cli: --disable-proto=warn

doc/api/cli.md

@@ -706,7 +706,9 @@ added:
 
 Disable the `Object.prototype.__proto__` property. If `mode` is `delete`, the
 property is removed entirely. If `mode` is `throw`, accesses to the
-property throw an exception with the code `ERR_PROTO_ACCESS`.
+property throw an exception with the code `ERR_PROTO_ACCESS`. If `mode` is
+`warn`, accesses to the property emit a deprecation warning while preserving
+existing behavior.
 
 ### `--disable-sigusr1`
 

src/api/environment.cc

@@ -788,6 +788,48 @@ void ProtoThrower(const FunctionCallbackInfo<Value>& info) {
   THROW_ERR_PROTO_ACCESS(info.GetIsolate());
 }
 
+void EmitProtoWarn(Isolate* isolate) {
+  Local<Context> context = isolate->GetCurrentContext();
+  Environment* env = Environment::GetCurrent(context);
+  if (env->options()->pending_deprecation && env->EmitProtoWarning()) {
+    if (ProcessEmitDeprecationWarning(
+        env,
+        "Object.prototype.__proto__ is deprecated and will be removed in a future"
+        " version of Node.js. Use Object.getPrototypeOf() and "
+        "Object.setPrototypeOf() instead.",
+        "DeprecationWarning").IsNothing())
+    return;
+  }
+}
+
+void ProtoGetterWarn(Local<Name> property,
+                       const PropertyCallbackInfo<Value>& info) {
+  Isolate* isolate = info.GetIsolate();
+  EmitProtoWarn(isolate);
+
+  Local<Context> context = isolate->GetCurrentContext();
+  Local<Value> receiver = info.This();
+
+  Local<Value> prototype;
+  if (!receiver->ToObject(context).ToLocalChecked()
+           ->GetPrototypeV2()
+           .ToLocal(&prototype)) {
+    return;
+  }
+
+  info.GetReturnValue().Set(prototype);
+}
+
+void ProtoSetterWarn(Local<Name> property,
+                       Local<Value> value,
+                       const PropertyCallbackInfo<void>& info) {
+  Isolate* isolate = info.GetIsolate();
+  EmitProtoWarn(isolate);
+
+  Local<Context> context = isolate->GetCurrentContext();
+  USE(info.This()->SetPrototypeV2(context, value));
+}
+
 // This runs at runtime, regardless of whether the context
 // is created from a snapshot.
 Maybe<void> InitializeContextRuntime(Local<Context> context) {
@@ -861,6 +903,15 @@ Maybe<void> InitializeContextRuntime(Local<Context> context) {
         .IsNothing()) {
       return Nothing<void>();
     }
+  } else if (per_process::cli_options->disable_proto == "warn") {
+    PropertyDescriptor descriptor(ProtoGetterWarn, ProtoSetterWarn);
+    descriptor.set_enumerable(false);
+    descriptor.set_configurable(true);
+    if (prototype
+        ->DefineProperty(context, proto_string, descriptor)
+        .IsNothing()) {
+      return Nothing<void>();
+    }
   } else if (per_process::cli_options->disable_proto != "") {
     // Validated in ProcessGlobalArgs
     UNREACHABLE("invalid --disable-proto mode");

src/env.h

@@ -934,6 +934,12 @@ class Environment final : public MemoryRetainer {
     return current_value;
   }
 
+  inline bool EmitProtoWarning() {
+    bool current_value = emit_proto_warning_;
+    emit_proto_warning_ = false;
+    return current_value;
+  }
+
   // cb will be called as cb(env) on the next event loop iteration.
   // Unlike the JS setImmediate() function, nested SetImmediate() calls will
   // be run without returning control to the event loop, similar to nextTick().
@@ -1117,6 +1123,7 @@ class Environment final : public MemoryRetainer {
   bool trace_sync_io_ = false;
   bool emit_env_nonstring_warning_ = true;
   bool emit_err_name_warning_ = true;
+  bool emit_proto_warning_ = true;
   bool source_maps_enabled_ = false;
 
   size_t async_callback_scope_depth_ = 0;

src/node.cc

@@ -753,6 +753,7 @@ static ExitCode ProcessGlobalArgsInternal(std::vector<std::string>* args,
 
   if (per_process::cli_options->disable_proto != "delete" &&
       per_process::cli_options->disable_proto != "throw" &&
+      per_process::cli_options->disable_proto != "warn" &&
       per_process::cli_options->disable_proto != "") {
     errors->emplace_back("invalid mode passed to --disable-proto");
     // TODO(joyeecheung): merge into kInvalidCommandLineArgument.

test/parallel/test-disable-proto-warn.js

@@ -0,0 +1,46 @@
+// Flags: --disable-proto=warn
+
+'use strict';
+
+const common = require('../common');
+const assert = require('assert');
+const vm = require('vm');
+const { Worker, isMainThread } = require('worker_threads');
+
+common.expectWarning(
+  'DeprecationWarning',
+  'Object.prototype.__proto__ is deprecated and will be removed in a future ' +
+    'version of Node.js. Use Object.getPrototypeOf() and ' +
+    'Object.setPrototypeOf() instead.'
+);
+
+// eslint-disable-next-line no-proto
+const proto1 = ({}).__proto__;
+assert.strictEqual(proto1, Object.prototype);
+
+const obj = {};
+const newProto = { a: 1 };
+// eslint-disable-next-line no-proto
+obj.__proto__ = newProto;
+assert.strictEqual(Object.getPrototypeOf(obj), newProto);
+
+const ctx = vm.createContext();
+assert.strictEqual(
+  vm.runInContext('({}).__proto__ === Object.prototype', ctx),
+  true
+);
+assert.strictEqual(
+  vm.runInContext(`
+    const obj = {};
+    const newProto = { a: 1 };
+    obj.__proto__ = newProto;
+    Object.getPrototypeOf(obj) === newProto;
+  `, ctx),
+  true
+);
+
+if (isMainThread) {
+  new Worker(__filename);
+} else {
+  process.exit();
+}