fix(browser): Ensure source is set correctly when updating span name in-place in beforeStartSpan (#17501)

When using `beforeStartSpan` in `browserTracingIntgration`, we document
that users should return a new options object rather than mutating the
passed in object in-place. However, nothing keeps users from setting the
name in-place like so:'

```js
browserTracingIntegration({
  beforeStartSpan: opts => {
    opts.name = 'changed';
    return opts;
  },
}),
```

In such a case, we previously didn't catch the mutation and wouldn't
adjust the span source attribute to `'custom'` accordingly. This PR
ensures we take care of this case, too. (found while writing a test for
#17500)

Author: Lms24

packages/browser/src/tracing/browserTracingIntegration.ts

@@ -356,6 +356,7 @@ export const browserTracingIntegration = ((_options: Partial<BrowserTracingOptio
   function _createRouteSpan(client: Client, startSpanOptions: StartSpanOptions, makeActive = true): void {
     const isPageloadTransaction = startSpanOptions.op === 'pageload';
 
+    const initialSpanName = startSpanOptions.name;
     const finalStartSpanOptions: StartSpanOptions = beforeStartSpan
       ? beforeStartSpan(startSpanOptions)
       : startSpanOptions;
@@ -364,7 +365,7 @@ export const browserTracingIntegration = ((_options: Partial<BrowserTracingOptio
 
     // If `finalStartSpanOptions.name` is different than `startSpanOptions.name`
     // it is because `beforeStartSpan` set a custom name. Therefore we set the source to 'custom'.
-    if (startSpanOptions.name !== finalStartSpanOptions.name) {
+    if (initialSpanName !== finalStartSpanOptions.name) {
       attributes[SEMANTIC_ATTRIBUTE_SENTRY_SOURCE] = 'custom';
       finalStartSpanOptions.attributes = attributes;
     }

packages/browser/test/tracing/browserTracingIntegration.test.ts

@@ -446,7 +446,44 @@ describe('browserTracingIntegration', () => {
     setCurrentClient(client);
     client.init();
 
-    startBrowserTracingPageLoadSpan(client, { name: 'test span' });
+    startBrowserTracingPageLoadSpan(client, {
+      name: 'test span',
+      attributes: {
+        [SEMANTIC_ATTRIBUTE_SENTRY_SOURCE]: 'url',
+      },
+    });
+
+    const pageloadSpan = getActiveSpan();
+
+    expect(spanToJSON(pageloadSpan!).description).toBe('changed');
+    expect(spanToJSON(pageloadSpan!).data[SEMANTIC_ATTRIBUTE_SENTRY_SOURCE]).toBe('custom');
+  });
+
+  it('sets source to "custom" if name is changed in-place in beforeStartSpan', () => {
+    const client = new BrowserClient(
+      getDefaultBrowserClientOptions({
+        tracesSampleRate: 0,
+        integrations: [
+          browserTracingIntegration({
+            instrumentPageLoad: false,
+            instrumentNavigation: false,
+            beforeStartSpan: opts => {
+              opts.name = 'changed';
+              return opts;
+            },
+          }),
+        ],
+      }),
+    );
+    setCurrentClient(client);
+    client.init();
+
+    startBrowserTracingPageLoadSpan(client, {
+      name: 'test span',
+      attributes: {
+        [SEMANTIC_ATTRIBUTE_SENTRY_SOURCE]: 'url',
+      },
+    });
 
     const pageloadSpan = getActiveSpan();