Surface ffmpeg startup failures

src/ffmpeg.ts

@@ -3,7 +3,7 @@ import type { ChildProcessByStdio } from 'node:child_process';
 import { createRequire } from 'node:module';
 import type { Readable } from 'node:stream';
 
-import { AudioManagerConfigError } from './errors';
+import { AudioManagerConfigError, FfmpegProcessError } from './errors';
 import type { FfmpegOptions } from './types';
 
 const requireFromCurrentModule = createRequire(__filename);
@@ -11,9 +11,11 @@ const requireFromCurrentModule = createRequire(__filename);
 const DEFAULT_INPUT_ARGS = ['-hide_banner', '-loglevel', 'error', '-nostdin'] as const;
 const DEFAULT_OUTPUT_ARGS = ['-vn', '-f', 's16le', '-ar', '48000', '-ac', '2', 'pipe:1'] as const;
 const FORCE_KILL_TIMEOUT_MS = 2_000;
+const STDERR_TAIL_BYTES = 4_096;
 
 export type FfmpegProcessHandle = {
     process: ChildProcessByStdio<null, Readable, Readable>;
+    ready: Promise<void>;
     stop(): void;
 };
 
@@ -50,18 +52,65 @@ export function startFfmpeg(input: string, options: FfmpegOptions = {}): FfmpegP
         ...(options.outputArgs ?? DEFAULT_OUTPUT_ARGS),
     ];
     const childProcess = spawn(executable, args, { stdio: ['ignore', 'pipe', 'pipe'] });
+    const ready = waitForFfmpegOutput(childProcess);
 
-    childProcess.once('error', () => undefined);
     childProcess.stderr.resume();
 
     return {
         process: childProcess,
+        ready,
         stop: (): void => {
             stopProcess(childProcess);
         },
     };
 }
 
+function waitForFfmpegOutput(childProcess: ChildProcessByStdio<null, Readable, Readable>): Promise<void> {
+    let stderrTail = '';
+
+    const appendStderr = (chunk: Buffer | string): void => {
+        stderrTail = (stderrTail + String(chunk)).slice(-STDERR_TAIL_BYTES);
+    };
+
+    childProcess.stderr.on('data', appendStderr);
+
+    return new Promise((resolve, reject) => {
+        const cleanup = (): void => {
+            childProcess.off('error', onError);
+            childProcess.off('exit', onExit);
+            childProcess.stdout.off('readable', onReadable);
+        };
+
+        const fail = (message: string, cause?: unknown): void => {
+            cleanup();
+            reject(new FfmpegProcessError(addStderrTail(message, stderrTail), cause));
+        };
+
+        const onError = (error: Error): void => {
+            fail(`Unable to start ffmpeg. Cause: ${error.message}`, error);
+        };
+
+        const onExit = (code: number | null, signal: NodeJS.Signals | null): void => {
+            fail(`ffmpeg exited before producing audio. Exit code: ${code ?? 'none'}, signal: ${signal ?? 'none'}.`);
+        };
+
+        const onReadable = (): void => {
+            cleanup();
+            resolve();
+        };
+
+        childProcess.once('error', onError);
+        childProcess.once('exit', onExit);
+        childProcess.stdout.once('readable', onReadable);
+    });
+}
+
+function addStderrTail(message: string, stderrTail: string): string {
+    const trimmedTail = stderrTail.trim();
+
+    return trimmedTail ? `${message} stderr: ${trimmedTail}` : message;
+}
+
 function stopProcess(childProcess: ChildProcessByStdio<null, Readable, Readable>): void {
     childProcess.stdout.destroy();
     childProcess.stderr.destroy();