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n-api: Handle fatal exception in async callback

 - Create a handle scope before invoking the async completion
   callback, because it is basically always needed, easy for user
   code to forget, and this makes it more consistent with ordinary
   N-API function callbacks.

 - Check for an unhandled JS exception after invoking an async
   completion callback, and report it via `node::FatalException()`.

 - Add a corresponding test case for an exception in async callback.

Previously, any unhandled JS exception thrown from a
`napi_async_complete_callback` would be silently ignored. Among other
things this meant assertions in some test cases could be undetected.

PR-URL: #12838
Reviewed-By: Anna Henningsen <>
Reviewed-By: Benjamin Gruenbaum <>
Reviewed-By: Michael Dawson <>
Reviewed-By: James M Snell <>
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jasongin authored and mhdawson committed May 4, 2017
1 parent 2bbabb1 commit 2e3fef7628ed10aeb2c2eddec8bec68661d2836e
Showing with 38 additions and 27 deletions.
  1. +0 −7 doc/api/
  2. +20 −1 src/
  3. +18 −0 test/addons-napi/test_async/test.js
  4. +0 −19 test/addons-napi/test_async/
@@ -2855,13 +2855,6 @@ will be invoked with a status value of `napi_cancelled`.
The work should not be deleted before the `complete`
callback invocation, even when it was cancelled.

**Note:** As mentioned in the section on memory management, if
the code to be run in the callbacks will create N-API values, then
N-API handle scope functions must be used to create/destroy a
`napi_handle_scope` such that the scope is active when
objects can be created.

### napi_create_async_work
<!-- YAML
added: v8.0.0
@@ -2766,7 +2766,26 @@ class Work {
Work* work = static_cast<Work*>(req->data);

if (work->_complete != nullptr) {
work->_complete(work->_env, ConvertUVErrorCode(status), work->_data);
napi_env env = work->_env;

// Establish a handle scope here so that every callback doesn't have to.
// Also it is needed for the exception-handling below.
v8::HandleScope scope(env->isolate);

work->_complete(env, ConvertUVErrorCode(status), work->_data);

// Note: Don't access `work` after this point because it was
// likely deleted by the complete callback.

// If there was an unhandled exception in the complete callback,
// report it as a fatal exception. (There is no JavaScript on the
// callstack that can possibly handle it.)
if (!env->last_exception.IsEmpty()) {
v8::TryCatch try_catch;
v8::Local<v8::Value>::New(env->isolate, env->last_exception));
node::FatalException(env->isolate, try_catch);

@@ -1,12 +1,30 @@
'use strict';
const common = require('../../common');
const assert = require('assert');
const child_process = require('child_process');
const test_async = require(`./build/${common.buildType}/test_async`);

const testException = 'test_async_cb_exception';

// Exception thrown from async completion callback.
// (Tested in a spawned process because the exception is fatal.)
if (process.argv[2] === 'child') {
test_async.Test(1, common.mustCall(function(err, val) {
throw new Error(testException);
const p = child_process.spawnSync(
process.execPath, [ '--napi-modules', __filename, 'child' ]);

// Successful async execution and completion callback.
test_async.Test(5, common.mustCall(function(err, val) {
assert.strictEqual(err, null);
assert.strictEqual(val, 10);

// Async work item cancellation with callback.
@@ -20,22 +20,6 @@ typedef struct {
carrier the_carrier;
carrier async_carrier[MAX_CANCEL_THREADS];

struct AutoHandleScope {
explicit AutoHandleScope(napi_env env)
: _env(env),
_scope(nullptr) {
napi_open_handle_scope(_env, &_scope);
~AutoHandleScope() {
napi_close_handle_scope(_env, _scope);
AutoHandleScope() { }

napi_env _env;
napi_handle_scope _scope;

void Execute(napi_env env, void* data) {
#if defined _WIN32
@@ -53,7 +37,6 @@ void Execute(napi_env env, void* data) {

void Complete(napi_env env, napi_status status, void* data) {
AutoHandleScope scope(env);
carrier* c = static_cast<carrier*>(data);

if (c != &the_carrier) {
@@ -116,13 +99,11 @@ napi_value Test(napi_env env, napi_callback_info info) {

void BusyCancelComplete(napi_env env, napi_status status, void* data) {
AutoHandleScope scope(env);
carrier* c = static_cast<carrier*>(data);
NAPI_CALL_RETURN_VOID(env, napi_delete_async_work(env, c->_request));

void CancelComplete(napi_env env, napi_status status, void* data) {
AutoHandleScope scope(env);
carrier* c = static_cast<carrier*>(data);

if (status == napi_cancelled) {

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