Handle all possible cases of double listen

fastify.js

@@ -167,13 +167,29 @@ function build (options) {
     const _cb = (hasAddress) ? cb : address
     fastify.ready(function (err) {
       if (err) return _cb(err)
+      if (listening) {
+        return _cb(new Error('Fastify is already listening'))
+      }
+
+      server.on('error', wrap)
       if (hasAddress) {
-        server.listen(port, address, _cb)
+        server.listen(port, address, wrap)
       } else {
-        server.listen(port, _cb)
+        server.listen(port, wrap)
       }
       listening = true
     })
+
+    function wrap (err) {
+      server.removeListener('error', wrap)
+      if (_cb) {
+        _cb(err)
+      } else {
+        // this will crash the process
+        // it will go to 'uncaughtException'
+        throw err
+      }
+    }
   }
 
   function startHooks (req, res, params, store) {

test/listen.test.js

@@ -2,23 +2,26 @@
 
 const test = require('tap').test
 const Fastify = require('..')
-const fastify = Fastify()
 
 test('listen accepts a port and a callback', t => {
   t.plan(2)
+  const fastify = Fastify()
   fastify.listen(0, (err) => {
     fastify.server.unref()
     t.error(err)
     t.pass()
+    fastify.close()
   })
 })
 
 test('listen accepts a port, address, and callback', t => {
   t.plan(2)
+  const fastify = Fastify()
   fastify.listen(0, '127.0.0.1', (err) => {
     fastify.server.unref()
     t.error(err)
     t.pass()
+    fastify.close()
   })
 })
 
@@ -31,6 +34,7 @@ test('listen after Promise.resolve()', t => {
         f.server.unref()
         t.error(err)
         t.pass()
+        f.close()
       })
     })
 })
@@ -56,3 +60,28 @@ test('register after listen using Promise.resolve()', t => {
       })
     })
 })
+
+test('double listen errors', t => {
+  t.plan(2)
+  const fastify = Fastify()
+  fastify.listen(0, (err) => {
+    t.error(err)
+    fastify.listen(fastify.server.address().port, (err) => {
+      t.ok(err)
+      fastify.close()
+    })
+  })
+})
+
+test('listen twice on the same port', t => {
+  t.plan(2)
+  const fastify = Fastify()
+  fastify.listen(0, (err) => {
+    t.error(err)
+    const s2 = Fastify()
+    s2.listen(fastify.server.address().port, (err) => {
+      fastify.close()
+      t.ok(err)
+    })
+  })
+})

test/route.test.js

@@ -76,14 +76,9 @@ fastify.listen(0, function (err) {
       t.deepEqual(JSON.parse(body), { hello: 'world' })
     })
   })
-})
-
-test('path can be specified in place of uri', t => {
-  t.plan(3)
 
-  fastify.listen(0, function (err) {
-    if (err) t.error(err)
-    fastify.server.unref()
+  test('path can be specified in place of uri', t => {
+    t.plan(3)
 
     fastify.route({
       method: 'GET',