process: handle rejections only when needed

[gurgunday]

After:

./node benchmark/timers/timers-timeout-nexttick.js
timers/timers-timeout-nexttick.js n=50000: 2,807,740.108387753
timers/timers-timeout-nexttick.js n=5000000: 9,041,959.898481071

Before:

./node benchmark/timers/timers-timeout-nexttick.js
timers/timers-timeout-nexttick.js n=50000: 2,127,372.8770254664
timers/timers-timeout-nexttick.js n=5000000: 6,926,259.012697572

• Skips noop processPromiseRejections() calls in processTicksAndRejections() when there is no pending rejection work

---

We already do a check before entering the loop through runNextTicks():

node/lib/internal/process/task_queues.js

Lines 62 to 70 in 926162a

	// Should be in sync with RunNextTicksNative in node_task_queue.cc
	function runNextTicks() {
	if (!hasTickScheduled() && !hasRejectionToWarn())
	runMicrotasks();
	if (!hasTickScheduled() && !hasRejectionToWarn())
	return;
	processTicksAndRejections();
	}

But once we are already inside processTicksAndRejections() because of tick work, the internal do..while loop still calls processPromiseRejections() on every iteration (where we have a Map instantiation, method calls, etc.) without checking if we should even call it:

node/lib/internal/process/task_queues.js

Line 105 in 926162a

} while (!queue.isEmpty() || processPromiseRejections());

node/lib/internal/process/promises.js

Lines 377 to 424 in 012bf70

	function processPromiseRejections() {
	let maybeScheduledTicksOrMicrotasks = !asyncHandledRejections.isEmpty();
	while (!asyncHandledRejections.isEmpty()) {
	const { promise, warning } = asyncHandledRejections.shift();
	if (!process.emit('rejectionHandled', promise)) {
	process.emitWarning(warning);
	}
	}
	let needPop = true;
	let promiseAsyncId;
	const pending = pendingUnhandledRejections;
	pendingUnhandledRejections = new SafeMap();
	for (const { 0: promise, 1: promiseInfo } of pending.entries()) {
	maybeUnhandledPromises.set(promise, promiseInfo);
	promiseInfo.warned = true;
	// We need to check if async_hooks are enabled
	// don't use enabledHooksExist as a Promise could
	// come from a vm.* context and not have an async id
	promiseAsyncId = promise[kAsyncIdSymbol];
	if (promiseAsyncId !== undefined) {
	pushAsyncContext(
	promiseAsyncId,
	promise[kTriggerAsyncIdSymbol],
	promise,
	);
	}
	const { contextFrame } = promiseInfo;
	const priorContextFrame = AsyncContextFrame.exchange(contextFrame);
	try {
	needPop = unhandledRejectionsMode(promise, promiseInfo, promiseAsyncId);
	} finally {
	AsyncContextFrame.set(priorContextFrame);
	needPop &&
	promiseAsyncId !== undefined &&
	popAsyncContext(promiseAsyncId);
	}
	maybeScheduledTicksOrMicrotasks = true;
	}
	return maybeScheduledTicksOrMicrotasks ||
	pendingUnhandledRejections.size !== 0;
	}

Now the name hasRejectionToWarn is inaccurate because we use it for any type of unhandled rejection:

node/lib/internal/process/promises.js

Lines 193 to 206 in 012bf70

	/**
	* @param {Promise} promise
	* @param {Error} reason
	*/
	function unhandledRejection(promise, reason) {
	pendingUnhandledRejections.set(promise, {
	reason,
	uid: ++lastPromiseId,
	warned: false,
	domain: process.domain,
	contextFrame: AsyncContextFrame.current(),
	});
	setHasRejectionToWarn(true);
	}

So skipping covers all types of rejections

[gurgunday]

Cc @nodejs/performance since they weren't tagged

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.63%. Comparing base (1266013) to head (a02e782).
⚠️ Report is 109 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #62919      +/-   ##
==========================================
- Coverage   89.68%   89.63%   -0.05%     
==========================================
  Files         706      706              
  Lines      218223   219206     +983     
  Branches    41768    42006     +238     
==========================================
+ Hits       195703   196488     +785     
- Misses      14426    14610     +184     
- Partials     8094     8108      +14     

Files with missing lines	Coverage Δ
lib/internal/process/task_queues.js	100.00% <100.00%> (ø)

... and 68 files with indirect coverage changes

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[mcollina]

lgtm

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/72924/

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/72925/