feat: handle synchronous errors in errorHandler (#5445)

lib/error-handler.js

@@ -65,13 +65,17 @@ function handleError (reply, error, cb) {
     return
   }
 
-  const result = func(error, reply.request, reply)
-  if (result !== undefined) {
-    if (result !== null && typeof result.then === 'function') {
-      wrapThenable(result, reply)
-    } else {
-      reply.send(result)
+  try {
+    const result = func(error, reply.request, reply)
+    if (result !== undefined) {
+      if (result !== null && typeof result.then === 'function') {
+        wrapThenable(result, reply)
+      } else {
+        reply.send(result)
+      }
     }
+  } catch (err) {
+    reply.send(err)
   }
 }
 

lib/wrapThenable.js

@@ -39,6 +39,8 @@ function wrapThenable (thenable, reply) {
     // try-catch allow to re-throw error in error handler for async handler
     try {
       reply.send(err)
+      // The following should not happen
+      /* c8 ignore next 3 */
     } catch (err) {
       reply.send(err)
     }

test/500s.test.js

@@ -188,3 +188,35 @@ test('cannot set childLoggerFactory after binding', t => {
     }
   })
 })
+
+test('catch synchronous errors', t => {
+  t.plan(3)
+
+  const fastify = Fastify()
+  t.teardown(fastify.close.bind(fastify))
+
+  fastify.setErrorHandler((_, req, reply) => {
+    throw new Error('kaboom2')
+  })
+
+  fastify.post('/', function (req, reply) {
+    reply.send(new Error('kaboom'))
+  })
+
+  fastify.inject({
+    method: 'POST',
+    url: '/',
+    headers: {
+      'Content-Type': 'application/json'
+    },
+    payload: JSON.stringify({ hello: 'world' }).substring(0, 5)
+  }, (err, res) => {
+    t.error(err)
+    t.equal(res.statusCode, 500)
+    t.same(res.json(), {
+      error: 'Internal Server Error',
+      message: 'kaboom2',
+      statusCode: 500
+    })
+  })
+})