Restrict parallel execution of load hook.

src/ModuleLoader.ts

@@ -220,9 +220,9 @@ export class ModuleLoader {
 		timeStart('load modules', 3);
 		let source: string | SourceDescription;
 		try {
-			source =
-				(await this.pluginDriver.hookFirst('load', [id])) ??
-				(await this.readQueue.run(async () => readFile(id)));
+			source = await this.readQueue.run(
+				async () => (await this.pluginDriver.hookFirst('load', [id])) ?? (await readFile(id))
+			);
 		} catch (err) {
 			timeEnd('load modules', 3);
 			let msg = `Could not load ${id}`;

test/function/samples/max-parallel-file-reads-with-plugin/1.js

@@ -0,0 +1 @@
+export const x1 = 1;

test/function/samples/max-parallel-file-reads-with-plugin/2.js

@@ -0,0 +1 @@
+export const x2 = 2;

test/function/samples/max-parallel-file-reads-with-plugin/3.js

@@ -0,0 +1 @@
+export const x3 = 3;

test/function/samples/max-parallel-file-reads-with-plugin/4.js

@@ -0,0 +1 @@
+export const x4 = 4;

test/function/samples/max-parallel-file-reads-with-plugin/5.js

@@ -0,0 +1 @@
+export const x5 = 5;

test/function/samples/max-parallel-file-reads-with-plugin/_config.js

@@ -0,0 +1,37 @@
+const assert = require('assert');
+const fs = require('fs');
+
+const fsReadFile = fs.readFile;
+let currentReads = 0;
+let maxReads = 0;
+
+module.exports = {
+	description: 'maxParallelFileReads with plugin',
+	options: {
+		maxParallelFileReads: 3,
+		plugins: [
+			{
+				async load(id) {
+					return new Promise((fulfil, reject) =>
+						fs.readFile(id, 'utf-8', (err, contents) => (err ? reject(err) : fulfil(contents)))
+					);
+				}
+			}
+		]
+	},
+	before() {
+		fs.readFile = (path, options, callback) => {
+			console.log('mock read');
+			currentReads++;
+			maxReads = Math.max(maxReads, currentReads);
+			fsReadFile(path, options, (err, data) => {
+				currentReads--;
+				callback(err, data);
+			});
+		};
+	},
+	after() {
+		fs.readFile = fsReadFile;
+		assert.strictEqual(maxReads, 3, 'Wrong number of parallel file reads: ' + maxReads);
+	}
+};

test/function/samples/max-parallel-file-reads-with-plugin/main.js

@@ -0,0 +1,5 @@
+export * from './1';
+export * from './2';
+export * from './3';
+export * from './4';
+export * from './5';