fix: Make headers of cloned requests independent

integration-tests/js-compute/fixtures/app/src/request-clone.js

@@ -46,6 +46,36 @@ routes.set('/request/clone/valid', async () => {
   assert(newRequest.bodyUsed, false, 'newRequest.bodyUsed');
   assert(newRequest.body, null, 'newRequest.body');
 });
+routes.set('/request/clone/headers-are-independent', () => {
+  const request = new Request('https://www.fastly.com', {
+    headers: { 'x-foo': 'original' },
+    method: 'get',
+  });
+  const cloned = request.clone();
+
+  // Mutating the clone's headers must not affect the original
+  cloned.headers.set('x-foo', 'mutated');
+  assert(
+    request.headers.get('x-foo'),
+    'original',
+    'original header unchanged after mutating clone',
+  );
+
+  // Mutating the original's headers must not affect the clone
+  request.headers.set('x-bar', 'added');
+  assert(
+    cloned.headers.get('x-bar'),
+    null,
+    'clone does not see header added to original',
+  );
+
+  // Clone must carry the headers that existed at clone time
+  assert(
+    cloned.headers.get('x-foo'),
+    'mutated',
+    'clone has header value set on it',
+  );
+});
 routes.set('/request/clone/invalid', async () => {
   const request = new Request('https://www.fastly.com', {
     headers: {

integration-tests/js-compute/fixtures/app/tests.json

@@ -1222,6 +1222,7 @@
   "GET /request/clone/called-as-constructor": {},
   "GET /request/clone/called-unbound": {},
   "GET /request/clone/valid": {},
+  "GET /request/clone/headers-are-independent": {},
   "GET /request/clone/invalid": {},
   "POST /request/body-async-simple/no-workaround": {
     "environments": ["viceroy", "compute"],