cppgc: Add sample program showing cppgc usage for V8 embedders

Bug: chromium:1056170
Change-Id: I7cb19e19de63a74e5407665c28f1b5c0f23d7d0e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2226563
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Anton Bikineev <bikineev@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68139}

BUILD.gn

@@ -4698,6 +4698,23 @@ if (want_v8_shell) {
   }
 }
 
+v8_executable("cppgc_for_v8_embedders") {
+  sources = [ "samples/cppgc/cppgc-for-v8-embedders.cc" ]
+
+  configs = [
+    # Note: don't use :internal_config here because this target will get
+    # the :external_config applied to it by virtue of depending on :v8, and
+    # you can't have both applied to the same target.
+    ":internal_config_base",
+  ]
+
+  deps = [
+    ":cppgc",
+    ":v8_libplatform",
+    "//build/win:default_exe_manifest",
+  ]
+}
+
 template("v8_fuzzer") {
   name = target_name
   forward_variables_from(invoker, "*")

samples/cppgc/cppgc-for-v8-embedders.cc

@@ -0,0 +1,106 @@
+// Copyright 2020 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include <include/cppgc/allocation.h>
+#include <include/cppgc/garbage-collected.h>
+#include <include/cppgc/heap.h>
+#include <include/cppgc/member.h>
+#include <include/cppgc/platform.h>
+#include <include/cppgc/visitor.h>
+#include <include/libplatform/libplatform.h>
+#include <include/v8.h>
+
+#include <iostream>
+#include <memory>
+#include <string>
+
+/**
+ * This sample program shows how to set up a stand-alone cppgc heap as an
+ * embedder of V8. Most importantly, this example shows how to reuse V8's
+ * platform for cppgc.
+ */
+
+/**
+ * Platform used by cppgc. Can just redirect to v8::Platform for most calls.
+ * Exception: GetForegroundTaskRunner(), see below.
+ *
+ * This example uses V8's default platform implementation to drive the cppgc
+ * platform.
+ */
+class Platform final : public cppgc::Platform {
+ public:
+  Platform() : v8_platform_(v8::platform::NewDefaultPlatform()) {}
+
+  cppgc::PageAllocator* GetPageAllocator() final {
+    return v8_platform_->GetPageAllocator();
+  }
+
+  double MonotonicallyIncreasingTime() final {
+    return v8_platform_->MonotonicallyIncreasingTime();
+  }
+
+  std::shared_ptr<cppgc::TaskRunner> GetForegroundTaskRunner() final {
+    // V8's default platform creates a new task runner when passed the
+    // v8::Isolate pointer the first time. For non-default platforms this will
+    // require getting the appropriate task runner.
+    return v8_platform_->GetForegroundTaskRunner(nullptr);
+  }
+
+  std::unique_ptr<cppgc::JobHandle> PostJob(
+      cppgc::TaskPriority priority,
+      std::unique_ptr<cppgc::JobTask> job_task) final {
+    return v8_platform_->PostJob(priority, std::move(job_task));
+  }
+
+ private:
+  std::unique_ptr<v8::Platform> v8_platform_;
+};
+
+/**
+ * Simple string rope to illustrate allocation and garbage collection below. The
+ * rope keeps the next parts alive via regular managed reference.
+ */
+class Rope final : public cppgc::GarbageCollected<Rope> {
+ public:
+  explicit Rope(std::string part, Rope* next = nullptr)
+      : part_(part), next_(next) {}
+
+  void Trace(cppgc::Visitor* visitor) const { visitor->Trace(next_); }
+
+ private:
+  std::string part_;
+  cppgc::Member<Rope> next_;
+
+  friend std::ostream& operator<<(std::ostream& os, const Rope& rope);
+};
+
+std::ostream& operator<<(std::ostream& os, const Rope& rope) {
+  os << rope.part_;
+  if (rope.next_) {
+    os << *rope.next_;
+  }
+  return os;
+}
+
+int main(int argc, char* argv[]) {
+  // Create a platform that is used by cppgc::Heap for execution and backend
+  // allocation.
+  auto cppgc_platform = std::make_shared<Platform>();
+  // Initialize the process. This must happen before any cppgc::Heap::Create()
+  // calls.
+  cppgc::InitializeProcess(cppgc_platform->GetPageAllocator());
+  // Create a managed heap.
+  std::unique_ptr<cppgc::Heap> heap = cppgc::Heap::Create(cppgc_platform);
+  // Allocate a string rope on the managed heap.
+  auto* greeting = cppgc::MakeGarbageCollected<Rope>(
+      heap.get(), "Hello ",
+      cppgc::MakeGarbageCollected<Rope>(heap.get(), "World!"));
+  // Manually trigger garbage collection. The object greeting is held alive
+  // through conservative stack scanning.
+  heap->ForceGarbageCollectionSlow("V8 embedders example", "Testing");
+  std::cout << *greeting << std::endl;
+  // Gracefully shutdown the process.
+  cppgc::ShutdownProcess();
+  return 0;
+}