fix: avoid setup global dispatcher in advance

Author: himself65

lib/global.js

@@ -6,22 +6,36 @@ const globalDispatcher = Symbol.for('undici.globalDispatcher.1')
 const { InvalidArgumentError } = require('./core/errors')
 const Agent = require('./dispatcher/agent')
 
-if (getGlobalDispatcher() === undefined) {
-  setGlobalDispatcher(new Agent())
-}
+let currentDispatcher = null
 
-function setGlobalDispatcher (agent) {
-  if (!agent || typeof agent.dispatch !== 'function') {
-    throw new InvalidArgumentError('Argument agent must implement Agent')
+if (Object.hasOwn(globalThis, globalDispatcher)) {
+  const existing = Reflect.get(globalThis, globalDispatcher)
+  if (existing && typeof existing.dispatch === 'function') {
+    currentDispatcher = existing
   }
+} else {
   Object.defineProperty(globalThis, globalDispatcher, {
-    value: agent,
-    writable: true,
+    get () {
+      if (currentDispatcher === null) {
+        currentDispatcher = new Agent()
+      }
+      return currentDispatcher
+    },
+    set (agent) {
+      setGlobalDispatcher(agent)
+    },
     enumerable: false,
     configurable: false
   })
 }
 
+function setGlobalDispatcher (agent) {
+  if (!agent || typeof agent.dispatch !== 'function') {
+    throw new InvalidArgumentError('Argument agent must implement Agent')
+  }
+  currentDispatcher = agent
+}
+
 function getGlobalDispatcher () {
   return globalThis[globalDispatcher]
 }