[Jobs API]: Expose JobDelegate::IsJoiningThread.

To let the user do special handling on the main thread e.g. Scavenging
uses different tracing categories for background/foreground threads.

Change-Id: I6c9187fd6201b5b81cd83727727fda49fcf7ff68
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2405797Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Etienne Pierre-Doray <etiennep@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69978}

include/v8-platform.h

@@ -177,6 +177,13 @@ class JobDelegate {
    * reused by a different thread after a worker_task returns.
    */
   virtual uint8_t GetTaskId() = 0;
+
+  /**
+   * Returns true if the current task is called from the thread currently
+   * running JobHandle::Join().
+   * TODO(etiennep): Make pure virtual once custom embedders implement it.
+   */
+  virtual bool IsJoiningThread() const { return false; }
 };
 
 /**

src/heap/cppgc/default-platform.cc

@@ -114,6 +114,7 @@ std::unique_ptr<cppgc::JobHandle> DefaultPlatform::PostJob(
       bool ShouldYield() override { return false; }
       void NotifyConcurrencyIncrease() override {}
       uint8_t GetTaskId() override { return 0; }
+      bool IsJoiningThread() const override { return false; }
     } delegate;
 
     if (task) task->Run(&delegate);

src/libplatform/default-job.cc

@@ -104,7 +104,7 @@ void DefaultJobState::Join() {
     ++active_workers_;
     can_run = WaitForParticipationOpportunityLockRequired();
   }
-  DefaultJobState::JobDelegate delegate(this);
+  DefaultJobState::JobDelegate delegate(this, true);
   while (can_run) {
     job_task_->Run(&delegate);
     base::MutexGuard guard(&mutex_);

src/libplatform/default-job.h

@@ -21,7 +21,8 @@ class V8_PLATFORM_EXPORT DefaultJobState
  public:
   class JobDelegate : public v8::JobDelegate {
    public:
-    explicit JobDelegate(DefaultJobState* outer) : outer_(outer) {}
+    explicit JobDelegate(DefaultJobState* outer, bool is_joining_thread = false)
+        : outer_(outer), is_joining_thread_(is_joining_thread) {}
     ~JobDelegate();
 
     void NotifyConcurrencyIncrease() override {
@@ -32,6 +33,7 @@ class V8_PLATFORM_EXPORT DefaultJobState
       return outer_->is_canceled_.load(std::memory_order_relaxed);
     }
     uint8_t GetTaskId() override;
+    bool IsJoiningThread() const override { return is_joining_thread_; }
 
    private:
     static constexpr uint8_t kInvalidTaskId =
@@ -39,6 +41,7 @@ class V8_PLATFORM_EXPORT DefaultJobState
 
     DefaultJobState* outer_;
     uint8_t task_id_ = kInvalidTaskId;
+    bool is_joining_thread_;
   };
 
   DefaultJobState(Platform* platform, std::unique_ptr<JobTask> job_task,

test/unittests/heap/cppgc/test-platform.h

@@ -96,6 +96,7 @@ class TestPlatform : public Platform {
         bool ShouldYield() override { return false; }
         void NotifyConcurrencyIncrease() override {}
         uint8_t GetTaskId() override { return 0; }
+        bool IsJoiningThread() const override { return false; }
       } delegate;
 
       if (task_) task_->Run(&delegate);