fix: default clientError replies on reused connection (#4101)

fastify.js

@@ -621,7 +621,7 @@ function fastify (options) {
     // https://github.com/nodejs/node/blob/6ca23d7846cb47e84fd344543e394e50938540be/lib/_http_server.js#L666
 
     // If the socket is not writable, there is no reason to try to send data.
-    if (socket.writable && socket.bytesWritten === 0) {
+    if (socket.writable) {
       socket.write(`HTTP/1.1 400 Bad Request\r\nContent-Length: ${body.length}\r\nContent-Type: application/json\r\n\r\n${body}`)
     }
     socket.destroy(err)

test/request-error.test.js

@@ -150,7 +150,7 @@ test('default clientError handler ignores sockets in destroyed state', t => {
 })
 
 test('default clientError handler destroys sockets in writable state', t => {
-  t.plan(1)
+  t.plan(2)
 
   const fastify = Fastify({
     bodyLimit: 1,
@@ -166,6 +166,9 @@ test('default clientError handler destroys sockets in writable state', t => {
     },
     destroy () {
       t.pass('destroy should be called')
+    },
+    write (response) {
+      t.match(response, /^HTTP\/1.1 400 Bad Request/)
     }
   })
 })
@@ -186,6 +189,9 @@ test('default clientError handler destroys http sockets in non-writable state',
     },
     destroy () {
       t.pass('destroy should be called')
+    },
+    write (response) {
+      t.fail('write should not be called')
     }
   })
 })
@@ -270,3 +276,40 @@ test('encapsulated error handler binding', t => {
     t.equal(fastify.hello, undefined)
   })
 })
+
+test('default clientError replies with bad request on reused keep-alive connection', t => {
+  t.plan(2)
+
+  let response = ''
+
+  const fastify = Fastify({
+    bodyLimit: 1,
+    keepAliveTimeout: 100
+  })
+
+  fastify.get('/', (request, reply) => {
+    reply.send('OK\n')
+  })
+
+  fastify.listen({ port: 0 }, function (err) {
+    t.error(err)
+    fastify.server.unref()
+
+    const client = connect(fastify.server.address().port)
+
+    client.on('data', chunk => {
+      response += chunk.toString('utf-8')
+    })
+
+    client.on('end', () => {
+      t.match(response, /^HTTP\/1.1 200 OK.*HTTP\/1.1 400 Bad Request/s)
+    })
+
+    client.resume()
+    client.write('GET / HTTP/1.1\r\n')
+    client.write('\r\n\r\n')
+    client.write('GET /?a b HTTP/1.1\r\n')
+    client.write('Connection: close\r\n')
+    client.write('\r\n\r\n')
+  })
+})