Fail clearly on empty downloads; friendlier create errors (#43)

src/main/core/net.ts

@@ -99,6 +99,15 @@ export async function downloadFile(url: string, dest: string, opts: DownloadOpts
   const source = Readable.fromWeb(r.body as Parameters<typeof Readable.fromWeb>[0])
   await pipeline(source, tap, createWriteStream(dest))
 
+  // A 0-byte body (dead mirror, 404-that-redirects-to-nothing, changed API shape)
+  // would otherwise be hashed to the well-known SHA-256 of empty input
+  // (e3b0c442…) and reported as a baffling "checksum mismatch". Fail honestly and
+  // early instead — this guards every provider, not just the one that regressed.
+  if (received === 0) {
+    await rm(dest, { force: true })
+    throw new Error(`empty-download: ${url} returned 0 bytes`)
+  }
+
   if (hash) {
     const digest = hash.digest('hex').toLowerCase()
     const expected = (opts.sha256 ?? opts.sha1 ?? '').toLowerCase()

src/main/smoke.ts

@@ -16,6 +16,8 @@ import type { Product } from '@shared/web'
 import { getProvider } from './core/versions'
 import { createServer } from './core/createServer'
 import { pickForgeRunJar } from './core/serverDetect'
+import { downloadFile } from './core/net'
+import { createServer as httpCreateServer } from 'node:http'
 import { removeServer } from './core/serverRegistry'
 import * as sf from './core/serverFiles'
 import * as playersMod from './core/players'
@@ -2675,6 +2677,42 @@ export async function runWizardSmoke(): Promise<void> {
     console.log('WIZARD-SMOKE: forge run-jar fallback OK (universal/loader/none, installer excluded)')
   }
 
+  // 4. net.ts empty-download guard: a reachable 200 with a 0-byte body must fail
+  //    as `empty-download`, not as a baffling checksum mismatch against the hash
+  //    of empty input (the confusing Mohist symptom in #43). #43
+  {
+    const srv = httpCreateServer((_req, res) => {
+      res.writeHead(200, { 'Content-Type': 'application/java-archive' })
+      res.end() // zero bytes, chunked (no content-length) so fetch still yields a body
+    })
+    await new Promise<void>((r) => srv.listen(0, '127.0.0.1', () => r()))
+    const addr = srv.address()
+    const port = typeof addr === 'object' && addr ? addr.port : 0
+    const dest = join(app.getPath('temp'), 'msms-empty-dl-test.jar')
+    let msg = ''
+    try {
+      await downloadFile(`http://127.0.0.1:${port}/x.jar`, dest, {
+        sha256: '5ad74546004d0e5b9a5b0f6f8e2b1c3d4e5f60718293a4b5c6d7e8f9011223344'
+      })
+    } catch (e) {
+      msg = String((e as Error)?.message ?? e)
+    }
+    srv.close()
+    try {
+      rmSync(dest, { force: true })
+    } catch {
+      /* ignore */
+    }
+    if (!msg.includes('empty-download')) {
+      return fail('0-byte download should throw empty-download, got: ' + (msg || '(no error)'))
+    }
+    if (/checksum mismatch/i.test(msg)) {
+      return fail('0-byte download surfaced as a checksum mismatch instead of empty-download')
+    }
+    if (existsSync(dest)) return fail('empty-download must not leave a partial file behind')
+    console.log('WIZARD-SMOKE: empty-download guard OK (0-byte body fails clearly, no checksum confusion, dest removed)')
+  }
+
   console.log('WIZARD-SMOKE: PASS')
   app.exit(0)
 }

