feat(audio): implement ffmpeg-based audio operations backend

Adds AudioOperations.js with pure argument builders (convert/trim/extract/merge)
plus one executeOperation that spawns ffmpeg via a dependency-injected execFileFn,
so tests never invoke a real binary. Wires process-audio-operation in main.js and
updates preload.js's ALLOWED_SEND_CHANNELS to replace the 5 stale audio-* entries.

Amit Haridas

src/main.js

@@ -6,6 +6,7 @@ const { execFile } = require('child_process');
 const WordTemplateExporter = require('./wordTemplateExporter');
 const PDFOperations = require('./main/PDFOperations');
 const ImageOperations = require('./main/ImageOperations');
+const AudioOperations = require('./main/AudioOperations');
 const GitOperations = require('./main/GitOperations');
 const PdfFontHeader = require('./main/PdfFontHeader');
 const MonospaceFontConfig = require('./main/MonospaceFontConfig');
@@ -4636,6 +4637,20 @@ ipcMain.handle('process-image-operation', async (event, { operation, data }) =>
   }
 });
 
+// ========================================
+// AUDIO OPERATIONS — delegates to main/AudioOperations.js
+// ========================================
+
+ipcMain.handle('process-audio-operation', async (event, { operation, data }) => {
+  try {
+    return await AudioOperations.executeOperation(operation, data, {
+      ffmpegPath: getFFmpegPath(),
+    });
+  } catch (error) {
+    return { success: false, error: sanitizeErrorMessage(error.message) };
+  }
+});
+
 // IPC Handler for folder selection (for batch image operations)
 ipcMain.on('select-image-folder', (event, inputId) => {
   const folder = dialog.showOpenDialogSync(mainWindow, {

src/main/AudioOperations.js

@@ -0,0 +1,171 @@
+/**
+ * Audio Operations Module
+ *
+ * Handles audio manipulation via `ffmpeg`: format conversion, trim, extract (audio
+ * track from video/audio), and merge (concat demuxer). Because ffmpeg is an external
+ * binary, this module is split into pure/testable argument-builder functions and a
+ * single `executeOperation` that is the only piece which actually spawns ffmpeg —
+ * the ffmpeg binary path and the `execFile` implementation are both injected so tests
+ * can replace them with fakes, without invoking a real binary.
+ *
+ * @module AudioOperations
+ */
+
+const fs = require('fs');
+const os = require('os');
+const path = require('path');
+const { execFile } = require('child_process');
+
+/**
+ * Build args for a straight format conversion. ffmpeg infers the output format from
+ * outputPath's extension; only pass -f explicitly when format is given and differs
+ * from that extension.
+ */
+function buildConvertArgs({ inputPath, outputPath, format }) {
+  const args = ['-i', inputPath, '-y'];
+
+  if (format) {
+    const ext = path.extname(outputPath).replace(/^\./, '').toLowerCase();
+    if (format.toLowerCase() !== ext) {
+      args.push('-f', format);
+    }
+  }
+
+  args.push(outputPath);
+  return args;
+}
+
+/**
+ * Build args to trim inputPath to [startTime, startTime + duration) seconds.
+ * startTime/duration must be finite, non-negative numbers — they become argv
+ * elements passed straight to execFile with no shell involved, so there's no
+ * injection risk, but malformed values should still fail fast rather than reach
+ * ffmpeg with garbage.
+ */
+function buildTrimArgs({ inputPath, outputPath, startTime, duration }) {
+  const isValid = (n) => typeof n === 'number' && Number.isFinite(n) && n >= 0;
+
+  if (!isValid(startTime) || !isValid(duration)) {
+    throw new Error('Invalid trim range');
+  }
+
+  return ['-i', inputPath, '-ss', String(startTime), '-t', String(duration), '-y', outputPath];
+}
+
+/**
+ * Build args to extract the audio track (stream copy, no video, no re-encode).
+ * If codec copy fails, executeOperation's 'extract' case retries without -acodec copy.
+ */
+function buildExtractArgs({ inputPath, outputPath }) {
+  return ['-i', inputPath, '-vn', '-acodec', 'copy', '-y', outputPath];
+}
+
+/**
+ * Fallback extract args that let ffmpeg transcode instead of stream-copying.
+ */
+function buildExtractFallbackArgs({ inputPath, outputPath }) {
+  return ['-i', inputPath, '-vn', '-y', outputPath];
+}
+
+/**
+ * Build args to merge 2+ files via the concat demuxer. Returns both concatListContent
+ * (the `file '<path>'` lines the caller writes to a temp list file) and args — the
+ * caller doesn't know the temp list file's path until it creates it, so tempListPath
+ * is an optional param: executeOperation's 'merge' case calls this once to obtain
+ * concatListContent, writes it to disk, then calls it again with the real
+ * tempListPath to obtain the final args referencing that file.
+ */
+function buildMergeArgs({ inputPaths, outputPath, tempListPath = null }) {
+  if (!Array.isArray(inputPaths) || inputPaths.length < 2) {
+    throw new Error('inputPaths must contain at least 2 files');
+  }
+
+  const concatListContent = inputPaths.map((p) => `file '${p}'`).join('\n') + '\n';
+
+  const args = ['-f', 'concat', '-safe', '0', '-i', tempListPath, '-c', 'copy', '-y', outputPath];
+
+  return { args, concatListContent };
+}
+
+/**
+ * Run ffmpeg with the given args via the injected execFileFn, wrapped in a Promise.
+ */
+function runFfmpeg(ffmpegPath, args, execFileFn) {
+  return new Promise((resolve, reject) => {
+    execFileFn(ffmpegPath, args, { maxBuffer: 10 * 1024 * 1024 }, (error, stdout, stderr) => {
+      if (error) {
+        reject(new Error(stderr || error.message));
+        return;
+      }
+      resolve();
+    });
+  });
+}
+
+async function executeOperation(operation, data, { ffmpegPath, execFileFn } = {}) {
+  const resolvedFfmpegPath = ffmpegPath || 'ffmpeg';
+  const resolvedExecFileFn = execFileFn || execFile;
+
+  switch (operation) {
+    case 'convert': {
+      const { inputPath, outputPath, format } = data || {};
+      const args = buildConvertArgs({ inputPath, outputPath, format });
+      await runFfmpeg(resolvedFfmpegPath, args, resolvedExecFileFn);
+      return { success: true, outputPath };
+    }
+
+    case 'trim': {
+      const { inputPath, outputPath, startTime, duration } = data || {};
+      const args = buildTrimArgs({ inputPath, outputPath, startTime, duration });
+      await runFfmpeg(resolvedFfmpegPath, args, resolvedExecFileFn);
+      return { success: true, outputPath };
+    }
+
+    case 'extract': {
+      const { inputPath, outputPath } = data || {};
+      try {
+        const args = buildExtractArgs({ inputPath, outputPath });
+        await runFfmpeg(resolvedFfmpegPath, args, resolvedExecFileFn);
+      } catch {
+        // Codec copy can fail when the source audio codec isn't valid in the target
+        // container — fall back to letting ffmpeg transcode instead.
+        const fallbackArgs = buildExtractFallbackArgs({ inputPath, outputPath });
+        await runFfmpeg(resolvedFfmpegPath, fallbackArgs, resolvedExecFileFn);
+      }
+      return { success: true, outputPath };
+    }
+
+    case 'merge': {
+      const { inputPaths, outputPath } = data || {};
+      const { concatListContent } = buildMergeArgs({ inputPaths, outputPath });
+
+      const tempDir = fs.mkdtempSync(path.join(os.tmpdir(), 'audio-merge-'));
+      const tempListPath = path.join(tempDir, 'concat-list.txt');
+
+      try {
+        fs.writeFileSync(tempListPath, concatListContent, 'utf8');
+        const { args } = buildMergeArgs({ inputPaths, outputPath, tempListPath });
+        await runFfmpeg(resolvedFfmpegPath, args, resolvedExecFileFn);
+      } finally {
+        try {
+          fs.rmSync(tempDir, { recursive: true, force: true });
+        } catch {
+          /* best-effort cleanup */
+        }
+      }
+
+      return { success: true, outputPath };
+    }
+
+    default:
+      throw new Error(`Unknown operation: ${operation}`);
+  }
+}
+
+module.exports = {
+  executeOperation,
+  buildConvertArgs,
+  buildTrimArgs,
+  buildExtractArgs,
+  buildMergeArgs,
+};

