test: add test to ensure copyDirectory copies files atomically

Assisted-by: Claude Opus 4.6
Signed-off-by: David Sanders <dsanders11@ucsbalum.com>

Author: dsanders11

test/test-copy-directory.js

@@ -0,0 +1,86 @@
+'use strict'
+
+const { describe, it, afterEach } = require('mocha')
+const assert = require('assert')
+const path = require('path')
+const fs = require('fs')
+const { promises: fsp } = fs
+const os = require('os')
+const { FULL_TEST, platformTimeout } = require('./common')
+const copyDirectory = require('../lib/copy-directory')
+
+describe('copyDirectory', function () {
+  let timer
+  let tmpDir
+
+  afterEach(async () => {
+    if (tmpDir) {
+      await fsp.rm(tmpDir, { recursive: true, force: true })
+      tmpDir = null
+    }
+    clearInterval(timer)
+  })
+
+  it('large file appears atomically (no partial writes visible)', async function () {
+    if (!FULL_TEST) {
+      return this.skip('Skipping due to test environment configuration')
+    }
+
+    this.timeout(platformTimeout(5, { win32: 10 }))
+
+    tmpDir = await fsp.mkdtemp(path.join(os.tmpdir(), 'node-gyp-copy-test-'))
+    const srcDir = path.join(tmpDir, 'src')
+    const destDir = path.join(tmpDir, 'dest')
+    await fsp.mkdir(srcDir)
+
+    const fileName = 'large.bin'
+    const srcFile = path.join(srcDir, fileName)
+    const destFile = path.join(destDir, fileName)
+
+    // Create a 5 GB sparse file — instant to create, consumes no real
+    // disk, but fs.copyFile still has to process the full extent map so
+    // the destination file is visible at size 0 and grows over time.
+    // fs.rename() is atomic at the VFS level: the file either does not
+    // exist at the destination or appears at its full size in one step.
+    const fileSize = 5 * 1024 * 1024 * 1024
+    const handle = await fsp.open(srcFile, 'w')
+    await handle.truncate(fileSize)
+    await handle.close()
+
+    // Tight synchronous poll: stat the destination on every event-loop
+    // turn while copyDirectory runs concurrently.
+    //
+    // With the old fs.copyFile approach the dest file is created at
+    // size 0 and grows → this loop catches it at a partial size.
+    //
+    // With fs.rename the file either isn't there yet (ENOENT) or is
+    // already at its full size → zero violations.
+    let polls = 0
+    const violations = []
+
+    timer = setInterval(() => {
+      try {
+        const stat = fs.statSync(destFile)
+        polls++
+        if (stat.size !== fileSize) {
+          violations.push({ poll: polls, size: stat.size })
+        }
+      } catch (err) {
+        if (err.code !== 'ENOENT') throw err
+      }
+    }, 0)
+
+    await copyDirectory(srcDir, destDir)
+
+    clearInterval(timer)
+    timer = undefined
+
+    console.log(`        ${polls} stats observed the file during the operation`)
+
+    assert.strictEqual(violations.length, 0, 'file must never be observed at a partial size')
+
+    const finalStat = await fsp.stat(destFile)
+    assert.strictEqual(finalStat.size, fileSize,
+      'destination file should have the correct final size')
+  })
+})