fix(mergeConfig): don't recreate server.hmr.server instance (#17763)

Author: Vija02

packages/vite/src/node/__tests__/config.spec.ts

@@ -1,3 +1,4 @@
+import http from 'node:http'
 import { describe, expect, test } from 'vitest'
 import type { InlineConfig } from '..'
 import type { PluginOption, UserConfig, UserConfigExport } from '../config'
@@ -185,6 +186,18 @@ describe('mergeConfig', () => {
     expect(mergeConfig(newConfig, baseConfig)).toEqual(mergedConfig)
   })
 
+  test('handles server.hmr.server', () => {
+    const httpServer = http.createServer()
+
+    const baseConfig = { server: { hmr: { server: httpServer } } }
+    const newConfig = { server: { hmr: { server: httpServer } } }
+
+    const mergedConfig = mergeConfig(baseConfig, newConfig)
+
+    // Server instance should not be recreated
+    expect(mergedConfig.server.hmr.server).toBe(httpServer)
+  })
+
   test('throws error with functions', () => {
     const baseConfig = defineConfig(() => ({ base: 'base' }))
     const newConfig = defineConfig(() => ({ base: 'new' }))

packages/vite/src/node/utils.ts

@@ -1110,6 +1110,9 @@ function mergeConfigRecursively(
         ...backwardCompatibleWorkerPlugins(value),
       ]
       continue
+    } else if (key === 'server' && rootPath === 'server.hmr') {
+      merged[key] = value
+      continue
     }
 
     if (Array.isArray(existing) || Array.isArray(value)) {