server-youtube: diagnostic detail + PATH fallback when bundled yt-dlp won't run

probeVersion() now captures stderr/exit-code/signal/spawn-error instead of
returning a bare boolean, and ensureYtDlp() tries the bundled binary first,
falls back to `yt-dlp(.exe)` on PATH if the bundled one won't execute (AV
block, missing libc symbol, broken download), and only then re-downloads.
The final user-facing error includes the per-attempt diagnostics so we can
actually see WHY verification failed instead of the opaque
"yt-dlp 다운로드는 됐지만 실행 검증에 실패했습니다." message.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/server/youtube.ts

@@ -40,40 +40,25 @@ export function getYtDlpInstallPath(): string {
 /** 한 번에 한 다운로드만 진행하도록 락 (서버 동시 요청 보호). */
 let installPromise: Promise<string> | null = null
 
+type ProbeResult = { ok: true } | { ok: false; detail: string }
+
 /**
  * %appdata%/.mc_custom/ 에 yt-dlp 가 준비됐는지 확인하고, 없으면 GitHub Releases 에서
  * 현재 OS/아키텍처용 바이너리를 자동으로 받아 설치한다. 성공 시 실행 경로 반환.
+ * 다운로드된 바이너리가 실행되지 않는 환경(antivirus, 권한 문제 등)이라면
+ * PATH 의 `yt-dlp(.exe)` 로 폴백하고, 그것도 실패하면 진단정보가 포함된 에러를 던진다.
  */
 export async function ensureYtDlp(): Promise<string> {
   const target = getYtDlpInstallPath()
-  // 이미 설치돼 있고 실행 가능하면 그대로 사용
-  if (await canExecute(target)) return target
+  // Fast path: 이미 설치돼 있고 실행도 잘 되면 그대로 사용
+  if (await fileExists(target)) {
+    const probe = await probeVersion(target)
+    if (probe.ok) return target
+  }
   if (installPromise) return installPromise
   installPromise = (async () => {
     try {
-      const dir = getMcCustomDir()
-      await fs.mkdir(dir, { recursive: true })
-      const asset = getYtDlpAssetName()
-      const url = `https://github.com/yt-dlp/yt-dlp/releases/latest/download/${asset}`
-      await downloadToFile(url, target)
-      // POSIX 계열은 실행 권한 부여
-      if (process.platform !== 'win32') {
-        await fs.chmod(target, 0o755)
-      }
-      // 검증
-      const okVersion = await probeVersion(target)
-      if (!okVersion) {
-        throw new YtDlpUnavailableError(t('youtube.ytdlpVerifyFailed'))
-      }
-      return target
-    } catch (err) {
-      // 실패 흔적(부분 다운로드) 삭제
-      try { await fs.unlink(target) } catch { /* noop */ }
-      throw err instanceof YtDlpUnavailableError
-        ? err
-        : new YtDlpUnavailableError(
-            t('youtube.ytdlpInstallFailed', { message: err instanceof Error ? err.message : String(err) })
-          )
+      return await prepareYtDlp(target)
     } finally {
       installPromise = null
     }
@@ -81,31 +66,85 @@ export async function ensureYtDlp(): Promise<string> {
   return installPromise
 }
 
-async function canExecute(filePath: string): Promise<boolean> {
+async function prepareYtDlp(target: string): Promise<string> {
+  const diagnostics: string[] = []
+
+  // 1. 기존 파일이 있으면 우선 그걸로 시도
+  if (await fileExists(target)) {
+    const probe = await probeVersion(target)
+    if (probe.ok) return target
+    diagnostics.push(`기존 ${path.basename(target)} 검증 실패: ${probe.detail}`)
+  }
+
+  // 2. PATH 에 yt-dlp(.exe) 가 시스템 전역으로 설치돼 있으면 그걸 사용
+  const pathCmd = process.platform === 'win32' ? 'yt-dlp.exe' : 'yt-dlp'
+  const pathProbe = await probeVersion(pathCmd)
+  if (pathProbe.ok) return pathCmd
+  diagnostics.push(`PATH 의 ${pathCmd} 사용 불가: ${pathProbe.detail}`)
+
+  // 3. 최후 수단: 새로 다운로드해서 시도
+  try {
+    await fs.mkdir(getMcCustomDir(), { recursive: true })
+    const asset = getYtDlpAssetName()
+    const url = `https://github.com/yt-dlp/yt-dlp/releases/latest/download/${asset}`
+    try { await fs.unlink(target) } catch { /* noop */ }
+    await downloadToFile(url, target)
+    if (process.platform !== 'win32') {
+      await fs.chmod(target, 0o755)
+    }
+    const probe = await probeVersion(target)
+    if (probe.ok) return target
+    diagnostics.push(`새로 받은 ${asset} 검증 실패: ${probe.detail}`)
+    try { await fs.unlink(target) } catch { /* noop */ }
+  } catch (err) {
+    diagnostics.push(`다운로드 실패: ${err instanceof Error ? err.message : String(err)}`)
+    try { await fs.unlink(target) } catch { /* noop */ }
+  }
+
+  throw new YtDlpUnavailableError(
+    t('youtube.ytdlpVerifyFailedDetail', { detail: diagnostics.join(' | ') })
+  )
+}
+
+async function fileExists(filePath: string): Promise<boolean> {
   try {
     await fs.access(filePath, fsConst.F_OK)
+    return true
   } catch {
     return false
   }
-  // POSIX 면 X 비트도 확인
-  if (process.platform !== 'win32') {
-    try {
-      await fs.access(filePath, fsConst.X_OK)
-    } catch {
-      return false
-    }
-  }
-  // 실제로 --version 으로 한 번 더 확인
-  return probeVersion(filePath)
 }
 
-function probeVersion(bin: string): Promise<boolean> {
+function probeVersion(bin: string): Promise<ProbeResult> {
   return new Promise((resolve) => {
-    const child = spawn(bin, ['--version'], { stdio: ['ignore', 'pipe', 'pipe'] })
-    let ok = false
-    child.stdout.on('data', () => { ok = true })
-    child.on('error', () => resolve(false))
-    child.on('close', (code) => resolve(ok && code === 0))
+    let child: ReturnType<typeof spawn>
+    try {
+      child = spawn(bin, ['--version'], { stdio: ['ignore', 'pipe', 'pipe'], windowsHide: true })
+    } catch (err) {
+      resolve({ ok: false, detail: `spawn throw: ${err instanceof Error ? err.message : String(err)}` })
+      return
+    }
+    let stdout = ''
+    let stderr = ''
+    child.stdout?.on('data', (chunk: Buffer) => { stdout += chunk.toString('utf8') })
+    child.stderr?.on('data', (chunk: Buffer) => { stderr += chunk.toString('utf8') })
+    child.on('error', (err: NodeJS.ErrnoException) => {
+      const code = err.code ? `${err.code} ` : ''
+      resolve({ ok: false, detail: `spawn error: ${code}${err.message}` })
+    })
+    child.on('close', (code, signal) => {
+      const out = stdout.trim()
+      if (out && code === 0) {
+        resolve({ ok: true })
+        return
+      }
+      const parts: string[] = []
+      parts.push(`exit=${code === null ? `signal:${signal}` : code}`)
+      if (!out) parts.push('stdout=(empty)')
+      const errLine = stderr.trim().split('\n')[0]
+      if (errLine) parts.push(`stderr="${errLine.slice(0, 200)}"`)
+      resolve({ ok: false, detail: parts.join(', ') })
+    })
   })
 }