Fixed an error that occurred when sending an empty body as a buffer

lib/contentTypeParser.js

@@ -281,7 +281,7 @@ function getDefaultJsonParser (onProtoPoisoning, onConstructorPoisoning) {
   return defaultJsonParser
 
   function defaultJsonParser (req, body, done) {
-    if (body === '' || body == null) {
+    if (body === '' || body == null || (Buffer.isBuffer(body) && body.length === 0)) {
       return done(new FST_ERR_CTP_EMPTY_JSON_BODY(), undefined)
     }
     let json

test/buffer.test.js

@@ -0,0 +1,51 @@
+const t = require('tap')
+const test = t.test
+const Fastify = require('..')
+
+test('Buffer test', async t => {
+  const fastify = Fastify()
+  fastify.addContentTypeParser('application/json', { parseAs: 'buffer' }, fastify.getDefaultJsonParser('error', 'ignore'))
+
+  fastify.delete('/', async (request) => {
+    return request.body
+  })
+
+  test('should return 200 if the body is not empty', async t => {
+    t.plan(3)
+
+    const response = await fastify.inject({
+      method: 'DELETE',
+      url: '/',
+      payload: Buffer.from('{"hello":"world"}'),
+      headers: {
+        'content-type': 'application/json'
+      }
+    })
+
+    t.error(response.error)
+    t.equal(response.statusCode, 200)
+    t.same(response.payload.toString(), '{"hello":"world"}')
+  })
+
+  test('should return 400 if the body is empty', async t => {
+    t.plan(3)
+
+    const response = await fastify.inject({
+      method: 'DELETE',
+      url: '/',
+      payload: Buffer.alloc(0),
+      headers: {
+        'content-type': 'application/json'
+      }
+    })
+
+    t.error(response.error)
+    t.equal(response.statusCode, 400)
+    t.same(JSON.parse(response.payload.toString()), {
+      error: 'Bad Request',
+      code: 'FST_ERR_CTP_EMPTY_JSON_BODY',
+      message: 'Body cannot be empty when content-type is set to \'application/json\'',
+      statusCode: 400
+    })
+  })
+})