[FIX] Preserve user baggage header when setting Session ID

packages/core/src/rum/instrumentation/resourceTracking/distributedTracing/distributedTracing.tsx

@@ -26,12 +26,14 @@ export type DdRumResourceTracingAttributes =
           rulePsr: number;
           propagatorTypes: PropagatorType[];
           rumSessionId?: string;
+          baggageHeaders?: Set<string>;
       }
     | {
           tracingStrategy: 'DISCARD';
           traceId?: void;
           spanId?: void;
           samplingPriorityHeader: '0';
+          baggageHeaders?: Set<string>;
       };
 
 const DISCARDED_TRACE_ATTRIBUTES: DdRumResourceTracingAttributes = {

packages/core/src/rum/instrumentation/resourceTracking/distributedTracing/distributedTracingHeaders.ts

@@ -138,9 +138,22 @@ export const getTracingHeadersFromAttributes = (
             }
         }
         if (tracingAttributes.rumSessionId) {
+            if (!tracingAttributes.baggageHeaders) {
+                tracingAttributes.baggageHeaders = new Set<string>();
+            }
+
+            tracingAttributes.baggageHeaders?.add(
+                `${DD_RUM_SESSION_ID_TAG}=${tracingAttributes.rumSessionId}`
+            );
+        }
+
+        const baggageHeader = tracingAttributes.baggageHeaders
+            ? Array.from(tracingAttributes.baggageHeaders).join(', ')
+            : null;
+        if (baggageHeader) {
             headers.push({
                 header: BAGGAGE_HEADER_KEY,
-                value: `${DD_RUM_SESSION_ID_TAG}=${tracingAttributes.rumSessionId}`
+                value: baggageHeader
             });
         }
     });

packages/core/src/rum/instrumentation/resourceTracking/requestProxy/XHRProxy/XHRProxy.ts

@@ -6,7 +6,10 @@
 
 import { Timer } from '../../../../../utils/Timer';
 import { getCachedSessionId } from '../../../../sessionId/sessionIdHelper';
-import { getTracingHeadersFromAttributes } from '../../distributedTracing/distributedTracingHeaders';
+import {
+    BAGGAGE_HEADER_KEY,
+    getTracingHeadersFromAttributes
+} from '../../distributedTracing/distributedTracingHeaders';
 import type { DdRumResourceTracingAttributes } from '../../distributedTracing/distributedTracing';
 import { getTracingAttributes } from '../../distributedTracing/distributedTracing';
 import {
@@ -226,6 +229,14 @@ const proxySetRequestHeader = (providers: XHRProxyProviders): void => {
             }
         }
 
+        if (header.toLowerCase() === BAGGAGE_HEADER_KEY) {
+            if (!this._datadog_xhr.tracingAttributes.baggageHeaders) {
+                this._datadog_xhr.tracingAttributes.baggageHeaders = new Set<string>();
+            }
+
+            this._datadog_xhr.tracingAttributes.baggageHeaders?.add(value);
+        }
+
         // eslint-disable-next-line prefer-rest-params
         return originalXhrSetRequestHeader.apply(this, arguments as any);
     };

packages/core/src/rum/instrumentation/resourceTracking/requestProxy/XHRProxy/__tests__/XHRProxy.test.ts

@@ -839,6 +839,55 @@ describe('XHRProxy', () => {
             // THEN
             expect(xhr.requestHeaders[BAGGAGE_HEADER_KEY]).toBeUndefined();
         });
+
+        it('rum session id does not overwrite existing baggage headers', async () => {
+            // GIVEN
+            const method = 'GET';
+            const url = 'https://api.example.com:443/v2/user';
+            xhrProxy.onTrackingStart({
+                tracingSamplingRate: 100,
+                firstPartyHostsRegexMap: firstPartyHostsRegexMapBuilder([
+                    {
+                        match: 'api.example.com',
+                        propagatorTypes: [
+                            PropagatorType.DATADOG,
+                            PropagatorType.TRACECONTEXT
+                        ]
+                    },
+                    {
+                        match: 'example.com',
+                        propagatorTypes: [
+                            PropagatorType.B3,
+                            PropagatorType.B3MULTI
+                        ]
+                    }
+                ])
+            });
+
+            setCachedSessionId('TEST-SESSION-ID');
+
+            // WHEN
+            const xhr = new XMLHttpRequestMock();
+            xhr.open(method, url);
+            xhr.setRequestHeader('baggage', 'existing.key=existing-value');
+            xhr.send();
+            xhr.notifyResponseArrived();
+            xhr.complete(200, 'ok');
+            await flushPromises();
+
+            // THEN
+            expect(xhr.requestHeaders[BAGGAGE_HEADER_KEY]).not.toBeUndefined();
+            expect(xhr.requestHeaders[BAGGAGE_HEADER_KEY]).toContain(
+                'existing.key=existing-value'
+            );
+
+            const values = xhr.requestHeaders[BAGGAGE_HEADER_KEY].split(
+                ', '
+            ).sort();
+
+            expect(values[0]).toBe('existing.key=existing-value');
+            expect(values[1]).toBe('session.id=TEST-SESSION-ID');
+        });
     });
 
     describe('DdRum.startResource calls', () => {