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events: change EventTarget handler exception behavior
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Change the behavior of EventTarget, instead of emitting
'error' on handler exception, emit 'uncaughtException'.

Fixes: #36770
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Nitzan Uziely committed Feb 5, 2021
1 parent fe43bd8 commit 7d9cab1
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Showing 3 changed files with 19 additions and 14 deletions.
9 changes: 6 additions & 3 deletions doc/api/events.md
Original file line number Diff line number Diff line change
Expand Up @@ -1137,6 +1137,9 @@ setMaxListeners(5, target, emitter);
<!-- YAML
added: v14.5.0
changes:
- version: REPLACEME
pr-url: xxx
description: changed EventTarget error handling.
- version: v15.4.0
pr-url: https://github.com/nodejs/node/pull/35949
description: No longer experimental.
Expand Down Expand Up @@ -1247,9 +1250,9 @@ target.addEventListener('foo', handler4, { once: true });
### `EventTarget` error handling

When a registered event listener throws (or returns a Promise that rejects),
by default the error is forwarded to the `process.on('error')` event
on `process.nextTick()`. Throwing within an event listener will *not* stop
the other registered handlers from being invoked.
by default the error is forwarded to the `process.on('uncaughtException')`
event on `process.nextTick()`. Throwing within an event listener will *not*
stop the other registered handlers from being invoked.

The `EventTarget` does not implement any special default handling for
`'error'` type events.
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12 changes: 6 additions & 6 deletions lib/internal/event_target.js
Original file line number Diff line number Diff line change
Expand Up @@ -420,9 +420,9 @@ class EventTarget {
}
const result = FunctionPrototypeCall(handler.callback, this, arg);
if (result !== undefined && result !== null)
addCatch(this, result, createEvent());
addCatch(result);
} catch (err) {
emitUnhandledRejectionOrErr(this, err, createEvent());
emitUncaughtException(err);
}

handler = next;
Expand Down Expand Up @@ -582,19 +582,19 @@ function isEventTarget(obj) {
return obj?.constructor?.[kIsEventTarget];
}

function addCatch(that, promise, event) {
function addCatch(promise) {
const then = promise.then;
if (typeof then === 'function') {
FunctionPrototypeCall(then, promise, undefined, function(err) {
// The callback is called with nextTick to avoid a follow-up
// rejection from this promise.
process.nextTick(emitUnhandledRejectionOrErr, that, err, event);
emitUncaughtException(err);
});
}
}

function emitUnhandledRejectionOrErr(that, err, event) {
process.emit('error', err, event);
function emitUncaughtException(err) {
process.nextTick(() => { throw err; });
}

function makeEventHandler(handler) {
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12 changes: 7 additions & 5 deletions test/parallel/test-eventtarget.js
Original file line number Diff line number Diff line change
Expand Up @@ -176,14 +176,16 @@ let asyncTest = Promise.resolve();
}

{
const uncaughtException = common.mustCall((err, event) => {
const uncaughtException = common.mustCall((err, origin) => {
strictEqual(err.message, 'boom');
strictEqual(event.type, 'foo');
strictEqual(origin, 'uncaughtException');
}, 4);

// Whether or not the handler function is async or not, errors
// are routed to uncaughtException
process.on('error', uncaughtException);
// Make sure that we no longer call 'error' on error.
process.on('error', common.mustNotCall());
// Don't call rejection even for async handlers.
process.on('unhandledRejection', common.mustNotCall());
process.on('uncaughtException', uncaughtException);

const eventTarget = new EventTarget();

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