fix: correctly handle multi-value headers in fetch when rawHeaders contains array

lib/web/fetch/index.js

@@ -2110,7 +2110,15 @@ async function httpNetworkFetch (
           const headersList = new HeadersList()
 
           for (let i = 0; i < rawHeaders.length; i += 2) {
-            headersList.append(bufferToLowerCasedHeaderName(rawHeaders[i]), rawHeaders[i + 1].toString('latin1'), true)
+            const nameStr = bufferToLowerCasedHeaderName(rawHeaders[i])
+            const value = rawHeaders[i + 1]
+            if (Array.isArray(value) && !Buffer.isBuffer(rawHeaders[i + 1])) {
+              for (const val of value) {
+                headersList.append(nameStr, val.toString('latin1'), true)
+              }
+            } else {
+              headersList.append(nameStr, value.toString('latin1'), true)
+            }
           }
           const contentEncoding = headersList.get('content-encoding', true)
           if (contentEncoding) {

test/issue-4389.js

@@ -0,0 +1,45 @@
+const { describe, test } = require('node:test')
+const assert = require('node:assert')
+const { once } = require('node:events')
+const { createServer: createHttpServer } = require('node:http')
+const { createServer: createSocketServer } = require('node:net')
+const { fetch, Agent } = require('..')
+
+describe('Interceptor should not break multi-value headers', () => {
+  test('Interceptor should not break Set-Cookie', async (t) => {
+    const server = createHttpServer({ joinDuplicateHeaders: true }, async (_req, res) => {
+      const headers = [
+        ['set-cookie', 'a=1'],
+        ['set-cookie', 'b=2'],
+        ['set-cookie', 'c=3']
+      ]
+      res.writeHead(200, headers)
+      res.end()
+    }).listen(0)
+
+    await once(server, 'listening')
+    t.after(() => server.close())
+
+    const dispatcher = new Agent().compose((dispatch) => dispatch)
+
+    const { headers } = await fetch(`http://localhost:${server.address().port}`, { dispatcher })
+    assert.deepStrictEqual(headers.getSetCookie(), ['a=1', 'b=2', 'c=3'])
+  })
+
+  test('Interceptor should not break other multi-value header', async (t) => {
+    const server = createSocketServer((socket) => {
+      socket.write('HTTP/1.0 204 No Content\r\n')
+      socket.write('X-Test-Header: 1\r\n')
+      socket.write('X-Test-Header: 2\r\n')
+      socket.write('X-Test-Header: 3\r\n')
+      socket.end('\r\n\r\n')
+    }).listen(0)
+    t.after(() => server.close())
+    await once(server, 'listening')
+
+    const dispatcher = new Agent().compose((dispatch) => dispatch)
+
+    const { headers } = await fetch(`http://localhost:${server.address().port}`, { dispatcher })
+    assert.deepStrictEqual(headers.get('x-test-header'), '1, 2, 3')
+  })
+})