src/preload.js

@@ -49,11 +49,7 @@ const ALLOWED_SEND_CHANNELS = [
   'select-image-folder',
 
   // Audio converter
-  'audio-convert',
-  'audio-batch-convert',
-  'audio-extract',
-  'audio-trim',
-  'audio-merge',
+  'process-audio-operation',
 
   // Video converter
   'video-convert',

tests/main/AudioOperations.test.js

@@ -0,0 +1,63 @@
+const AudioOperations = require('../../src/main/AudioOperations');
+
+describe('AudioOperations argument builders', () => {
+  test('buildConvertArgs builds correct ffmpeg args', () => {
+    const args = AudioOperations.buildConvertArgs({ inputPath: '/a.wav', outputPath: '/b.mp3' });
+    expect(args).toEqual(['-i', '/a.wav', '-y', '/b.mp3']);
+  });
+
+  test('buildTrimArgs builds correct trim args', () => {
+    const args = AudioOperations.buildTrimArgs({
+      inputPath: '/a.mp3',
+      outputPath: '/b.mp3',
+      startTime: 5,
+      duration: 10,
+    });
+    expect(args).toEqual(['-i', '/a.mp3', '-ss', '5', '-t', '10', '-y', '/b.mp3']);
+  });
+
+  test('buildTrimArgs rejects non-finite startTime', () => {
+    expect(() =>
+      AudioOperations.buildTrimArgs({
+        inputPath: '/a.mp3',
+        outputPath: '/b.mp3',
+        startTime: NaN,
+        duration: 10,
+      })
+    ).toThrow('Invalid trim range');
+  });
+
+  test('buildMergeArgs builds concat-demuxer args and list content', () => {
+    const { args, concatListContent } = AudioOperations.buildMergeArgs({
+      inputPaths: ['/a.mp3', '/b.mp3'],
+      outputPath: '/out.mp3',
+    });
+    expect(concatListContent).toContain("file '/a.mp3'");
+    expect(concatListContent).toContain("file '/b.mp3'");
+    expect(args).toContain('-f');
+    expect(args).toContain('concat');
+  });
+});
+
+describe('AudioOperations.executeOperation', () => {
+  test('convert calls execFileFn with ffmpeg path and args, resolves success', async () => {
+    const execFileFn = (cmd, args, opts, cb) => cb(null, '', '');
+    const result = await AudioOperations.executeOperation(
+      'convert',
+      { inputPath: '/a.wav', outputPath: '/b.mp3' },
+      { ffmpegPath: '/usr/bin/ffmpeg', execFileFn }
+    );
+    expect(result.success).toBe(true);
+    expect(result.outputPath).toBe('/b.mp3');
+  });
+
+  test('unknown operation rejects', async () => {
+    await expect(
+      AudioOperations.executeOperation(
+        'bogus',
+        {},
+        { ffmpegPath: '/usr/bin/ffmpeg', execFileFn: () => {} }
+      )
+    ).rejects.toThrow();
+  });
+});