src: add initial shadow realm support

Add initial shadow realm support behind an off-by-default flag
`--experimental-shadow-realm`.

node.gyp

@@ -531,6 +531,7 @@
         'src/node_report_module.cc',
         'src/node_report_utils.cc',
         'src/node_serdes.cc',
+        'src/node_shadow_realm.cc',
         'src/node_snapshotable.cc',
         'src/node_sockaddr.cc',
         'src/node_stat_watcher.cc',

src/api/environment.cc

@@ -5,6 +5,7 @@
 #include "node_native_module_env.h"
 #include "node_options-inl.h"
 #include "node_platform.h"
+#include "node_shadow_realm.h"
 #include "node_v8_platform-inl.h"
 #include "node_wasm_web_api.h"
 #include "uv.h"
@@ -261,6 +262,12 @@ void SetIsolateMiscHandlers(v8::Isolate* isolate, const IsolateSettings& s) {
     isolate->SetWasmStreamingCallback(wasm_web_api::StartStreamingCompilation);
   }
 
+  if (per_process::cli_options->get_per_isolate_options()
+          ->experimental_shadow_realm) {
+    isolate->SetHostCreateShadowRealmContextCallback(
+        shadow_realm::HostCreateShadowRealmContextCallback);
+  }
+
   if ((s.flags & SHOULD_NOT_SET_PROMISE_REJECTION_CALLBACK) == 0) {
     auto* promise_reject_cb = s.promise_reject_callback ?
       s.promise_reject_callback : PromiseRejectCallback;

src/node_options.cc

@@ -713,6 +713,15 @@ PerIsolateOptionsParser::PerIsolateOptionsParser(
   Implies("--harmony-top-level-await", "--experimental-top-level-await");
   ImpliesNot("--no-harmony-top-level-await", "--experimental-top-level-await");
 
+  AddOption("--experimental-shadow-realm",
+            "",
+            &PerIsolateOptions::experimental_shadow_realm,
+            kAllowedInEnvironment);
+  AddOption("--harmony-shadow-realm", "", V8Option{});
+  Implies("--experimental-shadow-realm", "--harmony-shadow-realm");
+  Implies("--harmony-shadow-realm", "--experimental-shadow-realm");
+  ImpliesNot("--no-harmony-shadow-realm", "--experimental-shadow-realm");
+
   Insert(eop, &PerIsolateOptions::get_per_env_options);
 }
 

src/node_options.h

@@ -207,6 +207,7 @@ class PerIsolateOptions : public Options {
   bool report_uncaught_exception = false;
   bool report_on_signal = false;
   bool experimental_top_level_await = true;
+  bool experimental_shadow_realm = false;
   std::string report_signal = "SIGUSR2";
   inline EnvironmentOptions* get_per_env_options();
   void CheckOptions(std::vector<std::string>* errors) override;

src/node_shadow_realm.cc

@@ -0,0 +1,16 @@
+#include "node_shadow_realm.h"
+
+namespace node {
+namespace shadow_realm {
+using v8::Context;
+using v8::Local;
+using v8::MaybeLocal;
+
+// static
+MaybeLocal<Context> HostCreateShadowRealmContextCallback(
+    Local<Context> initiator_context) {
+  return Context::New(initiator_context->GetIsolate());
+}
+
+}  // namespace shadow_realm
+}  // namespace node

src/node_shadow_realm.h

@@ -0,0 +1,19 @@
+#ifndef SRC_NODE_SHADOW_REALM_H_
+#define SRC_NODE_SHADOW_REALM_H_
+
+#if defined(NODE_WANT_INTERNALS) && NODE_WANT_INTERNALS
+
+#include "v8.h"
+
+namespace node {
+namespace shadow_realm {
+
+v8::MaybeLocal<v8::Context> HostCreateShadowRealmContextCallback(
+    v8::Local<v8::Context> initiator_context);
+
+}  // namespace shadow_realm
+}  // namespace node
+
+#endif  // defined(NODE_WANT_INTERNALS) && NODE_WANT_INTERNALS
+
+#endif  // SRC_NODE_SHADOW_REALM_H_

test/parallel/test-shadow-realm.js

@@ -0,0 +1,12 @@
+// Flags: --harmony-shadow-realm
+'use strict';
+
+require('../common');
+const assert = require('assert');
+
+// Validates we can construct ShadowRealm successfully.
+const shadowRealm = new ShadowRealm();
+
+const getter = shadowRealm.evaluate('globalThis.realmValue = "inner"; () => globalThis.realmValue;');
+assert.strictEqual(getter(), 'inner');
+assert.strictEqual('realmValue' in globalThis, false);