test: reuse shared temp dir roots

src/test-helpers/temp-dir.ts

@@ -3,6 +3,51 @@ import fs from "node:fs/promises";
 import os from "node:os";
 import path from "node:path";
 
+const asyncPrefixRoots = new Map<string, string>();
+const pendingAsyncPrefixRoots = new Map<string, Promise<string>>();
+const syncPrefixRoots = new Map<string, string>();
+let nextAsyncDirIndex = 0;
+let nextSyncDirIndex = 0;
+
+function getRootKey(options: { prefix: string; parentDir?: string }): string {
+  return `${options.parentDir ?? os.tmpdir()}\u0000${options.prefix}`;
+}
+
+async function ensureAsyncPrefixRoot(options: {
+  prefix: string;
+  parentDir?: string;
+}): Promise<string> {
+  const key = getRootKey(options);
+  const cached = asyncPrefixRoots.get(key);
+  if (cached) {
+    return cached;
+  }
+  const pending = pendingAsyncPrefixRoots.get(key);
+  if (pending) {
+    return await pending;
+  }
+  const create = fs.mkdtemp(path.join(options.parentDir ?? os.tmpdir(), options.prefix));
+  pendingAsyncPrefixRoots.set(key, create);
+  try {
+    const root = await create;
+    asyncPrefixRoots.set(key, root);
+    return root;
+  } finally {
+    pendingAsyncPrefixRoots.delete(key);
+  }
+}
+
+function ensureSyncPrefixRoot(options: { prefix: string; parentDir?: string }): string {
+  const key = getRootKey(options);
+  const cached = syncPrefixRoots.get(key);
+  if (cached) {
+    return cached;
+  }
+  const root = fsSync.mkdtempSync(path.join(options.parentDir ?? os.tmpdir(), options.prefix));
+  syncPrefixRoots.set(key, root);
+  return root;
+}
+
 export async function withTempDir<T>(
   options: {
     prefix: string;
@@ -11,7 +56,10 @@ export async function withTempDir<T>(
   },
   run: (dir: string) => Promise<T>,
 ): Promise<T> {
-  const base = await fs.mkdtemp(path.join(options.parentDir ?? os.tmpdir(), options.prefix));
+  const root = await ensureAsyncPrefixRoot(options);
+  const base = path.join(root, `dir-${String(nextAsyncDirIndex)}`);
+  nextAsyncDirIndex += 1;
+  await fs.mkdir(base, { recursive: true });
   const dir = options.subdir ? path.join(base, options.subdir) : base;
   if (options.subdir) {
     await fs.mkdir(dir, { recursive: true });
@@ -68,7 +116,10 @@ export function withTempDirSync<T>(
   },
   run: (dir: string) => T,
 ): T {
-  const base = fsSync.mkdtempSync(path.join(options.parentDir ?? os.tmpdir(), options.prefix));
+  const root = ensureSyncPrefixRoot(options);
+  const base = path.join(root, `dir-${String(nextSyncDirIndex)}`);
+  nextSyncDirIndex += 1;
+  fsSync.mkdirSync(base, { recursive: true });
   const dir = options.subdir ? path.join(base, options.subdir) : base;
   if (options.subdir) {
     fsSync.mkdirSync(dir, { recursive: true });