fix(dns): preserve header shape for [string,string][] and ensure Host

lib/interceptor/dns.js

@@ -5,6 +5,17 @@ const DecoratorHandler = require('../handler/decorator-handler')
 const { InvalidArgumentError, InformationalError } = require('../core/errors')
 const maxInt = Math.pow(2, 31) - 1
 
+function addHostHeader (headers = {}, host) {
+  if (Array.isArray(headers)) {
+    const header = ['host', host]
+    if (Array.isArray(headers[0])) {
+      return [header, ...headers]
+    }
+    return [...header, ...headers]
+  }
+  return { host, ...headers }
+}
+
 class DNSInstance {
   #maxTTL = 0
   #maxItems = 0
@@ -411,10 +422,7 @@ module.exports = interceptorOpts => {
           ...origDispatchOpts,
           servername: origin.hostname, // For SNI on TLS
           origin: newOrigin.origin,
-          headers: {
-            host: origin.host,
-            ...origDispatchOpts.headers
-          }
+          headers: addHostHeader(origDispatchOpts.headers, origin.host)
         }
 
         dispatch(

test/interceptors/dns.js

@@ -1936,3 +1936,59 @@ test('#3951 - Should handle lookup errors correctly', async t => {
     origin: 'http://localhost'
   }), new Error('lookup error'))
 })
+
+test('Various (parameterized) header shapes should work with DNS interceptor', async t => {
+  const server = createServer({ joinDuplicateHeaders: true })
+  server.on('request', (req, res) => {
+    t.equal(req.headers.foo, 'bar')
+    t.equal(typeof req.headers['0'], 'undefined')
+    t.match(req.headers.host, /^localhost:\d+$/)
+    res.end('ok')
+  })
+
+  server.listen(0)
+  await once(server, 'listening')
+
+  const origin = `http://localhost:${server.address().port}`
+
+  const { cache: cacheInterceptor, dns: dnsInterceptor } = interceptors
+
+  // will expand
+  const cases = [
+    {
+      name: 'array of pairs',
+      headers: [['foo', 'bar']],
+      interceptors: [cacheInterceptor(), dnsInterceptor()]
+    },
+    {
+      name: 'record',
+      headers: { foo: 'bar' },
+      interceptors: [cacheInterceptor(), dnsInterceptor()]
+    },
+    {
+      name: 'flat array',
+      headers: ['foo', 'bar'],
+      // note: cacheInterceptor cannot accept flat arrays as it calls normalizeHeaders
+      // possibly need to fix
+      interceptors: [dnsInterceptor()]
+    },
+    {
+    //   name: 'record with multi-value',
+      headers: { foo: ['bar'] },
+      interceptors: [cacheInterceptor(), dnsInterceptor()]
+    }
+  ]
+
+  t = tspl(t, { plan: cases.length * 4 })
+
+  for (const c of cases) {
+    const agent = new Agent().compose(c.interceptors)
+    const r = await agent.request({ origin, path: '/', method: 'GET', headers: c.headers })
+    t.equal(r.statusCode, 200, c.name)
+    await r.body.text()
+    await agent.close()
+  }
+
+  server.close()
+  await once(server, 'close')
+})