fix: pass logger instances via loggerInstance (fastify v5 rejects instances in logger)

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) {

test/data/custom-logger-instance.js

@@ -0,0 +1,5 @@
+'use strict'
+
+const pino = require('pino')
+
+module.exports = pino({ level: 'warn' })

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)