Refactor copy to use opendir (#1028)

* refactor(copy): backport nodejs/node#41351

* perf(copy): parallel copy

* perf(copy): run filter in parallel as well

lib/copy/copy-sync.js

@@ -106,7 +106,17 @@ function mkDirAndCopy (srcMode, src, dest, opts) {
 }
 
 function copyDir (src, dest, opts) {
-  fs.readdirSync(src).forEach(item => copyDirItem(item, src, dest, opts))
+  const dir = fs.opendirSync(src)
+
+  try {
+    let dirent
+
+    while ((dirent = dir.readSync()) !== null) {
+      copyDirItem(dirent.name, src, dest, opts)
+    }
+  } finally {
+    dir.closeSync()
+  }
 }
 
 function copyDirItem (item, src, dest, opts) {

lib/copy/copy.js

@@ -113,23 +113,28 @@ async function onDir (srcStat, destStat, src, dest, opts) {
     await fs.mkdir(dest)
   }
 
-  const items = await fs.readdir(src)
+  const promises = []
 
   // loop through the files in the current directory to copy everything
-  await Promise.all(items.map(async item => {
-    const srcItem = path.join(src, item)
-    const destItem = path.join(dest, item)
-
-    // skip the item if it is matches by the filter function
-    const include = await runFilter(srcItem, destItem, opts)
-    if (!include) return
-
-    const { destStat } = await stat.checkPaths(srcItem, destItem, 'copy', opts)
+  for await (const item of await fs.opendir(src)) {
+    const srcItem = path.join(src, item.name)
+    const destItem = path.join(dest, item.name)
+
+    promises.push(
+      runFilter(srcItem, destItem, opts).then(include => {
+        if (include) {
+          // only copy the item if it matches the filter function
+          return stat.checkPaths(srcItem, destItem, 'copy', opts).then(({ destStat }) => {
+            // If the item is a copyable file, `getStatsAndPerformCopy` will copy it
+            // If the item is a directory, `getStatsAndPerformCopy` will call `onDir` recursively
+            return getStatsAndPerformCopy(destStat, srcItem, destItem, opts)
+          })
+        }
+      })
+    )
+  }
 
-    // If the item is a copyable file, `getStatsAndPerformCopy` will copy it
-    // If the item is a directory, `getStatsAndPerformCopy` will call `onDir` recursively
-    return getStatsAndPerformCopy(destStat, srcItem, destItem, opts)
-  }))
+  await Promise.all(promises)
 
   if (!destStat) {
     await fs.chmod(dest, srcStat.mode)