improve native isolation

src/fsutil.js

@@ -199,6 +199,49 @@ async function chmodTree(root) {
   return { files, dirs }
 }
 
+// One file, streamed, with byte progress: copyFile says nothing until a
+// multi-gigabyte copy is over. The bytes land on a .part name and are renamed
+// at the end, so nothing can ever read a half-written document.
+async function copyFileProgress(src, dst, onProgress) {
+  const stat = await fsp.stat(src)
+  if (!stat.isFile()) throw new Error('not a file: ' + src)
+  const gate = onProgress ? progressGate(onProgress) : null
+  await fsp.mkdir(path.dirname(dst), { recursive: true })
+  const part = dst + '.part'
+  let bytes = 0
+  try {
+    await new Promise((resolve, reject) => {
+      const input = fs.createReadStream(src)
+      const output = fs.createWriteStream(part)
+      const fail = err => {
+        input.destroy()
+        output.destroy()
+        reject(err)
+      }
+      input.on('error', fail)
+      output.on('error', fail)
+      input.on('data', chunk => {
+        bytes += chunk.length
+        if (gate) gate({ done: 0, bytes }, false)
+      })
+      output.on('finish', resolve)
+      input.pipe(output)
+    })
+    await fsp.chmod(part, stat.mode & 0o777)
+    await fsp.rename(part, dst)
+  } catch (err) {
+    try {
+      await fsp.rm(part, { force: true })
+    } catch {}
+    throw err
+  }
+  if (gate) {
+    gate({ done: 1, bytes: stat.size }, true)
+    gate.done()
+  }
+  return { files: 1, bytes: stat.size }
+}
+
 // Delete deepest-first with progress through onProgress; symlinks are
 // unlinked, never followed.
 async function removeTree(root, onProgress) {
@@ -266,5 +309,5 @@ async function removeTree(root, onProgress) {
   return { files, dirs, bytes }
 }
 
-module.exports = { pathExists, dedupeDir, copyTree, treeSize, removeTree, chmodTree, progressGate, mapLimit }
+module.exports = { pathExists, dedupeDir, copyTree, copyFileProgress, treeSize, removeTree, chmodTree, progressGate, mapLimit }