fix: handle postcss load unhandled rejections (#18886)

Author: sapphi-red

packages/vite/bin/vite.js

@@ -7,6 +7,10 @@ if (!import.meta.url.includes('node_modules')) {
     // only available as dev dependency
     await import('source-map-support').then((r) => r.default.install())
   } catch {}
+
+  process.on('unhandledRejection', (err) => {
+    throw new Error('UNHANDLED PROMISE REJECTION', { cause: err })
+  })
 }
 
 global.__vite_start_time = performance.now()

packages/vite/src/node/plugins/css.ts

@@ -307,7 +307,9 @@ export function cssPlugin(config: ResolvedConfig): Plugin {
 
   // warm up cache for resolved postcss config
   if (config.css?.transformer !== 'lightningcss') {
-    resolvePostcssConfig(config)
+    resolvePostcssConfig(config).catch(() => {
+      /* will be handled later */
+    })
   }
 
   return {
@@ -1696,9 +1698,14 @@ async function resolvePostcssConfig(
       return null
     })
     // replace cached promise to result object when finished
-    result.then((resolved) => {
-      postcssConfigCache.set(config, resolved)
-    })
+    result.then(
+      (resolved) => {
+        postcssConfigCache.set(config, resolved)
+      },
+      () => {
+        /* keep as rejected promise, will be handled later */
+      },
+    )
   }
 
   postcssConfigCache.set(config, result)