fix(browser): Respect manually set sentry tracing headers in XHR requests

dev-packages/browser-integration-tests/suites/tracing/request/xhr-merged-baggage-headers/subject.js

@@ -0,0 +1,7 @@
+const xhr = new XMLHttpRequest();
+
+xhr.open('GET', 'http://sentry-test-site.example/1');
+xhr.setRequestHeader('X-Test-Header', 'existing-header');
+xhr.setRequestHeader('baggage', 'someVendor-foo=bar');
+
+xhr.send();

dev-packages/browser-integration-tests/suites/tracing/request/xhr-merged-baggage-headers/test.ts

@@ -0,0 +1,25 @@
+import { expect } from '@playwright/test';
+import { TRACEPARENT_REGEXP } from '@sentry/core';
+import { sentryTest } from '../../../../utils/fixtures';
+import { shouldSkipTracingTest } from '../../../../utils/helpers';
+
+sentryTest('merges `baggage` headers of pre-existing non-sentry XHR requests', async ({ getLocalTestUrl, page }) => {
+  if (shouldSkipTracingTest()) {
+    sentryTest.skip();
+  }
+
+  const url = await getLocalTestUrl({ testDir: __dirname });
+
+  const requestPromise = page.waitForRequest('http://sentry-test-site.example/1');
+
+  await page.goto(url);
+
+  const request = await requestPromise;
+
+  const requestHeaders = request.headers();
+  expect(requestHeaders).toMatchObject({
+    'sentry-trace': expect.stringMatching(TRACEPARENT_REGEXP),
+    baggage: expect.stringMatching(/^someVendor-foo=bar, sentry-.*$/),
+    'x-test-header': 'existing-header',
+  });
+});

dev-packages/browser-integration-tests/suites/tracing/request/xhr-with-custom-sentry-headers/subject.js

@@ -0,0 +1,8 @@
+const xhr = new XMLHttpRequest();
+
+xhr.open('GET', 'http://sentry-test-site.example/1');
+xhr.setRequestHeader('X-Test-Header', 'existing-header');
+xhr.setRequestHeader('sentry-trace', '123-abc-1');
+xhr.setRequestHeader('baggage', 'sentry-release=1.1.1');
+
+xhr.send();

dev-packages/browser-integration-tests/suites/tracing/request/xhr-with-custom-sentry-headers/test.ts

@@ -0,0 +1,27 @@
+import { expect } from '@playwright/test';
+import { sentryTest } from '../../../../utils/fixtures';
+import { shouldSkipTracingTest } from '../../../../utils/helpers';
+
+sentryTest(
+  'attaches manually passed in `sentry-trace` and `baggage` headers to XHR requests',
+  async ({ getLocalTestUrl, page }) => {
+    if (shouldSkipTracingTest()) {
+      sentryTest.skip();
+    }
+
+    const url = await getLocalTestUrl({ testDir: __dirname });
+
+    const requestPromise = page.waitForRequest('http://sentry-test-site.example/1');
+
+    await page.goto(url);
+
+    const request = await requestPromise;
+
+    const requestHeaders = request.headers();
+    expect(requestHeaders).toMatchObject({
+      'sentry-trace': '123-abc-1',
+      baggage: 'sentry-release=1.1.1',
+      'x-test-header': 'existing-header',
+    });
+  },
+);

packages/browser/src/tracing/request.ts

@@ -420,21 +420,36 @@ function setHeaderOnXhr(
   sentryTraceHeader: string,
   sentryBaggageHeader: string | undefined,
 ): void {
+  const originalHeaders = xhr.__sentry_xhr_v3__?.request_headers;
+
+  if (originalHeaders?.['sentry-trace']) {
+    // bail if a sentry-trace header is already set
+    return;
+  }
+
   try {
     // eslint-disable-next-line @typescript-eslint/no-non-null-assertion
     xhr.setRequestHeader!('sentry-trace', sentryTraceHeader);
     if (sentryBaggageHeader) {
-      // From MDN: "If this method is called several times with the same header, the values are merged into one single request header."
-      // We can therefore simply set a baggage header without checking what was there before
-      // https://developer.mozilla.org/en-US/docs/Web/API/XMLHttpRequest/setRequestHeader
-      // eslint-disable-next-line @typescript-eslint/no-non-null-assertion
-      xhr.setRequestHeader!('baggage', sentryBaggageHeader);
+      // bail if a pre-existing baggage header is set and already contains sentry values
+      const originalBaggageHeader = originalHeaders?.['baggage'];
+      if (!originalBaggageHeader || !baggageHeaderHasSentryValues(originalBaggageHeader)) {
+        // From MDN: "If this method is called several times with the same header, the values are merged into one single request header."
+        // We can therefore simply set a baggage header without checking what was there before
+        // https://developer.mozilla.org/en-US/docs/Web/API/XMLHttpRequest/setRequestHeader
+        // eslint-disable-next-line @typescript-eslint/no-non-null-assertion
+        xhr.setRequestHeader!('baggage', sentryBaggageHeader);
+      }
     }
   } catch (_) {
     // Error: InvalidStateError: Failed to execute 'setRequestHeader' on 'XMLHttpRequest': The object's state must be OPENED.
   }
 }
 
+function baggageHeaderHasSentryValues(baggageHeader: string): boolean {
+  return baggageHeader.split(',').some(value => value.startsWith('sentry-'));
+}
+
 function getFullURL(url: string): string | undefined {
   try {
     // By adding a base URL to new URL(), this will also work for relative urls