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fix(engine): write the audio mix filter graph to a file, not the command line (#1890)

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* fix(engine): write the audio mix filter graph to a file, not the command line

mixAudioTracks built the ffmpeg -filter_complex argument as one inline
string scaling linearly with track count. Reported in the wild at 146
timed audio clips: the resulting command line exceeded the OS length
limit and spawn failed with ENAMETOOLONG, dropping audio entirely until
the user manually consolidated clips to reduce the count.

FFmpeg supports -filter_complex_script specifically for this - the same
filter graph read from a file instead of inlined as an argument. The -i
pairs for each track still scale with count but stay short and fixed-size
each, so the one component that actually grew unbounded (the filter
string) no longer sits on the command line at all. The temp file is
cleaned up immediately after ffmpeg exits, matching the existing sibling
temp-file convention in audioVolumeEnvelope.ts.

Verified end-to-end against a real ffmpeg binary (not just mocked): a
two-track mix produced correct output audio with no leftover temp files.

* fix(engine): create audio filter scripts safely
This commit is contained in:
Miguel Ángel
2026-07-04 14:08:15 -07:00
committed by GitHub
parent 78069da140
commit 8a3227f548
2 changed files with 126 additions and 44 deletions
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packages/engine/src/services/audioMixer.ts
+30 -10
View File
@@ -4,7 +4,7 @@
* Processes and mixes audio tracks using FFmpeg.
*/
import { existsSync, mkdirSync, rmSync } from "fs";
import { closeSync, existsSync, mkdirSync, mkdtempSync, openSync, rmSync, writeFileSync } from "fs";
import { isAbsolute, join, dirname } from "path";
import { parseHTML } from "linkedom";
import { extractAudioMetadata } from "../utils/ffprobe.js";
@@ -384,11 +384,9 @@ async function mixAudioTracks(
const outputDir = dirname(outputPath);
if (!existsSync(outputDir)) mkdirSync(outputDir, { recursive: true });
const buildArgs = (ignoreAutomation: boolean): string[] => {
const inputs: string[] = [];
const buildFilterComplex = (ignoreAutomation: boolean): string => {
const filterParts: string[] = [];
tracks.forEach((track, i) => {
inputs.push("-i", track.srcPath);
const delayMs = Math.round(track.start * 1000);
const trimDuration = track.end - track.start;
const volumeFilter = buildVolumeExpression(track, ignoreAutomation);
@@ -403,12 +401,31 @@ async function mixAudioTracks(
// gain by track count so per-track volumes authored in data-volume are preserved.
const compensatedGain = masterOutputGain * tracks.length;
const postMixGainFilter = `[mixed]volume=${formatFilterNumber(compensatedGain)}[out]`;
const fullFilter = [...filterParts, mixFilter, postMixGainFilter].join(";");
return [...filterParts, mixFilter, postMixGainFilter].join(";");
};
return [
// A large track count (100+) makes the inline `-filter_complex <string>`
// argument scale linearly with track count until it exceeds the OS
// command-line length limit — spawn ENAMETOOLONG, seen in practice at 146
// tracks — even though every individual filter segment is short. FFmpeg's
// own `-filter_complex_script <file>` reads the same graph from disk
// instead, sidestepping the argv limit for the one component of this
// command line that actually grows with the composition.
const runMix = (ignoreAutomation: boolean) => {
const inputs: string[] = [];
tracks.forEach((track) => inputs.push("-i", track.srcPath));
const scriptDir = mkdtempSync(join(outputDir, ".filter-complex-"));
const scriptPath = join(scriptDir, "graph.txt");
const fd = openSync(scriptPath, "wx", 0o600);
try {
writeFileSync(fd, buildFilterComplex(ignoreAutomation));
} finally {
closeSync(fd);
}
const args = [
...inputs,
"-filter_complex",
fullFilter,
"-filter_complex_script",
scriptPath,
"-map",
"[out]",
"-acodec",
@@ -420,9 +437,12 @@ async function mixAudioTracks(
"-y",
outputPath,
];
return runFfmpeg(args, { signal, timeout: ffmpegProcessTimeout }).finally(() =>
rmSync(scriptDir, { recursive: true, force: true }),
);
};
let result = await runFfmpeg(buildArgs(false), { signal, timeout: ffmpegProcessTimeout });
let result = await runMix(false);
// Defense in depth: volume automation is folded into an FFmpeg `volume`
// expression whose evaluator limits are build-dependent (see
@@ -432,7 +452,7 @@ async function mixAudioTracks(
let degradedAutomation = false;
const hasAutomation = tracks.some((track) => (track.volumeKeyframes?.length ?? 0) > 0);
if (!result.success && !signal?.aborted && hasAutomation) {
const retry = await runFfmpeg(buildArgs(true), { signal, timeout: ffmpegProcessTimeout });
const retry = await runMix(true);
if (retry.success) {
result = retry;
degradedAutomation = true;