the License. A copy of the License is
+ *  located at
+ *
+ *  http://aws.amazon.com/apache2.0
+ *
+ *  or in the "license" file accompanying this file. This file is distributed on
+ *  an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either
+ *  express or implied. See the License for the specific language governing
+ *  permissions and limitations under the License.
+ */
+
+package io.temporal.common.interceptors;
+
+import io.temporal.api.common.v1.WorkflowExecution;
+import io.temporal.client.WorkflowOptions;
+
+public interface WorkflowStubOutboundCallsInterceptor {
+  final class WorkflowInput {
+    private final Header header;
+    private final Object[] arguments;
+    private final WorkflowOptions options;
+
+    public WorkflowInput(Header header, Object[] arguments, WorkflowOptions options) {
+      this.header = header;
+      this.arguments = arguments;
+      this.options = options;
+    }
+
+    public Header getHeader() {
+      return header;
+    }
+
+    public Object[] getArguments() {
+      return arguments;
+    }
+
+    public WorkflowOptions getOptions() {
+      return options;
+    }
+  }
+
+  final class WorkflowInputWithSignal {
+    private final WorkflowInput workflowInput;
+    private final String signalName;
+    private final Object[] signalArguments;
+
+    public WorkflowInputWithSignal(
+        WorkflowInput workflowInput, String signalName, Object[] signalArguments) {
+      this.workflowInput = workflowInput;
+      this.signalName = signalName;
+      this.signalArguments = signalArguments;
+    }
+
+    public WorkflowInput getWorkflowInput() {
+      return workflowInput;
+    }
+
+    public String getSignalName() {
+      return signalName;
+    }
+
+    public Object[] getSignalArguments() {
+      return signalArguments;
+    }
+  }
+
+  final class WorkflowOutput {
+    private final WorkflowExecution workflowExecution;
+
+    public WorkflowOutput(WorkflowExecution workflowExecution) {
+      this.workflowExecution = workflowExecution;
+    }
+
+    public WorkflowExecution getWorkflowExecution() {
+      return workflowExecution;
+    }
+  }
+
+  WorkflowOutput start(WorkflowInput input);
+
+  WorkflowOutput signalWithStart(WorkflowInputWithSignal input);
+}

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

@@ -409,9 +409,7 @@ private Promise<Optional<Payloads>> executeChildWorkflow(
     attributes.setCronSchedule(OptionsUtils.safeGet(options.getCronSchedule()));
     io.temporal.api.common.v1.Header grpcHeader =
         toHeaderGrpc(header, extractContextsAndConvertToBytes(propagators));
-    if (grpcHeader != null) {
-      attributes.setHeader(grpcHeader);
-    }
+    attributes.setHeader(grpcHeader);
     ParentClosePolicy parentClosePolicy = options.getParentClosePolicy();
     if (parentClosePolicy != null) {
       attributes.setParentClosePolicy(parentClosePolicy);