fix(xhr): clone the request before calculating its body size

src/interceptors/XMLHttpRequest/XMLHttpRequestController.ts

@@ -163,7 +163,7 @@ export class XMLHttpRequestController {
 
             // Delegate request handling to the consumer.
             const fetchRequest = this.toFetchApiRequest(requestBody)
-            this[kFetchRequest] = fetchRequest
+            this[kFetchRequest] = fetchRequest.clone()
 
             const onceRequestSettled =
               this.onRequest?.call(this, {
@@ -296,7 +296,7 @@ export class XMLHttpRequestController {
      */
     if (this[kFetchRequest]) {
       const totalRequestBodyLength = await getBodyByteLength(
-        this[kFetchRequest].clone()
+        this[kFetchRequest]
       )
 
       this.trigger('loadstart', this.request.upload, {

test/modules/XMLHttpRequest/regressions/xhr-request-body-length.test.ts

@@ -0,0 +1,41 @@
+// @vitest-environment jsdom
+import { vi, beforeAll, afterEach, afterAll, it, expect } from 'vitest'
+import { XMLHttpRequestInterceptor } from '../../../../src/interceptors/XMLHttpRequest'
+import { createXMLHttpRequest } from '../../../helpers'
+
+const interceptor = new XMLHttpRequestInterceptor()
+
+beforeAll(() => {
+  interceptor.apply()
+})
+
+afterEach(() => {
+  interceptor.removeAllListeners()
+})
+
+afterAll(() => {
+  interceptor.dispose()
+})
+
+it('does not lock the request body stream when calculating the body size', async () => {
+  interceptor.on('request', async ({ request, controller }) => {
+    // Read the request body in the interceptor.
+    const buffer = await request.arrayBuffer()
+    controller.respondWith(new Response(buffer))
+  })
+
+  const uploadLoadStartListener = vi.fn()
+  const request = await createXMLHttpRequest((request) => {
+    request.upload.addEventListener('loadstart', uploadLoadStartListener)
+    request.open('POST', '/resource')
+    request.send('request-body')
+  })
+
+  // Must calculate the total request body size for the upload event.
+  const progressEvent = uploadLoadStartListener.mock.calls[0][0]
+  expect(progressEvent.total).toBe(12)
+
+  // Must be able to read the request in the interceptor
+  // and use its body as the mocked response body.
+  expect(request.responseText).toBe('request-body')
+})