FileStore: write cache entries atomically

packages/metro-cache/src/stores/FileStore.js

@@ -9,6 +9,7 @@
  * @oncall react_native
  */
 
+import crypto from 'node:crypto';
 import fs from 'node:fs';
 import path from 'node:path';
 
@@ -65,7 +66,19 @@ export default class FileStore<T> {
     } else {
       content = JSON.stringify(value) ?? JSON.stringify(null);
     }
-    await fs.promises.writeFile(filePath, content);
+    // Write to a unique temporary file and rename it into place, so that a
+    // concurrent reader (or writer) of the same key never sees a partially
+    // written entry. Writing in place truncates the file first, and a reader
+    // could then see a run of null bytes, which looks like a binary entry.
+    const suffix = crypto.randomBytes(8).toString('hex');
+    const tempPath = `${filePath}.${process.pid}.${suffix}.tmp`;
+    try {
+      await fs.promises.writeFile(tempPath, content);
+      await fs.promises.rename(tempPath, filePath);
+    } catch (err) {
+      await fs.promises.rm(tempPath, {force: true});
+      throw err;
+    }
   }
 
   clear() {

packages/metro-cache/src/stores/__tests__/FileStore-test.js

@@ -67,4 +67,78 @@ describe('FileStore', () => {
     await fileStore.set(cache, data);
     expect(await fileStore.get(cache)).toEqual(data);
   });
+
+  test('never exposes a partially written entry to a reader', async () => {
+    const fileStore = new FileStore<unknown>({root: '/root'});
+    const cache = Buffer.from([0xfa, 0xce, 0xb0, 0x0c]);
+    const previous = {dependencies: [], output: ['previous']};
+    const next = {dependencies: [], output: ['x'.repeat(4096)]};
+    await fileStore.set(cache, previous);
+
+    // Simulate a write that is only part way done: the file has been opened
+    // (and truncated) and has zeros where its first half belongs. This is the
+    // state a concurrent writer of the same key can leave behind between the
+    // chunks of fs.promises.writeFile.
+    let resumeWrite: () => void = () => {};
+    let onWritePaused: () => void = () => {};
+    const writePaused = new Promise<void>(resolve => {
+      onWritePaused = resolve;
+    });
+    jest
+      .spyOn(fs.promises, 'writeFile')
+      .mockImplementationOnce(async (filePath, content) => {
+        const bytes = Buffer.from(content);
+        const half = Math.floor(bytes.length / 2);
+        const handle = await fs.promises.open(filePath, 'w');
+        try {
+          await handle.write(Buffer.alloc(half), 0, half, 0);
+          await handle.write(bytes, half, bytes.length - half, half);
+          await new Promise<void>(resolve => {
+            resumeWrite = resolve;
+            onWritePaused();
+          });
+          await handle.write(bytes, 0, half, 0);
+        } finally {
+          await handle.close();
+        }
+      });
+
+    const pendingSet = fileStore.set(cache, next);
+    await writePaused;
+    const duringWrite = await fileStore.get(cache);
+    resumeWrite();
+    await pendingSet;
+
+    expect(duringWrite).toEqual(previous);
+    expect(await fileStore.get(cache)).toEqual(next);
+  });
+
+  test('leaves no temporary files behind', async () => {
+    const fileStore = new FileStore<unknown>({root: '/root'});
+    const cache = Buffer.from([0xfa, 0xce, 0xb0, 0x0c]);
+
+    await Promise.all(
+      Array.from({length: 4}, (_, i) => fileStore.set(cache, {i})),
+    );
+
+    expect(fs.readdirSync('/root/fa')).toEqual(['ceb00c']);
+  });
+
+  test('removes the temporary file when a write fails', async () => {
+    const fileStore = new FileStore<unknown>({root: '/root'});
+    const cache = Buffer.from([0xfa, 0xce, 0xb0, 0x0c]);
+    await fileStore.set(cache, {foo: 42});
+    const writeError = new Error('Disk full');
+
+    jest
+      .spyOn(fs.promises, 'writeFile')
+      .mockImplementationOnce(async (filePath, content) => {
+        await fs.promises.appendFile(filePath, content.slice(0, 1));
+        throw writeError;
+      });
+
+    await expect(fileStore.set(cache, {foo: 43})).rejects.toBe(writeError);
+    expect(fs.readdirSync('/root/fa')).toEqual(['ceb00c']);
+    expect(await fileStore.get(cache)).toEqual({foo: 42});
+  });
 });