fix(rolldown): avoid duplicate concurrent log reads

Author: webfansplz

packages/rolldown/src/node/rolldown/__tests__/events-reader.test.ts

@@ -0,0 +1,40 @@
+import type { Event } from '@rolldown/debug'
+import { describe, expect, it, vi } from 'vitest'
+import { RolldownEventsReader } from '../events-reader'
+
+describe('rolldown events reader', () => {
+  it('does not process the same log bytes twice during concurrent reads', async () => {
+    const reader = RolldownEventsReader.get('/mock/logs.json')
+    let resolveRead!: () => void
+    const readComplete = new Promise<void>((resolve) => {
+      resolveRead = resolve
+    })
+    const event = { action: 'BuildStart', timestamp: '1772529438599', session_id: '5173' } satisfies Event
+
+    const readChanges = vi.fn(async () => {
+      reader.manager.handleEvent(event)
+      await readComplete
+    })
+    Object.defineProperty(reader, 'readChanges', { value: readChanges })
+
+    try {
+      const reads = [
+        reader.read(),
+        reader.read(),
+        reader.read(),
+      ]
+
+      expect(readChanges).toHaveBeenCalledTimes(1)
+      resolveRead()
+      await Promise.all(reads)
+
+      expect(reader.manager.events).toHaveLength(1)
+
+      await reader.read()
+      expect(readChanges).toHaveBeenCalledTimes(2)
+    }
+    finally {
+      reader.dispose()
+    }
+  })
+})

packages/rolldown/src/node/rolldown/events-reader.ts

@@ -10,6 +10,7 @@ export class RolldownEventsReader {
   lastTimestamp: number = 0
   manager = new RolldownEventsManager()
   meta: SessionMeta | undefined
+  private pendingRead: Promise<void> | undefined
 
   private constructor(
     readonly filepath: string,
@@ -26,6 +27,17 @@ export class RolldownEventsReader {
   }
 
   async read() {
+    if (this.pendingRead) {
+      return this.pendingRead
+    }
+
+    this.pendingRead = this.readChanges().finally(() => {
+      this.pendingRead = undefined
+    })
+    return this.pendingRead
+  }
+
+  private async readChanges() {
     const { mtime, size } = await fs.promises.stat(this.filepath)
     if (mtime.getTime() <= this.lastTimestamp) {
       return