fix: Deal with URL base

Author: AbhiPrasad

packages/core/src/fetch.ts

@@ -1,3 +1,4 @@
+/* eslint-disable complexity */
 import { getClient } from './currentScopes';
 import { SEMANTIC_ATTRIBUTE_SENTRY_OP, SEMANTIC_ATTRIBUTE_SENTRY_ORIGIN } from './semanticAttributes';
 import { SPAN_STATUS_ERROR, setHttpStatus, startInactiveSpan } from './tracing';
@@ -53,7 +54,20 @@ export function instrumentFetchRequest(
     return undefined;
   }
 
-  const parsedUrl = parseStringToURL(url);
+  // Curious about `thismessage:/`? See: https://www.rfc-editor.org/rfc/rfc2557.html
+  //  > When the methods above do not yield an absolute URI, a base URL
+  //  > of "thismessage:/" MUST be employed. This base URL has been
+  //  > defined for the sole purpose of resolving relative references
+  //  > within a multipart/related structure when no other base URI is
+  //  > specified.
+  //
+  // We need to provide a base URL to `parseStringToURL` because the fetch API gives us a
+  // relative URL sometimes.
+  //
+  // This is the only case where we need to provide a base URL to `parseStringToURL`
+  // because the relative URL is not valid on its own.
+  const parsedUrl = url.startsWith('/') ? parseStringToURL(url, 'thismessage:/') : parseStringToURL(url);
+  const fullUrl = url.startsWith('/') ? undefined : parsedUrl?.href;
 
   const hasParent = !!getActiveSpan();
 
@@ -65,10 +79,11 @@ export function instrumentFetchRequest(
             url,
             type: 'fetch',
             'http.method': method,
-            'http.url': parsedUrl?.href || url,
+            'http.url': parsedUrl?.href,
             [SEMANTIC_ATTRIBUTE_SENTRY_ORIGIN]: spanOrigin,
             [SEMANTIC_ATTRIBUTE_SENTRY_OP]: 'http.client',
-            ...(parsedUrl?.host && { 'server.address': parsedUrl.host }),
+            ...(fullUrl && { 'http.url': fullUrl }),
+            ...(fullUrl && parsedUrl?.host && { 'server.address': parsedUrl.host }),
             ...(parsedUrl?.search && { 'http.query': parsedUrl.search }),
             ...(parsedUrl?.hash && { 'http.fragment': parsedUrl.hash }),
           },

packages/core/src/utils-hoist/url.ts

@@ -8,7 +8,7 @@ type PartialURL = {
 };
 
 interface URLwithCanParse extends URL {
-  canParse: (url: string) => boolean;
+  canParse: (url: string, base?: string | URL | undefined) => boolean;
 }
 
 /**
@@ -17,16 +17,16 @@ interface URLwithCanParse extends URL {
  * @param url - The URL to parse
  * @returns The parsed URL object or undefined if the URL is invalid
  */
-export function parseStringToURL(url: string): URL | undefined {
+export function parseStringToURL(url: string, base?: string | URL | undefined): URL | undefined {
   try {
+    // Use `canParse` to short-circuit the URL constructor if it's not a valid URL
+    // This is faster than trying to construct the URL and catching the error
     // Node 20+, Chrome 120+, Firefox 115+, Safari 17+
-    if ('canParse' in URL) {
-      // Use `canParse` to short-circuit the URL constructor if it's not a valid URL
-      // This is faster than trying to construct the URL and catching the error
-      return (URL as unknown as URLwithCanParse).canParse(url) ? new URL(url) : undefined;
-    } else {
-      return new URL(url);
+    if ('canParse' in URL && !(URL as unknown as URLwithCanParse).canParse(url, base)) {
+      return undefined;
     }
+
+    return new URL(url, base);
   } catch {
     // empty body
   }