From de0a44e9f9a321d97af895be520e4b14ff217d64 Mon Sep 17 00:00:00 2001
From: Lukas Stracke <lukas.stracke@sentry.io>
Date: Mon, 15 Jan 2024 10:24:10 +0100
Subject: [PATCH] fix(tracing-internal): Prefer fetch options headers over
 fetch init headers

---
 .../request/fetch-with-request/init.js        | 10 +++++++
 .../request/fetch-with-request/subject.js     |  7 +++++
 .../request/fetch-with-request/test.ts        | 30 +++++++++++++++++++
 packages/tracing-internal/src/common/fetch.ts |  3 +-
 4 files changed, 49 insertions(+), 1 deletion(-)
 create mode 100644 dev-packages/browser-integration-tests/suites/tracing/request/fetch-with-request/init.js
 create mode 100644 dev-packages/browser-integration-tests/suites/tracing/request/fetch-with-request/subject.js
 create mode 100644 dev-packages/browser-integration-tests/suites/tracing/request/fetch-with-request/test.ts

diff --git a/dev-packages/browser-integration-tests/suites/tracing/request/fetch-with-request/init.js b/dev-packages/browser-integration-tests/suites/tracing/request/fetch-with-request/init.js
new file mode 100644
index 000000000000..505fab06b330
--- /dev/null
+++ b/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,
+});
diff --git a/dev-packages/browser-integration-tests/suites/tracing/request/fetch-with-request/subject.js b/dev-packages/browser-integration-tests/suites/tracing/request/fetch-with-request/subject.js
new file mode 100644
index 000000000000..70a1992e9f8b
--- /dev/null
+++ b/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' },
+});
diff --git a/dev-packages/browser-integration-tests/suites/tracing/request/fetch-with-request/test.ts b/dev-packages/browser-integration-tests/suites/tracing/request/fetch-with-request/test.ts
new file mode 100644
index 000000000000..eb2f354dba4a
--- /dev/null
+++ b/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 });
+  },
+);
diff --git a/packages/tracing-internal/src/common/fetch.ts b/packages/tracing-internal/src/common/fetch.ts
index a6f7d68ac920..e0a8e2ed9fa3 100644
--- a/packages/tracing-internal/src/common/fetch.ts
+++ b/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 };