src/renderer/src/locales/en.ts

@@ -325,7 +325,11 @@ export default {
     progressConfiguring: 'Configuring…',
     progressDone: 'Done!',
     createdOk: 'Server "{{name}}" created',
-    createFailed: 'Creation failed: {{error}}'
+    createFailed: 'Creation failed: {{error}}',
+    errNoBuild: 'No build is available for this Minecraft version yet — pick another version.',
+    errEmptyDownload: 'The download server returned an empty file. That build may be temporarily unavailable — try another build or version.',
+    errFolderExists: 'A server folder with that name already exists — choose a different name.',
+    errNoLauncher: 'The installer finished but produced no launchable server. This loader/version combination may be unsupported.'
   },
   players: {
     title: 'Players',

src/renderer/src/locales/tr.ts

@@ -328,7 +328,11 @@ const tr: typeof en = {
     progressConfiguring: 'Yapılandırılıyor…',
     progressDone: 'Tamamlandı!',
     createdOk: '"{{name}}" sunucusu oluşturuldu',
-    createFailed: 'Oluşturma başarısız: {{error}}'
+    createFailed: 'Oluşturma başarısız: {{error}}',
+    errNoBuild: 'Bu Minecraft sürümü için henüz bir yapı (build) yok — başka bir sürüm seçin.',
+    errEmptyDownload: 'İndirme sunucusu boş bir dosya döndürdü. O yapı geçici olarak erişilemez olabilir — başka bir yapı veya sürüm deneyin.',
+    errFolderExists: 'Bu adda bir sunucu klasörü zaten var — farklı bir ad seçin.',
+    errNoLauncher: 'Yükleyici tamamlandı ama çalıştırılabilir bir sunucu üretmedi. Bu yükleyici/sürüm birleşimi desteklenmiyor olabilir.'
   },
   players: {
     title: 'Oyuncular',

src/renderer/src/views/CreateView.tsx

@@ -1,5 +1,6 @@
 import { useEffect, useMemo, useState } from 'react'
 import { useTranslation } from 'react-i18next'
+import type { TFunction } from 'i18next'
 import { Sparkles, Download, Loader2, ExternalLink, ChevronRight, Check } from 'lucide-react'
 import { useStore } from '../store'
 import { CREATABLE_TYPES, HAS_BUILDS } from '@shared/versions'
@@ -9,6 +10,22 @@ import type { JavaPreset, ServerType } from '@shared/types'
 const PROXY_TYPES: ServerType[] = ['velocity', 'waterfall', 'bungeecord']
 const PRESETS: JavaPreset[] = ['aikars', 'aikars-large', 'basic']
 
+/**
+ * Turn a raw createServer error code into a human message. The backend surfaces
+ * short codes (e.g. `no-mohist-build` when the upstream API lists a version but
+ * has no build for it, or `empty-download` when a mirror serves a 0-byte body);
+ * shown verbatim these read as gibberish to a non-technical user. Unknown codes
+ * fall through unchanged.
+ */
+function friendlyCreateError(error: string | undefined, t: TFunction): string {
+  const e = error ?? '?'
+  if (/^no-[a-z]+-build$/.test(e)) return t('wizard.errNoBuild')
+  if (e.startsWith('empty-download')) return t('wizard.errEmptyDownload')
+  if (e === 'folder-exists') return t('wizard.errFolderExists')
+  if (e === 'installer-args-not-found') return t('wizard.errNoLauncher')
+  return e
+}
+
 export function CreateView(): JSX.Element {
   const { t } = useTranslation()
   const config = useStore((s) => s.config)
@@ -111,7 +128,7 @@ export function CreateView(): JSX.Element {
       await selectServer(res.server.id)
       setView('console')
     } else {
-      toast('error', 'wizard.createFailed', { error: res.error ?? '?' })
+      toast('error', 'wizard.createFailed', { error: friendlyCreateError(res.error, t) })
       setProgress({ stage: 'error', message: res.error })
     }
   }
@@ -130,7 +147,7 @@ export function CreateView(): JSX.Element {
       case 'done':
         return t('wizard.progressDone')
       case 'error':
-        return t('wizard.createFailed', { error: progress.message ?? '?' })
+        return t('wizard.createFailed', { error: friendlyCreateError(progress.message, t) })
     }
   }, [progress, t])