code refactor & sync bugfixes

src/fsutil.js

@@ -4,6 +4,16 @@ const fs = require('fs')
 const fsp = require('fs').promises
 const path = require('path')
 
+// accessSync instead of existsSync: same answer, one syscall fewer.
+function pathExists(p) {
+  try {
+    fs.accessSync(p)
+    return true
+  } catch {
+    return false
+  }
+}
+
 // Returns a non-existing path in `base` named `name` (or `name (2)`, `name (3)`...).
 function dedupeDir(base, name) {
   const candidate = path.join(base, name)
@@ -50,6 +60,22 @@ function progressGate(emit) {
   return push
 }
 
+// Run `fn` over `items` with at most `limit` calls in flight, results in
+// order. Walking a tree is dominated by stat/readlink/open syscalls: a few in
+// flight keep the disk and cpu busy without exhausting file descriptors.
+async function mapLimit(items, limit, fn) {
+  const out = new Array(items.length)
+  let next = 0
+  const workers = Array.from({ length: Math.min(limit, items.length) }, async () => {
+    while (next < items.length) {
+      const i = next++
+      out[i] = await fn(items[i], i)
+    }
+  })
+  await Promise.all(workers)
+  return out
+}
+
 // What a tree copy will have to move. Symlinks have no size of their own
 // (they are recreated, not read), so they only count for the file total.
 // `skipDir` is a single directory name to leave out (a game folder's
@@ -218,5 +244,5 @@ async function removeTree(root, onProgress) {
   return { files, dirs, bytes }
 }
 
-module.exports = { dedupeDir, copyTree, treeSize, removeTree, progressGate }
+module.exports = { pathExists, dedupeDir, copyTree, treeSize, removeTree, progressGate, mapLimit }