[base] Add destination checks in TaskTraits to Java traits as well

Java was never migrated to explicit-destination-required like
was done on the C++ side. So there are lots of callers using
default=>THREAD_POOL :
https://source.chromium.org/search?q=%5CbTaskTraits%5C.(BEST_EFFORT%7CUSER_VISIBLE%7CUSER_BLOCKING)&ss=chromium

This is a problem now that we want to make the C++ checks
runtime (applies to Java as well) in
https://chromium-review.googlesource.com/c/chromium/src/+/1978821

Decouple the C++ and the Java migration by promoting implicit
Java destination to an explicit C++ request to run on the thread
pool.

Bug: 1026641
Change-Id: Ifcd18a53643154393fbb1338ea574c0ba219a914
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1983461
Commit-Queue: Gabriel Charette <gab@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Sami Kyöstilä <skyostil@chromium.org>
Cr-Commit-Position: refs/heads/master@{#730536}

base/android/java/src/org/chromium/base/task/PostTask.java

@@ -87,10 +87,11 @@ public class PostTask {
             if (sPreNativeTaskRunners != null || taskTraits.mIsChoreographerFrame) {
                 getTaskExecutorForTraits(taskTraits).postDelayedTask(taskTraits, task, delay);
             } else {
-                PostTaskJni.get().postDelayedTask(taskTraits.mPrioritySetExplicitly,
-                        taskTraits.mPriority, taskTraits.mMayBlock, taskTraits.mUseThreadPool,
-                        taskTraits.mUseCurrentThread, taskTraits.mExtensionId,
-                        taskTraits.mExtensionData, task, delay);
+                TaskTraits postedTraits = taskTraits.withExplicitDestination();
+                PostTaskJni.get().postDelayedTask(postedTraits.mPrioritySetExplicitly,
+                        postedTraits.mPriority, postedTraits.mMayBlock, postedTraits.mUseThreadPool,
+                        postedTraits.mUseCurrentThread, postedTraits.mExtensionId,
+                        postedTraits.mExtensionData, task, delay);
             }
         }
     }

base/android/java/src/org/chromium/base/task/TaskRunnerImpl.java

@@ -57,7 +57,7 @@ public class TaskRunnerImpl implements TaskRunner {
      */
     protected TaskRunnerImpl(
             TaskTraits traits, String traceCategory, @TaskRunnerType int taskRunnerType) {
-        mTaskTraits = traits;
+        mTaskTraits = traits.withExplicitDestination();
         mTraceEvent = traceCategory + ".PreNativeTask.run";
         mTaskRunnerType = taskRunnerType;
         if (!PostTask.registerPreNativeTaskRunnerLocked(this)) initNativeTaskRunner();

base/android/java/src/org/chromium/base/task/TaskTraits.java

@@ -22,7 +22,9 @@ public class TaskTraits {
     // Keep in sync with base::TaskTraitsExtensionStorage::kStorageSize
     public static final int EXTENSION_STORAGE_SIZE = 8;
 
-    // Convenience variables explicitly specifying common priorities
+    // Convenience variables explicitly specifying common priorities.
+    // These also imply THREAD_POOL unless explicitly overwritten.
+    // TODO(1026641): Make destination explicit in Java too.
 
     // This task will only be scheduled when machine resources are available. Once
     // running, it may be descheduled if higher priority work arrives (in this
@@ -176,6 +178,21 @@ public class TaskTraits {
         return taskTraits;
     }
 
+    /**
+     * Returns a TaskTraits with an explicit destination.
+     *
+     * The C++ side enforces that a destination _must_ be specified. The Java
+     * side loosely considers lack of destination as implying THREAD_POOL
+     * destination.
+     * TODO(1026641): Bring the Java side inline with the C++ side.
+     */
+    public TaskTraits withExplicitDestination() {
+        if (!mUseThreadPool && !mUseCurrentThread && !hasExtension()) {
+            return this.threadPool();
+        }
+        return this;
+    }
+
     @Override
     public boolean equals(@Nullable Object object) {
         if (object == this) {

base/task/task_traits.h

@@ -386,6 +386,16 @@ class BASE_EXPORT TaskTraits {
         use_thread_pool_(use_thread_pool),
         use_current_thread_(use_current_thread) {
     static_assert(sizeof(TaskTraits) == 16, "Keep this constructor up to date");
+
+    // Same assert as in the regular TaskTraits constructor.
+    const bool has_extension =
+        (extension_.extension_id !=
+         TaskTraitsExtensionStorage::kInvalidExtensionId);
+    DCHECK(!use_current_thread_ || !use_thread_pool_)
+        << "base::CurrentThread is mutually exclusive with base::ThreadPool";
+    DCHECK(use_thread_pool_ ^ has_extension || use_current_thread_)
+        << "Traits must explicitly specify a destination (e.g. ThreadPool or a "
+           "named thread like BrowserThread, or CurrentThread)";
   }
 
   // This bit is set in |priority_|, |shutdown_behavior_| and |thread_policy_|