feat: defer AsyncLocalStorage creation for v8 startup snapshots (#1946)

## Summary

- Defer `AsyncLocalStorage` creation when
`v8.startupSnapshot.isBuildingSnapshot()` is true, making Koa compatible
with Node.js startup snapshots
- Register a `v8.startupSnapshot.addDeserializeCallback` to properly
initialize `ctxStorage` after snapshot restoration
- Extract `getAsyncLocalStorage()` helper to consolidate creation logic

## Test plan

- [x] All 429 existing tests pass with 0 failures
- [x] `currentContext` tests verify both `asyncLocalStorage: true` and
custom `AsyncLocalStorage` instance paths work correctly
- [x] Normal (non-snapshot) code path is unchanged —
`v8.startupSnapshot?.isBuildingSnapshot?.()` returns `undefined` in
regular execution

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: killagu

__tests__/application/currentContext.test.js

@@ -1,6 +1,7 @@
 'use strict'
 
-const { describe, it } = require('node:test')
+const { describe, it, beforeEach, afterEach } = require('node:test')
+const v8 = require('node:v8')
 const request = require('supertest')
 const assert = require('node:assert/strict')
 const Koa = require('../..')
@@ -113,4 +114,85 @@ describe('app.currentContext', () => {
     await request(app.callback()).get('/').expect('ok')
     assert(app.currentContext === undefined)
   })
+
+  describe('v8 startup snapshot', () => {
+    let originalStartupSnapshot
+
+    beforeEach(() => {
+      originalStartupSnapshot = v8.startupSnapshot
+    })
+
+    afterEach(() => {
+      v8.startupSnapshot = originalStartupSnapshot
+    })
+
+    it('should defer AsyncLocalStorage creation when building snapshot', () => {
+      let deserializeCallback
+      v8.startupSnapshot = {
+        isBuildingSnapshot: () => true,
+        addDeserializeCallback: (cb, data) => {
+          deserializeCallback = { cb, data }
+        }
+      }
+
+      const app = new Koa({ asyncLocalStorage: true })
+      assert.strictEqual(app.ctxStorage, null)
+      assert(deserializeCallback, 'deserialize callback should be registered')
+
+      // simulate snapshot deserialization
+      deserializeCallback.cb(deserializeCallback.data)
+      assert(app.ctxStorage instanceof AsyncLocalStorage)
+    })
+
+    it('should defer with custom AsyncLocalStorage when building snapshot', () => {
+      const customStorage = new AsyncLocalStorage()
+      let deserializeCallback
+      v8.startupSnapshot = {
+        isBuildingSnapshot: () => true,
+        addDeserializeCallback: (cb, data) => {
+          deserializeCallback = { cb, data }
+        }
+      }
+
+      const app = new Koa({ asyncLocalStorage: customStorage })
+      assert.strictEqual(app.ctxStorage, null)
+
+      // simulate snapshot deserialization
+      deserializeCallback.cb(deserializeCallback.data)
+      assert(app.ctxStorage instanceof AsyncLocalStorage)
+      assert.strictEqual(app.ctxStorage, customStorage)
+    })
+
+    it('should work normally after deserialization', async () => {
+      let deserializeCallback
+      v8.startupSnapshot = {
+        isBuildingSnapshot: () => true,
+        addDeserializeCallback: (cb, data) => {
+          deserializeCallback = { cb, data }
+        }
+      }
+
+      const app = new Koa({ asyncLocalStorage: true })
+
+      // simulate snapshot deserialization
+      deserializeCallback.cb(deserializeCallback.data)
+
+      app.use(async ctx => {
+        assert(ctx === app.currentContext)
+        ctx.body = 'ok'
+      })
+
+      await request(app.callback()).get('/').expect('ok')
+      assert(app.currentContext === undefined)
+    })
+
+    it('should not defer when not building snapshot', () => {
+      v8.startupSnapshot = {
+        isBuildingSnapshot: () => false
+      }
+
+      const app = new Koa({ asyncLocalStorage: true })
+      assert(app.ctxStorage instanceof AsyncLocalStorage)
+    })
+  })
 })

lib/application.js

@@ -4,6 +4,7 @@
  * Module dependencies.
  */
 const util = require('node:util')
+const v8 = require('node:v8')
 const debug = util.debuglog('koa:application')
 const Emitter = require('node:events')
 const Stream = require('node:stream')
@@ -40,6 +41,13 @@ const only = require('./only.js')
  * Inherits from `Emitter.prototype`.
  */
 
+function getAsyncLocalStorage (options) {
+  if (options.asyncLocalStorage instanceof AsyncLocalStorage) {
+    return options.asyncLocalStorage
+  }
+  return new AsyncLocalStorage()
+}
+
 module.exports = class Application extends Emitter {
   /**
    * Initialize a new `Application`.
@@ -81,10 +89,13 @@ module.exports = class Application extends Emitter {
       this[util.inspect.custom] = this.inspect
     }
     if (options.asyncLocalStorage) {
-      if (options.asyncLocalStorage instanceof AsyncLocalStorage) {
-        this.ctxStorage = options.asyncLocalStorage
+      if (v8.startupSnapshot?.isBuildingSnapshot?.()) {
+        this.ctxStorage = null
+        v8.startupSnapshot.addDeserializeCallback(({ app, options }) => {
+          app.ctxStorage = getAsyncLocalStorage(options)
+        }, { app: this, options })
       } else {
-        this.ctxStorage = new AsyncLocalStorage()
+        this.ctxStorage = getAsyncLocalStorage(options)
       }
     }
   }