[Prototype] Node 24: count await continuations that block on their dispatch fiber (port of #12)

fibers_async.js

@@ -99,10 +99,24 @@ function asyncContextFrameInUse() {
   return inUse;
 }
 
+// Code after an `await` that blocked on the fiber we gave it (Future.wait, Promise.await, a sync
+// Meteor call). Without this dispatch it would have thrown "Can't wait without a fiber", so each one
+// marks code that still depends on fibers after an `await`. Counted once per job, when it first
+// parks; nothing is counted for the jobs that don't block. Kept on the native module, which every
+// copy of this file shares (an app can load more than one).
+_Fiber.__awaitContinuationBlocks = _Fiber.__awaitContinuationBlocks || 0;
+
 function dispatchOnNewFiber(runDispatched, token) {
-  const fiber = Fiber(() => runDispatched(token));
+  let finished = false;
+  const fiber = Fiber(() => {
+    runDispatched(token);
+    finished = true;
+  });
   fiber[kHidden] = true;
   fiber.run();
+  if (!finished) {
+    _Fiber.__awaitContinuationBlocks++;
+  }
 }
 
 // Runs jobs on one long-lived hidden fiber instead of a new fiber per job. After each job it keeps
@@ -148,6 +162,7 @@ function reusingDispatcher(runNextDispatchable) {
     if (fiber.run(token) !== kIdle && dispatchFiber === fiber) {
       // A job blocked on this fiber; it belongs to that job now.
       dispatchFiber = null;
+      _Fiber.__awaitContinuationBlocks++;
     }
   };
 }
@@ -158,4 +173,10 @@ Object.defineProperty(Fiber, 'microtasksBatched', {
   }
 })
 
+Object.defineProperty(Fiber, 'awaitContinuationBlocks', {
+  get() {
+    return _Fiber.__awaitContinuationBlocks || 0;
+  }
+})
+
 module.exports = _Fiber.Fiber = Fiber;

test/await-continuation-blocks.js

@@ -0,0 +1,86 @@
+'use strict';
+// Fiber.awaitContinuationBlocks counts code after an `await` that blocked on the fiber the
+// dispatcher ran it on: once per blocking job, never for jobs that don't block or for awaits made
+// outside a fiber. Needs a node with the microtask dispatch patch; elsewhere nothing is dispatched.
+var Fiber = require('fibers');
+var Future;
+try {
+	Future = require('fibers/future');
+} catch (err) {
+	Future = require('future');
+}
+
+var enabled = Fiber.awaitDispatchEnabled !== undefined
+	? Fiber.awaitDispatchEnabled
+	: typeof Fiber.microtasksDispatched === 'number' && process.env.FIBERS_AWAIT_DISPATCH !== '0';
+if (!enabled) {
+	console.log('pass');
+	return;
+}
+
+function sleep(ms) {
+	var future = new Future();
+	setTimeout(function() {
+		future.return();
+	}, ms);
+	return future.wait();
+}
+
+function inFiber(fn) {
+	return new Promise(function(resolve, reject) {
+		Fiber(function() {
+			fn().then(resolve, reject);
+		}).run();
+	});
+}
+
+async function blocks(fn) {
+	var before = Fiber.awaitContinuationBlocks;
+	await fn();
+	return Fiber.awaitContinuationBlocks - before;
+}
+
+async function main() {
+	var cases = [
+		['awaits that never block', 0, function() {
+			return inFiber(async function() {
+				await null;
+				await new Promise(function(resolve) {
+					setTimeout(resolve, 1);
+				});
+			});
+		}],
+		['one continuation blocking twice', 1, function() {
+			return inFiber(async function() {
+				await null;
+				sleep(1);
+				sleep(1);
+			});
+		}],
+		['two continuations that block', 2, function() {
+			return inFiber(async function() {
+				await null;
+				sleep(1);
+				await null;
+				sleep(1);
+			});
+		}],
+		['awaits outside a fiber', 0, async function() {
+			await null;
+			await new Promise(function(resolve) {
+				setTimeout(resolve, 1);
+			});
+		}],
+	];
+	for (var ii = 0; ii < cases.length; ii++) {
+		var got = await blocks(cases[ii][2]);
+		if (got !== cases[ii][1]) {
+			throw new Error(cases[ii][0] + ': expected ' + cases[ii][1] + ' blocks, counted ' + got);
+		}
+	}
+	console.log('pass');
+}
+
+main().catch(function(err) {
+	console.log(err.stack);
+});