Fix race condition in FileStorage dequeue

src/storage/file.ts

@@ -1,4 +1,4 @@
-import { mkdir, readdir, readFile, unlink, watch, rm } from 'node:fs/promises'
+import { mkdir, readdir, readFile, rename, unlink, watch, rm } from 'node:fs/promises'
 import { join } from 'node:path'
 import { EventEmitter } from 'node:events'
 import fastWriteAtomic from 'fast-write-atomic'
@@ -194,25 +194,27 @@ export class FileStorage implements Storage {
     const files = await this.#getQueueFiles()
     if (files.length === 0) return null
 
-    // Try to claim the first file
-    const file = files[0]
-    const srcPath = join(this.#queuePath, file)
-    const id = this.#extractIdFromFilename(file)
-    const dstPath = join(this.#processingPath, workerId, `${id}.msg`)
+    // Ensure processing directory exists
+    await mkdir(join(this.#processingPath, workerId), { recursive: true })
 
-    try {
-      // Ensure processing directory exists
-      await mkdir(join(this.#processingPath, workerId), { recursive: true })
-
-      // Read and move atomically
-      const message = await readFile(srcPath)
-      await writeFileAtomic(dstPath, message)
-      await unlink(srcPath)
-      return message
-    } catch {
-      // Another worker may have claimed it
-      return null
+    // Try to claim files until one succeeds
+    for (const file of files) {
+      const srcPath = join(this.#queuePath, file)
+      const id = this.#extractIdFromFilename(file)
+      const dstPath = join(this.#processingPath, workerId, `${id}.msg`)
+
+      try {
+        // Atomic rename - only one worker can succeed on same filesystem
+        await rename(srcPath, dstPath)
+        // Only read after successful claim
+        return await readFile(dstPath)
+      } catch {
+        // Another worker claimed this file, try the next one
+        continue
+      }
     }
+
+    return null
   }
 
   async #getQueueFiles (): Promise<string[]> {