fix(tracing-internal): Prefer fetch init headers over fetch input headers

dev-packages/browser-integration-tests/suites/tracing/request/fetch-with-request/init.js

@@ -0,0 +1,10 @@
+import * as Sentry from '@sentry/browser';
+import { Integrations } from '@sentry/tracing';
+
+window.Sentry = Sentry;
+
+Sentry.init({
+  dsn: 'https://public@dsn.ingest.sentry.io/1337',
+  integrations: [new Integrations.BrowserTracing({ tracingOrigins: ['http://example.com'] })],
+  tracesSampleRate: 1,
+});

dev-packages/browser-integration-tests/suites/tracing/request/fetch-with-request/subject.js

@@ -0,0 +1,7 @@
+const request = new Request('http://example.com/api/test/', {
+  headers: { foo: '11' },
+});
+
+fetch(request, {
+  headers: { bar: '22' },
+});

dev-packages/browser-integration-tests/suites/tracing/request/fetch-with-request/test.ts

@@ -0,0 +1,30 @@
+import { expect } from '@playwright/test';
+
+import { sentryTest } from '../../../../utils/fixtures';
+import { shouldSkipTracingTest } from '../../../../utils/helpers';
+
+sentryTest(
+  'instrumentation should pass on headers from fetch options instead of init request, if set',
+  async ({ getLocalTestPath, page }) => {
+    if (shouldSkipTracingTest()) {
+      sentryTest.skip();
+    }
+
+    await page.route('**/api/test/', async route => {
+      const req = route.request();
+      const headers = await req.allHeaders();
+
+      // headers.bar was set in fetch options (and should be sent)
+      expect(headers.bar).toBe('22');
+      // headers.foo was set in init request object (and should be ignored)
+      expect(headers.foo).toBeUndefined();
+
+      return route.fulfill({
+        status: 200,
+        body: 'ok',
+      });
+    });
+
+    await getLocalTestPath({ testDir: __dirname });
+  },
+);

packages/tracing-internal/src/common/fetch.ts

@@ -149,7 +149,8 @@ export function addTracingHeadersToFetchRequest(
   const sentryBaggageHeader = dynamicSamplingContextToSentryBaggageHeader(dynamicSamplingContext);
 
   const headers =
-    typeof Request !== 'undefined' && isInstanceOf(request, Request) ? (request as Request).headers : options.headers;
+    options.headers ||
+    (typeof Request !== 'undefined' && isInstanceOf(request, Request) ? (request as Request).headers : undefined);
 
   if (!headers) {
     return { 'sentry-trace': sentryTraceHeader, baggage: sentryBaggageHeader };