Fix content length (fastify#2546)

* fix(reply): add content-length to supress error.headers['content-length']

* tests: add tests when content-length is wrong at errorHandler

* fix(reply): add content-length always on onSendEnd

* fix(reply): add undefined to content-length before errorHandler

lib/reply.js

@@ -521,6 +521,7 @@ function handleError (reply, error, cb) {
     reply[kReplySent] = false
     reply[kReplyIsError] = false
     reply[kReplyErrorHandlerCalled] = true
+    reply[kReplyHeaders]['content-length'] = undefined
     var result = errorHandler(error, reply.request, reply)
     if (result && typeof result.then === 'function') {
       wrapThenable(result, reply)

test/content-length.test.js

@@ -160,3 +160,30 @@ test('#2214 - wrong content-length', t => {
       t.end()
     })
 })
+
+test('#2543 - wrong content-length with errorHandler', t => {
+  const fastify = Fastify()
+
+  fastify.setErrorHandler((_error, _request, reply) => {
+    reply.code(500).send({ message: 'longer than 2 bytes' })
+  })
+
+  fastify.get('/', async () => {
+    const error = new Error('MY_ERROR_MESSAGE')
+    error.headers = {
+      'content-length': 2
+    }
+    throw error
+  })
+
+  fastify.inject({
+    method: 'GET',
+    path: '/'
+  })
+    .then(res => {
+      t.strictEqual(res.statusCode, 500)
+      t.strictEqual(res.headers['content-length'], '' + res.rawPayload.length)
+      t.deepEqual(JSON.parse(res.payload), { message: 'longer than 2 bytes' })
+      t.end()
+    })
+})