Fix #607: use deferred promise to prevent fetch response body race condition

- Replace `responseBodyPromise = Promise.resolve(undefined)` with a
  deferred promise whose resolver is captured before `Promise.all` runs.
- Inside the fetch `.then` handler, call `responseBodyResolve()` in both
  the recording and non-recording branches so `Promise.all` always settles.
- Export `initFetchObserver` and add a regression test that asserts the
  `cb` receives a populated `responseBody` (would have failed before the fix).

Fixes #607

Author: Copilot

package-lock.json

@@ -487,7 +487,7 @@ function initXhrObserver(cb, win, options) {
  * @param {Required<NetworkRecordOptions>} options
  * @returns {listenerHandler}
  */
-function initFetchObserver(cb, win, options) {
+export function initFetchObserver(cb, win, options) {
     if (!options.initiatorTypes.includes('fetch')) {
         return function() {
             //
@@ -508,7 +508,10 @@ function initFetchObserver(cb, win, options) {
 
             var originalFetchPromise;
             var requestBodyPromise = Promise.resolve(undefined);
-            var responseBodyPromise = Promise.resolve(undefined);
+            var responseBodyResolve;
+            var responseBodyPromise = new Promise(function(resolve) {
+                responseBodyResolve = resolve;
+            });
             try {
                 /** @type {Headers} */
                 var requestHeaders = {};
@@ -541,10 +544,12 @@ function initFetchObserver(cb, win, options) {
                     networkRequest.responseHeaders = responseHeaders;
 
                     if (shouldRecordBody('response', options.recordBodyUrls, req.url)) {
-                        responseBodyPromise = tryReadFetchBody(res)
-                            .then(function(body) {
-                                networkRequest.responseBody = body;
-                            });
+                        tryReadFetchBody(res).then(function(body) {
+                            networkRequest.responseBody = body;
+                            responseBodyResolve();
+                        });
+                    } else {
+                        responseBodyResolve();
                     }
 
                     return res;

src/recorder/rrweb-network-plugin.js

@@ -1,5 +1,5 @@
 import { expect } from 'chai';
-import { findLast, truncateBody, shouldRecordHeader, shouldRecordBody, patch } from '../../src/recorder/rrweb-network-plugin';
+import { findLast, truncateBody, shouldRecordHeader, shouldRecordBody, patch, initFetchObserver } from '../../src/recorder/rrweb-network-plugin';
 
 describe(`rrweb-network-plugin utils`, function() {
   describe(`findLast`, function() {
@@ -167,4 +167,48 @@ describe(`rrweb-network-plugin utils`, function() {
       expect(obj.val).to.equal(123);
     });
   });
+
+  describe(`initFetchObserver`, function() {
+    it(`populates responseBody before emitting the network event (regression for #607)`, function(done) {
+      const url = `https://example.com/api/data`;
+      const responseText = `{"hello":"world"}`;
+
+      const fakeEntry = {
+        name: url,
+        initiatorType: `fetch`,
+        entryType: `resource`,
+        startTime: 0,
+        responseEnd: 50,
+      };
+
+      const fakeResponse = new Response(responseText, { status: 200 });
+
+      const win = {
+        fetch: function() { return Promise.resolve(fakeResponse); },
+        performance: {
+          now: function() { return 0; },
+          getEntriesByName: function() { return [fakeEntry]; },
+        },
+      };
+
+      const options = {
+        initiatorTypes: [`fetch`],
+        recordHeaders: { request: [], response: [] },
+        recordBodyUrls: { request: [], response: [/example\.com/] },
+        ignoreRequestUrls: [],
+        ignoreRequestFn: function() { return false; },
+      };
+
+      initFetchObserver(function(data) {
+        try {
+          expect(data.requests[0].responseBody).to.equal(responseText);
+          done();
+        } catch (e) {
+          done(e);
+        }
+      }, win, options);
+
+      win.fetch(url);
+    });
+  });
 });