fix(libnpmversion): write package.json atomically to prevent corrupti…

workspaces/libnpmversion/lib/write-json.js

@@ -1,5 +1,6 @@
 // write the json back, preserving the line breaks and indent
-const { writeFile } = require('node:fs/promises')
+const { writeFile, rename, unlink } = require('node:fs/promises')
+const { randomBytes } = require('node:crypto')
 const kIndent = Symbol.for('indent')
 const kNewline = Symbol.for('newline')
 
@@ -11,5 +12,16 @@ module.exports = async (path, pkg) => {
   delete pkg._id
   const raw = JSON.stringify(pkg, null, indent) + '\n'
   const data = newline === '\n' ? raw : raw.split('\n').join(newline)
-  return writeFile(path, data)
+
+  // Write to a temp file and rename into place so a concurrent reader (e.g.
+  // another `npm version` process resolving workspaces) never observes a
+  // truncated or partially written package.json.
+  const tmpPath = `${path}.${randomBytes(6).toString('hex')}.tmp`
+  try {
+    await writeFile(tmpPath, data)
+    await rename(tmpPath, path)
+  } catch (err) {
+    await unlink(tmpPath).catch(() => {})
+    throw err
+  }
 }

workspaces/libnpmversion/test/write-json.js

@@ -1,7 +1,7 @@
 const t = require('tap')
 const path = require('node:path')
 const writeJson = require('../lib/write-json.js')
-const { readFile } = require('node:fs/promises')
+const { readFile, readdir } = require('node:fs/promises')
 
 const kIndent = Symbol.for('indent')
 const kNewline = Symbol.for('newline')
@@ -52,3 +52,44 @@ t.test('write json with newlines and indent set', async t => {
 
   t.end()
 })
+
+t.test('writes atomically, never leaving a truncated file for concurrent readers', async t => {
+  const dir = t.testdir()
+  const file = path.join(dir, 'package.json')
+
+  // A concurrent reader must always see either the old complete contents or
+  // the new complete contents, never a partial/empty file mid-write.
+  await writeJson(file, { a: 1 })
+  let sawTruncated = false
+  const reads = []
+  for (let i = 0; i < 20; i++) {
+    reads.push(readFile(file, 'utf8').then(str => {
+      if (str.length === 0 || !str.trim().endsWith('}')) {
+        sawTruncated = true
+      }
+      return null
+    }).catch(() => {}))
+  }
+  await writeJson(file, { a: 2, b: [1, 2, 3] })
+  await Promise.all(reads)
+
+  t.equal(sawTruncated, false, 'never observed a truncated file')
+  const final = JSON.parse(await readFile(file, 'utf8'))
+  t.same(final, { a: 2, b: [1, 2, 3] })
+
+  const leftover = (await readdir(dir)).filter(f => f.endsWith('.tmp'))
+  t.same(leftover, [], 'no leftover temp files')
+})
+
+t.test('cleans up the temp file when the write fails', async t => {
+  const dir = t.testdir()
+  // path is a directory, not a file, so renaming onto it will fail
+  const file = path.join(dir, 'package.json')
+  const { mkdir } = require('node:fs/promises')
+  await mkdir(file)
+
+  await t.rejects(writeJson(file, { a: 1 }))
+
+  const leftover = (await readdir(dir)).filter(f => f.endsWith('.tmp'))
+  t.same(leftover, [], 'temp file was cleaned up after failure')
+})