From 53f63298e8ed683b51266a029c220d88c26e7e19 Mon Sep 17 00:00:00 2001
From: kilisamemarisaaa <1798456934@qq.com>
Date: Fri, 28 Aug 2026 18:17:55 +0800
Subject: [PATCH] fix: pass logger instances via loggerInstance for fastify v5

fastify v5 throws FST_ERR_LOG_INVALID_LOGGER_CONFIG when a logger
instance is passed through the `logger` option (it only accepts a
configuration object there). fastify-cli converges user-provided logger
instances -- from --logging-module, --options files or server options --
into options.logger, so --logging-module with a real Pino instance
crashed at startup.

Route instances to loggerInstance after all option merges, and add a
regression test with a real Pino instance fixture: the existing tests
only used a config-object fixture, so the instance path had zero
coverage.

Co-Authored-By: EvoX <evox@evomap.ai>
---
 start.js                            |  7 +++++++
 test/data/custom-logger-instance.js |  5 +++++
 test/start.test.js                  | 11 +++++++++++
 3 files changed, 23 insertions(+)
 create mode 100644 test/data/custom-logger-instance.js

diff --git a/start.js b/start.js
index ef0d614c..b2ddb479 100755
--- a/start.js
+++ b/start.js
@@ -171,6 +171,13 @@ async function runFastify (args, additionalOptions, serverOptions, serverModule)
     options.trustProxy = opts.trustProxy
   }
 
+  // fastify v5 only accepts a configuration object in `logger`;
+  // logger instances must be passed via `loggerInstance`
+  if (options.logger && typeof options.logger.child === 'function') {
+    options.loggerInstance = options.logger
+    delete options.logger
+  }
+
   const fastify = Fastify(options)
 
   if (opts.prefix) {
diff --git a/test/data/custom-logger-instance.js b/test/data/custom-logger-instance.js
new file mode 100644
index 00000000..0f54c2bd
--- /dev/null
+++ b/test/data/custom-logger-instance.js
@@ -0,0 +1,5 @@
+'use strict'
+
+const pino = require('pino')
+
+module.exports = pino({ level: 'warn' })
diff --git a/test/start.test.js b/test/start.test.js
index 35300171..b50a5c77 100644
--- a/test/start.test.js
+++ b/test/start.test.js
@@ -981,6 +981,17 @@ test('should support custom logger configuration in ESM', async t => {
   t.pass('server closed')
 })
 
+test('should support a logger instance from a logging module', async t => {
+  t.plan(2)
+
+  const argv = ['-L', './test/data/custom-logger-instance.js', './examples/plugin.js']
+  const fastify = await start.start(argv)
+  t.equal(fastify.log.level, 'warn')
+
+  await fastify.close()
+  t.pass('server closed')
+})
+
 test('preloading a built-in module works', async t => {
   t.plan(1)