Refactor FFmpeg checker for improved reliability (#13)

These changes aim to improve the robustness and reliability of the FFmpeg binary checking process. By validating the `FFMPEG_PATH` environment variable and using `child_process.spawn` for the check, we enhance the overall stability and accuracy of the FFmpeg verification process, especially across different operating systems.

Signed-off-by: Ryuu Mitsuki <[email protected]>

lib/audioconv.js

@@ -34,7 +34,7 @@ const childProcess = require('node:child_process');
 const ffmpeg = require('fluent-ffmpeg');
 
 // Promisify the `exec` function
-const exec = promisify(childProcess.exec);
+const spawn = promisify(childProcess.exec);
 
 const { logger: log } = require('./utils');
 
@@ -150,15 +150,22 @@ function resolveOptions(options, useDefault=true) {
 async function checkFfmpeg(verbose=false) {
   verbose && log.debug('Checking `ffmpeg` binary...');
   if (process.env.FFMPEG_PATH) {
+    if ((await fs.promises.stat(process.env.FFMPEG_PATH)).isDirectory()) {
+      const msg = '[EISDIR] Please set the FFMPEG_PATH environment variable '
+        + 'to the path of the `ffmpeg` binary';
+      verbose && log.warn(msg);
+      throw new Error(msg);
+    }
+
     verbose && log.debug('`ffmpeg` installed on system');
     return true;
   }
 
   // This try-catch block to handle error,
   // in case the `ffmpeg` binary is not installed on system
   try {
-    const { stdout, stderr } = await exec('ffmpeg -version');
-    if (!stderr && stdout) {
+    const { status } = await spawn('ffmpeg -version', { shell: true });
+    if (status !== 0) {
       verbose && log.debug('`ffmpeg` installed on system');
       return true;
     }