Release Hermes WeakRef targets after draining jobs

Core/Node-API/Source/env_hermes.cc

@@ -164,6 +164,8 @@ namespace Napi
         // exceptions thrown FROM user callbacks are already handled in
         // AppRuntime::Dispatch's try/catch.
         (void)runtime->drainJobs();
+        // WeakRef targets are kept alive only until the current job finishes.
+        runtime->clearKeptObjects();
     }
 
     Napi::Value Eval(Napi::Env env, const char* source, const char* sourceUrl)

Tests/UnitTests/Source/Tests.NodeApi.cpp

@@ -1,10 +1,47 @@
 #include <Babylon/AppRuntime.h>
 #include <gtest/gtest.h>
+#include <napi/env.h>
+#include <chrono>
 #include <cstdint>
 #include <future>
+#include <stdexcept>
 #include <string>
 #include <string_view>
 
+TEST(NodeApi, HermesDrainJobsReleasesWeakRefTargets)
+{
+    if (std::string_view{NAPI_JAVASCRIPT_ENGINE} != "Hermes")
+    {
+        GTEST_SKIP() << "Uses Hermes heap statistics.";
+    }
+
+    std::promise<double> externalBytes;
+    auto completed = externalBytes.get_future();
+    Babylon::AppRuntime runtime{};
+    runtime.Dispatch([&](Napi::Env env) {
+        try
+        {
+            for (size_t i = 0; i < 256; ++i)
+            {
+                {
+                    Napi::HandleScope scope{env};
+                    Napi::Eval(env, "new WeakRef(new Uint8Array(1024 * 1024));", "weakref-job.js");
+                }
+                Napi::DrainJobs(env);
+            }
+            externalBytes.set_value(Napi::Eval(env,
+                "HermesInternal.getInstrumentedStats().js_externalBytes",
+                "weakref-heap.js").As<Napi::Number>().DoubleValue());
+        }
+        catch (const Napi::Error& error)
+        {
+            externalBytes.set_exception(std::make_exception_ptr(std::runtime_error{error.Message()}));
+        }
+    });
+    ASSERT_EQ(completed.wait_for(std::chrono::seconds{30}), std::future_status::ready);
+    EXPECT_LT(completed.get(), 64 * 1024 * 1024);
+}
+
 // The V8JSI Node-API shim does not implement napi_create_dataview /
 // napi_get_dataview_info (its DataView::New throws "TODO"), so this native test
 // only builds on the Chakra, V8, and JavaScriptCore backends. The size_t-width