refactor: extract writeFileAtomicSync to src/main/infra/fs-atomic + 6 tests

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/main.ts

@@ -6,6 +6,7 @@ import { connect as tlsConnect, TLSSocket } from 'node:tls';
 import axios from 'axios';
 import { autoUpdater } from 'electron-updater';
 import { compareUpdateVersions, isNewerUpdateVersion, normalizeUpdateVersion } from './main/domain/update-version-utils';
+import { writeFileAtomicSync } from './main/infra/fs-atomic';
 import { CustomClip, MergeGroupItem, MergeGroup, QueueItem, DownloadProgress, DownloadResult } from './types';
 import {
     setDebugLogFn, initToolDirs,
@@ -539,30 +540,6 @@ function loadConfig(): Config {
     return normalizeConfigTemplates(defaultConfig);
 }
 
-function writeFileAtomicSync(targetPath: string, payload: string | Buffer): void {
-    const buffer = Buffer.isBuffer(payload) ? payload : Buffer.from(payload, 'utf-8');
-    const tmpPath = targetPath + '.tmp';
-
-    let fd: number | null = null;
-    try {
-        fd = fs.openSync(tmpPath, 'w');
-        fs.writeSync(fd, buffer, 0, buffer.length, 0);
-        try { fs.fsyncSync(fd); } catch { /* fsync may fail on some FS; rename is still safer than nothing */ }
-    } finally {
-        if (fd !== null) {
-            try { fs.closeSync(fd); } catch { }
-        }
-    }
-
-    try {
-        fs.renameSync(tmpPath, targetPath);
-    } catch {
-        // On Windows, rename can fail if target exists or is locked. Fall back to copy.
-        fs.copyFileSync(tmpPath, targetPath);
-        try { fs.unlinkSync(tmpPath); } catch { }
-    }
-}
-
 function saveConfig(config: Config): void {
     try {
         writeFileAtomicSync(CONFIG_FILE, JSON.stringify(config, null, 2));

src/main/infra/fs-atomic.test.ts

@@ -0,0 +1,53 @@
+import { test, expect, describe, beforeEach, afterEach } from 'vitest';
+import * as fs from 'fs';
+import * as os from 'os';
+import * as path from 'path';
+import { writeFileAtomicSync } from './fs-atomic';
+
+let tmpDir: string;
+beforeEach(() => {
+    tmpDir = fs.mkdtempSync(path.join(os.tmpdir(), 'fsatomic-'));
+});
+afterEach(() => {
+    try { fs.rmSync(tmpDir, { recursive: true, force: true }); } catch { /* ignore */ }
+});
+
+describe('writeFileAtomicSync', () => {
+    test('writes a string payload', () => {
+        const target = path.join(tmpDir, 'a.txt');
+        writeFileAtomicSync(target, 'hello');
+        expect(fs.readFileSync(target, 'utf-8')).toBe('hello');
+    });
+
+    test('writes a buffer payload', () => {
+        const target = path.join(tmpDir, 'b.bin');
+        writeFileAtomicSync(target, Buffer.from([1, 2, 3, 4]));
+        expect(fs.readFileSync(target)).toEqual(Buffer.from([1, 2, 3, 4]));
+    });
+
+    test('overwrites existing file', () => {
+        const target = path.join(tmpDir, 'c.txt');
+        fs.writeFileSync(target, 'old');
+        writeFileAtomicSync(target, 'new');
+        expect(fs.readFileSync(target, 'utf-8')).toBe('new');
+    });
+
+    test('cleans up tmp file after success', () => {
+        const target = path.join(tmpDir, 'd.txt');
+        writeFileAtomicSync(target, 'x');
+        expect(fs.existsSync(target + '.tmp')).toBe(false);
+    });
+
+    test('utf-8 multibyte chars roundtrip', () => {
+        const target = path.join(tmpDir, 'e.txt');
+        writeFileAtomicSync(target, 'aeoeue-aeoeue');
+        expect(fs.readFileSync(target, 'utf-8')).toBe('aeoeue-aeoeue');
+    });
+
+    test('empty payload writes empty file', () => {
+        const target = path.join(tmpDir, 'f.txt');
+        writeFileAtomicSync(target, '');
+        expect(fs.readFileSync(target, 'utf-8')).toBe('');
+        expect(fs.statSync(target).size).toBe(0);
+    });
+});

src/main/infra/fs-atomic.ts

@@ -0,0 +1,29 @@
+import * as fs from 'fs';
+
+/**
+ * Atomic write via tmp + rename. Survives crash mid-write — either old or
+ * new content, never partial. Windows fallback: copy + unlink if rename
+ * fails (e.g. target locked by reader). fsync best-effort.
+ */
+export function writeFileAtomicSync(targetPath: string, payload: string | Buffer): void {
+    const buffer = Buffer.isBuffer(payload) ? payload : Buffer.from(payload, 'utf-8');
+    const tmpPath = targetPath + '.tmp';
+
+    let fd: number | null = null;
+    try {
+        fd = fs.openSync(tmpPath, 'w');
+        fs.writeSync(fd, buffer, 0, buffer.length, 0);
+        try { fs.fsyncSync(fd); } catch { /* fsync may fail on some FS; rename is still safer than nothing */ }
+    } finally {
+        if (fd !== null) {
+            try { fs.closeSync(fd); } catch { /* ignore */ }
+        }
+    }
+
+    try {
+        fs.renameSync(tmpPath, targetPath);
+    } catch {
+        fs.copyFileSync(tmpPath, targetPath);
+        try { fs.unlinkSync(tmpPath); } catch { /* ignore */ }
+    }
+}