perf(fetch): avoid redundant request state in the Request constructor

lib/web/fetch/request.js

@@ -153,7 +153,9 @@ class Request {
       }
 
       // 4. Set request to a new request whose URL is parsedURL.
-      request = makeRequest({ urlList: [parsedURL] })
+      // Note: Step 12 copies this request into a new one and makeRequest fills
+      // every missing field with its default, so the URL list is all it needs.
+      request = { urlList: [parsedURL] }
 
       // 5. Set fallbackMode to "cors".
       fallbackMode = 'cors'
@@ -486,29 +488,23 @@ class Request {
     }
 
     // 32. If init is not empty, then:
-    if (initHasKey) {
-      /** @type {HeadersList} */
-      const headersList = getHeadersList(this.#headers)
+    // Note: Without init["headers"], these steps empty the header list and
+    // append a copy of it back, which leaves it unchanged, so they are skipped.
+    // init["headers"] is never a Headers object here: the HeadersInit
+    // converter turns one into a list of its entries, which fill() appends.
+    if (initHasKey && init.headers !== undefined) {
       // 1. Let headers be a copy of this’s headers and its associated header
       // list.
       // 2. If init["headers"] exists, then set headers to init["headers"].
-      const headers = init.headers !== undefined ? init.headers : new HeadersList(headersList)
+      const headers = init.headers
 
       // 3. Empty this’s headers’s header list.
-      headersList.clear()
+      getHeadersList(this.#headers).clear()
 
       // 4. If headers is a Headers object, then for each header in its header
       // list, append header’s name/header’s value to this’s headers.
-      if (headers instanceof HeadersList) {
-        for (const { name, value } of headers.rawValues()) {
-          headersList.append(name, value, false)
-        }
-        // Note: Copy the `set-cookie` meta-data.
-        headersList.cookies = headers.cookies
-      } else {
-        // 5. Otherwise, fill this’s headers with headers.
-        fillHeaders(this.#headers, headers)
-      }
+      // 5. Otherwise, fill this’s headers with headers.
+      fillHeaders(this.#headers, headers)
     }
 
     // 33. Let inputBody be input’s request’s body if input is a Request

test/fetch/request.js

@@ -431,6 +431,31 @@ test('Clone the set-cookie header when Request is passed as the first parameter
   t.assert.strictEqual(request2.headers.getSetCookie().join(', '), request2.headers.get('set-cookie'))
 })
 
+test('Request with a Request input and an init without headers keeps a copy of the headers', (t) => {
+  const request = new Request('http://localhost', {
+    headers: [['Set-Cookie', 'a=1'], ['X-B', '2'], ['set-cookie', 'c=3'], ['x-a', '1']]
+  })
+  const request2 = new Request(request, { method: 'PUT' })
+
+  t.assert.strictEqual(request2.method, 'PUT')
+  t.assert.deepStrictEqual([...request2.headers], [...request.headers])
+  t.assert.deepStrictEqual(request2.headers.getSetCookie(), ['a=1', 'c=3'])
+
+  request2.headers.append('set-cookie', 'd=4')
+  request.headers.append('x-c', '3')
+  t.assert.deepStrictEqual(request.headers.getSetCookie(), ['a=1', 'c=3'])
+  t.assert.deepStrictEqual(request2.headers.getSetCookie(), ['a=1', 'c=3', 'd=4'])
+  t.assert.strictEqual(request2.headers.get('x-c'), null)
+})
+
+test('Request with a Headers object in init keeps all of its entries', (t) => {
+  const headers = new Headers([['X-A', '1'], ['set-cookie', 'a=1'], ['set-cookie', 'b=2']])
+  const request = new Request('http://localhost', { headers: { 'x-old': '0' } })
+  const request2 = new Request(request, { headers })
+
+  t.assert.deepStrictEqual([...request2.headers], [['set-cookie', 'a=1'], ['set-cookie', 'b=2'], ['x-a', '1']])
+})
+
 // Tests for optimization introduced in https://github.com/nodejs/undici/pull/2456
 test('keys to object prototypes method', (t) => {
   const request = new Request('http://localhost', { method: 'hasOwnProperty' })