Avoid reading entire file

src/util/mac.test.ts

@@ -2,18 +2,17 @@ import {describe, it} from 'node:test';
 import {deepStrictEqual, strictEqual} from 'node:assert';
 import {createHash} from 'node:crypto';
 
-import {machoAppLauncher, machoTypesData} from './mac';
+import {fixtureFile} from '../util.spec';
 
-const unhex = (hex: string) => Buffer.from(hex.replace(/\s/g, ''), 'hex');
+import {machoAppLauncher, machoTypesData, machoTypesFile} from './mac';
 
 function sha256(data: Buffer) {
 	return createHash('sha256').update(data).digest('hex');
 }
 
 const machoTypes = [
 	{
-		name: 'slim: ppc',
-		data: unhex('FE ED FA CE 00 00 00 12 00 00 00 0A'),
+		name: 'thin-ppc',
 		format: {
 			cpuType: 0x00000012,
 			cpuSubtype: 10
@@ -22,8 +21,7 @@ const machoTypes = [
 			'17414c123fe82ac74a89fad9c80e36d8b612ded5a520e35f3c33eabe75a023a7'
 	},
 	{
-		name: 'slim: i386',
-		data: unhex('CE FA ED FE 07 00 00 00 03 00 00 00'),
+		name: 'thin-i386',
 		format: {
 			cpuType: 0x00000007,
 			cpuSubtype: 3
@@ -32,14 +30,7 @@ const machoTypes = [
 			'e52e19fce336130824dcfd4731bf85db7e8e96628ef8c6a49769dc5247ef6ed0'
 	},
 	{
-		name: 'fat: ppc, i386',
-		data: unhex(
-			[
-				'CA FE BA BE 00 00 00 02',
-				'00 00 00 12 00 00 00 0A 00 00 00 00 00 00 00 00 00 00 00 00',
-				'00 00 00 07 00 00 00 03 00 00 00 00 00 00 00 00 00 00 00 00'
-			].join('')
-		),
+		name: 'fat-ppc-i386',
 		format: [
 			{
 				cpuType: 0x00000012,
@@ -56,10 +47,13 @@ const machoTypes = [
 ];
 
 void describe('util/mac', () => {
-	void describe('machoTypesData', () => {
-		for (const {name, data, format} of machoTypes) {
-			void it(name, () => {
-				deepStrictEqual(machoTypesData(data), format);
+	void describe('machoTypesFile', () => {
+		for (const {name, format} of machoTypes) {
+			void it(name, async () => {
+				deepStrictEqual(
+					await machoTypesFile(fixtureFile(`macho-head/${name}.bin`)),
+					format
+				);
 			});
 		}
 	});

src/util/mac.ts

@@ -1,4 +1,4 @@
-import {readFile} from 'node:fs/promises';
+import {open} from 'node:fs/promises';
 
 import {launcher} from '../util';
 
@@ -88,7 +88,27 @@ export function machoTypesData(data: Readonly<Buffer>) {
  * @returns Mach-O types.
  */
 export async function machoTypesFile(path: string) {
-	return machoTypesData(await readFile(path));
+	let data;
+	const f = await open(path, 'r');
+	try {
+		const h = 8;
+		const head = Buffer.alloc(h);
+		const {bytesRead} = await f.read(head, 0, h, 0);
+		if (bytesRead < h) {
+			data = head.subarray(0, bytesRead);
+		} else {
+			const n =
+				head.readUInt32BE(0) === FAT_MAGIC
+					? head.readUInt32BE(4) * 20
+					: 4;
+			const d = Buffer.alloc(n);
+			const {bytesRead} = await f.read(d, 0, n, h);
+			data = Buffer.concat([head, d.subarray(0, bytesRead)]);
+		}
+	} finally {
+		await f.close();
+	}
+	return machoTypesData(data);
 }
 
 /**