fix: mirror the legacy global dispatcher for built-in fetch

README.md

@@ -621,7 +621,7 @@ including undici that is bundled internally with node.js.
 
 Undici stores this dispatcher under `Symbol.for('undici.globalDispatcher.2')`.
 
-On Node.js 22, `setGlobalDispatcher()` also mirrors the configured dispatcher to
+`setGlobalDispatcher()` also mirrors the configured dispatcher to
 `Symbol.for('undici.globalDispatcher.1')` using `Dispatcher1Wrapper`, so Node.js built-in `fetch`
 can keep using the legacy handler contract while Undici uses the new handler API.
 

docs/docs/api/Dispatcher.md

@@ -237,7 +237,7 @@ Pause/resume now uses the controller:
 Undici now stores the global dispatcher under `Symbol.for('undici.globalDispatcher.2')`.
 This avoids conflicts with runtimes (such as Node.js built-in `fetch`) that still rely on the legacy dispatcher handler interface.
 
-On Node.js 22, `setGlobalDispatcher()` also mirrors the configured dispatcher to `Symbol.for('undici.globalDispatcher.1')` using a `Dispatcher1Wrapper`, so Node's built-in `fetch` can keep using the legacy handler contract.
+`setGlobalDispatcher()` also mirrors the configured dispatcher to `Symbol.for('undici.globalDispatcher.1')` using a `Dispatcher1Wrapper`, so Node's built-in `fetch` can keep using the legacy handler contract.
 
 If you need to expose a new dispatcher/agent to legacy v1 handler consumers (`onConnect/onHeaders/onData/onComplete/onError/onUpgrade`), use `Dispatcher1Wrapper`:
 

lib/global.js

@@ -8,8 +8,6 @@ const { InvalidArgumentError } = require('./core/errors')
 const Agent = require('./dispatcher/agent')
 const Dispatcher1Wrapper = require('./dispatcher/dispatcher1-wrapper')
 
-const nodeMajor = Number(process.versions.node.split('.', 1)[0])
-
 if (getGlobalDispatcher() === undefined) {
   setGlobalDispatcher(new Agent())
 }
@@ -26,16 +24,14 @@ function setGlobalDispatcher (agent) {
     configurable: false
   })
 
-  if (nodeMajor === 22) {
-    const legacyAgent = agent instanceof Dispatcher1Wrapper ? agent : new Dispatcher1Wrapper(agent)
+  const legacyAgent = agent instanceof Dispatcher1Wrapper ? agent : new Dispatcher1Wrapper(agent)
 
-    Object.defineProperty(globalThis, legacyGlobalDispatcher, {
-      value: legacyAgent,
-      writable: true,
-      enumerable: false,
-      configurable: false
-    })
-  }
+  Object.defineProperty(globalThis, legacyGlobalDispatcher, {
+    value: legacyAgent,
+    writable: true,
+    enumerable: false,
+    configurable: false
+  })
 }
 
 function getGlobalDispatcher () {

test/node-test/global-dispatcher-version.js

@@ -42,31 +42,59 @@ test('setGlobalDispatcher does not break Node.js global fetch', () => {
   assert.strictEqual(result.stdout, 'ok')
 })
 
-test('setGlobalDispatcher mirrors a v1-compatible dispatcher on Node.js 22', () => {
+test('setGlobalDispatcher mirrors a v1-compatible dispatcher that Node.js global fetch uses', () => {
   const script = `
     const { Agent, Dispatcher1Wrapper, setGlobalDispatcher } = require('./index.js')
-    const nodeMajor = Number(process.versions.node.split('.', 1)[0])
+    const http = require('node:http')
+    const { once } = require('node:events')
 
-    setGlobalDispatcher(new Agent())
+    ;(async () => {
+      const dispatcherV1Symbol = Symbol.for('undici.globalDispatcher.1')
+      const dispatcherV2Symbol = Symbol.for('undici.globalDispatcher.2')
+      const server = http.createServer((req, res) => res.end('ok'))
+      server.listen(0)
+      await once(server, 'listening')
+
+      let count = 0
+      class CountingAgent extends Agent {
+        dispatch (opts, handler) {
+          count++
+          return super.dispatch(opts, handler)
+        }
+      }
 
-    if (nodeMajor !== 22) {
-      process.stdout.write('skipped')
-    } else {
-      const dispatcherV1 = globalThis[Symbol.for('undici.globalDispatcher.1')]
+      const agent = new CountingAgent()
+      setGlobalDispatcher(agent)
 
+      const dispatcherV1 = globalThis[dispatcherV1Symbol]
       if (!(dispatcherV1 instanceof Dispatcher1Wrapper)) {
-        throw new Error('expected v1 global dispatcher to be a Dispatcher1Wrapper on Node.js 22')
+        throw new Error('expected v1 global dispatcher to be a Dispatcher1Wrapper')
       }
 
-      process.stdout.write('mirrored')
-    }
+      const url = 'http://127.0.0.1:' + server.address().port
+      const res = await fetch(url)
+      const body = await res.text()
+
+      process.stdout.write(JSON.stringify({
+        body,
+        count,
+        mirroredV2: globalThis[dispatcherV2Symbol] === agent
+      }))
+
+      server.close()
+    })().catch((err) => {
+      console.error(err?.cause?.stack || err?.stack || err)
+      process.exit(1)
+    })
   `
 
   const result = runNode(script)
   assert.strictEqual(result.status, 0, result.stderr)
 
-  const expected = Number(process.versions.node.split('.', 1)[0]) === 22 ? 'mirrored' : 'skipped'
-  assert.strictEqual(result.stdout, expected)
+  const payload = JSON.parse(result.stdout)
+  assert.strictEqual(payload.body, 'ok')
+  assert.strictEqual(payload.count, 1)
+  assert.strictEqual(payload.mirroredV2, true)
 })
 
 test('Dispatcher1Wrapper bridges legacy handlers to a new Agent', () => {