EventSnap/backend/src/services/video.rs

//! Poster-frame extraction, shared by the compression worker and the HTML export.
//!
//! Both used to spawn `ffmpeg` themselves with the same broken invocation:
//!
//! ```text
//! ffmpeg -i <src> -vframes 1 -ss 00:00:01 -vf scale=… -y <out>
//! ```
//!
//! `-ss` AFTER `-i` is an output-side seek. Against a clip of a second or less ffmpeg exits **0 and
//! writes nothing** — and both call sites gated on the exit status, so neither noticed. The worker
//! then wrote `thumbnail_path` for a file that was never created (404 in the live feed) and the
//! export listed the entry in `data.json` while the ZIP writer skipped it (a broken image tile in
//! the keepsake). Every server-side signal stayed green. Phones produce such clips constantly:
//! mis-taps, Live Photos, boomerangs.
//!
//! This module exists for the same reason `imaging.rs` does — that one was created when compression
//! and export duplicated decode logic, and it paid off immediately when the `max_alloc` fix landed
//! in both workers at once. Same duplication, same fix.

use std::path::Path;
use std::time::Duration;

use anyhow::{Context, Result};

/// A malformed video can hang `ffmpeg` indefinitely. In the compression worker that never releases
/// the semaphore permit and the pool eventually deadlocks; in the export worker it strands the job
/// at `running` so the keepsake never completes. `export.rs` had NO timeout at all before this
/// module — sharing the spawn fixes that too.
const FFMPEG_TIMEOUT: Duration = Duration::from_secs(120);

/// Seek positions to try, in order.
///
/// One second first: the opening frame of a real video is often black, a fade-in, or motion-blurred
/// as the camera settles, so it makes a poor poster. Zero second as the fallback, which is what
/// makes short clips work — and it is genuinely required, not defensive. Moving `-ss` before `-i`
/// (an input-side seek) is necessary but NOT sufficient: seeking to 1 s in a 1.000 s clip is still
/// past the last frame, and ffmpeg still exits 0 having written nothing. Verified against the real
/// production image.
const SEEK_POSITIONS: &[&str] = &["00:00:01", "0"];

/// Extract one poster frame from `src` into `dest`, scaled to `width` px wide.
///
/// `Ok(false)` means the video yielded no frame — a normal outcome for a very short or unusual
/// clip, NOT an error. Callers must degrade (no poster) rather than fail the upload: treating this
/// as an error would soft-delete every sub-second video, turning a cosmetic defect into data loss.
///
/// `Err` is reserved for something genuinely wrong — a hang we had to kill, or a failure to spawn.
pub async fn extract_poster_frame(src: &Path, dest: &Path, width: u32) -> Result<bool> {
    for seek in SEEK_POSITIONS {
        // A stale file from a previous attempt would be indistinguishable from a fresh success.
        let _ = tokio::fs::remove_file(dest).await;

        run_ffmpeg(src, dest, width, seek).await?;

        // THE CHECK BOTH CALL SITES WERE MISSING: ask the filesystem, not the exit status.
        // Non-empty, because a zero-byte file is not a poster either.
        if tokio::fs::metadata(dest)
            .await
            .map(|m| m.is_file() && m.len() > 0)
            .unwrap_or(false)
        {
            return Ok(true);
        }
    }

    // Leave nothing behind for a caller to mistake for a result.
    let _ = tokio::fs::remove_file(dest).await;
    Ok(false)
}

/// Run one ffmpeg attempt. A non-zero exit is NOT an error here — the artifact check above is the
/// authority, and a corrupt input that fails at 1 s may still yield a frame at 0.
async fn run_ffmpeg(src: &Path, dest: &Path, width: u32, seek: &str) -> Result<()> {
    let mut child = tokio::process::Command::new("ffmpeg")
        .args([
            // BEFORE -i: an input-side seek. See SEEK_POSITIONS.
            "-ss",
            seek,
            "-i",
            src.to_str().unwrap_or_default(),
            "-vframes",
            "1",
            "-vf",
            &format!("scale={width}:-1"),
            "-y",
            dest.to_str().unwrap_or_default(),
        ])
        .stdout(std::process::Stdio::piped())
        .stderr(std::process::Stdio::piped())
        .kill_on_drop(true)
        .spawn()
        .context("failed to spawn ffmpeg")?;

    match tokio::time::timeout(FFMPEG_TIMEOUT, child.wait()).await {
        Ok(res) => {
            res.context("ffmpeg wait failed")?;
        }
        Err(_) => {
            let _ = child.kill().await;
            anyhow::bail!("ffmpeg timed out after {}s", FFMPEG_TIMEOUT.as_secs());
        }
    }
    Ok(())
}

#[cfg(test)]
mod tests {
    use super::*;

    /// The order is the whole fix. `-ss` must precede `-i`, and 0 must be tried after 1 s.
    #[test]
    fn the_fallback_seek_exists_and_comes_last() {
        assert_eq!(
            SEEK_POSITIONS,
            &["00:00:01", "0"],
            "1s first for a better poster, 0 as the fallback that makes short clips work"
        );
    }

    /// A missing input yields no frame rather than an error: the caller must degrade to "no
    /// poster", never fail the upload. `Err` is reserved for a hang or a spawn failure.
    #[tokio::test]
    async fn a_missing_source_yields_no_frame_rather_than_an_error() {
        let dir = std::env::temp_dir().join(format!("es-video-{}", std::process::id()));
        std::fs::create_dir_all(&dir).unwrap();
        let dest = dir.join("out.jpg");

        let got = extract_poster_frame(Path::new("/nonexistent/clip.mp4"), &dest, 400).await;

        match got {
            Ok(false) => {}
            other => panic!("expected Ok(false) for a missing input, got {other:?}"),
        }
        assert!(
            !dest.exists(),
            "a failed extraction must leave nothing a caller could mistake for a poster"
        );
        let _ = std::fs::remove_dir_all(&dir);
    }
}