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// SPDX-License-Identifier: Apache-2.0
/**
* @module Utils/Fs/AtomicWrite
* @description Idempotent and atomic file write primitives.
*
* - `writeFileIfChanged` / `writeFileContent` preserve mtime when the
* on-disk bytes already match (idempotency contract for `aws s3 sync`).
* - `atomicWrite` writes via a unique temp file + rename with retries,
* tolerating concurrent writers on Windows-like filesystems.
* - `resolveUniqueFilePath` produces non-clobbering numeric suffixes.
*/
import { randomUUID } from 'crypto';
import fs from 'fs';
import path from 'path';
import { ensureDirectoryExists } from './directory.js';
/**
* Resolve a unique filename by appending a numeric suffix (-2, -3, …) before
* the file extension when the file already exists.
*
* This prevents repeated workflow runs from overwriting previously committed
* news articles.
*
* @param filepath - The preferred file path (e.g. `news/2026-04-02-breaking-en.html`)
* @returns The original path when the file doesn't exist, or a suffixed
* variant (e.g. `news/2026-04-02-breaking-en-2.html`) otherwise.
*/
export function resolveUniqueFilePath(filepath: string): string {
if (!fs.existsSync(filepath)) {
return filepath;
}
const dir = path.dirname(filepath);
const ext = path.extname(filepath);
const base = path.basename(filepath, ext);
let suffix = 2;
const MAX_SUFFIX = 100;
while (suffix <= MAX_SUFFIX) {
const candidate = path.join(dir, `${base}-${suffix}${ext}`);
if (!fs.existsSync(candidate)) {
return candidate;
}
suffix++;
}
return path.join(dir, `${base}-${randomUUID().slice(0, 8)}${ext}`);
}
/**
* Write `content` to `filepath` only if the existing on-disk bytes differ.
*
* Used to keep `aws s3 sync` (which compares size + mtime) from re-uploading
* files whose content the build pipeline regenerated identically — see the
* idempotency contract documented in `.github/workflows/deploy-s3.yml`.
*
* @param filepath - Output file path
* @param content - Desired file content (UTF-8 string or raw Buffer)
* @returns `true` when an actual write occurred, `false` when the file
* already had byte-identical content and was left untouched.
*/
export function writeFileIfChanged(filepath: string, content: string | Buffer): boolean {
const dir = path.dirname(filepath);
ensureDirectoryExists(dir);
const desired = Buffer.isBuffer(content) ? content : Buffer.from(content, 'utf-8');
if (fs.existsSync(filepath)) {
try {
const existing = fs.readFileSync(filepath);
if (existing.equals(desired)) {
return false;
}
} catch {
// Fall through to overwrite — read failures must not block deploy.
}
}
fs.writeFileSync(filepath, desired);
return true;
}
/**
* Write content to a file with UTF-8 encoding.
*
* Idempotent at the byte level: if the file already exists with identical
* content, the file is left untouched (mtime preserved). This keeps
* `aws s3 sync` from re-uploading regenerated-but-identical files.
*
* @param filepath - Output file path
* @param content - File content
*/
export function writeFileContent(filepath: string, content: string): void {
writeFileIfChanged(filepath, content);
}
/**
* Remove a file, ignoring ENOENT (file already deleted by another writer).
*
* @param filepath - Path to the file to remove
*/
function unlinkIfExists(filepath: string): void {
try {
fs.unlinkSync(filepath);
} catch (err: unknown) {
const code = err instanceof Error ? (err as NodeJS.ErrnoException).code : '';
Iif (code !== 'ENOENT') {
throw err;
}
}
}
/**
* Attempt to rename `src` to `dest` with a bounded retry loop.
*
* On each attempt the existing destination is removed first, then
* `renameSync` is retried. `EEXIST`/`EPERM` failures from concurrent
* writers are tolerated for up to `maxRetries` attempts.
*
* @param src - Source (temp) file path
* @param dest - Final destination path
* @param maxRetries - Maximum number of unlink-then-rename attempts
*/
function renameWithRetry(src: string, dest: string, maxRetries: number): void {
for (let attempt = 0; attempt < maxRetries; attempt++) {
unlinkIfExists(dest);
try {
fs.renameSync(src, dest);
return;
} catch (retryErr: unknown) {
const retryCode = retryErr instanceof Error ? (retryErr as NodeJS.ErrnoException).code : '';
if ((retryCode === 'EEXIST' || retryCode === 'EPERM') && attempt < maxRetries - 1) {
continue;
}
throw retryErr;
}
}
}
/**
* Best-effort removal of a temporary file. Ignores ENOENT (the file was
* already renamed or never created) but logs a warning for other errors
* (e.g. EBUSY, EACCES) so operators can detect leaked temp files.
*
* @param tempPath - Path to the temp file to remove
*/
function cleanupTempFile(tempPath: string): void {
try {
fs.unlinkSync(tempPath);
} catch (unlinkErr: unknown) {
const errno =
unlinkErr && typeof unlinkErr === 'object' ? (unlinkErr as NodeJS.ErrnoException) : undefined;
if (errno?.code !== 'ENOENT') {
const message =
errno && typeof errno.message === 'string' ? errno.message : String(unlinkErr);
const code = errno?.code ?? 'UNKNOWN';
console.warn(
`atomicWrite: failed to remove temporary file "${tempPath}" (code: ${code}): ${message}`
);
}
}
}
/**
* Write content to a file atomically.
*
* Writes to a uniquely-named temporary file in the same directory first, then
* renames it to the final path. The temp filename includes the PID and a random
* UUID so that concurrent callers targeting the same destination never collide
* on the intermediate file. If the rename fails the temp file is cleaned up in
* a `finally` block. On platforms where `renameSync` does not overwrite an
* existing destination (e.g. Windows), the error is caught and the target is
* removed before retrying the rename.
*
* @param filepath - Final output file path
* @param content - File content to write
*/
export function atomicWrite(filepath: string, content: string): void {
const dir = path.dirname(filepath);
ensureDirectoryExists(dir);
const uniqueSuffix = `${process.pid}-${randomUUID()}`;
const tempPath = `${filepath}.${uniqueSuffix}.tmp`;
try {
fs.writeFileSync(tempPath, content, 'utf-8');
try {
fs.renameSync(tempPath, filepath);
} catch (err: unknown) {
const code = err instanceof Error ? (err as NodeJS.ErrnoException).code : '';
if (code === 'EEXIST' || code === 'EPERM') {
renameWithRetry(tempPath, filepath, 3);
} else {
throw err;
}
}
} finally {
cleanupTempFile(tempPath);
}
}
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