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This example demonstrates how to gracefully shutdown FFmpeg operations, allowing the process to finalize the output file properly before terminating. This is crucial for producing valid, playable media files even when cancelled.

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Why Graceful Shutdown Matters

When you cancel a running FFmpeg process, you have two options:
  1. Immediate kill (SIGKILL): The process terminates instantly, but the output file may be corrupted or unplayable because headers weren’t finalized.
  2. Graceful shutdown: Send FFmpeg the q command via stdin, allowing it to:
    • Flush buffers
    • Write final metadata
    • Properly close the output file
    • Produce a valid, playable file up to the point of cancellation
The ffmpeg_graceful() function implements the second approach with a fallback: it sends the quit command and waits up to 5 seconds for clean exit, then falls back to SIGKILL if needed.

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Complete Example

use std::path::PathBuf;
use libffmpeg::ffmpeg::ffmpeg_graceful;
use libffmpeg::libcmd::CommandMonitor;
use tokio_util::sync::CancellationToken;

#[tokio::main]
async fn main() -> anyhow::Result<()> {
    let input = PathBuf::from("input.mp4");
    let output = PathBuf::from("output.mp4");
    
    // Create cancellation token
    let cancellation_token = CancellationToken::new();
    
    // Setup signal handling for Ctrl+C
    libsignal::cancel_after_signal(cancellation_token.clone());
    
    // Create monitor for output capture
    let monitor = CommandMonitor::with_capacity(100);
    
    // Spawn task to handle output
    let monitor_task = {
        let mut client = monitor.client.clone();
        tokio::spawn(async move {
            while let Some(Some(msg)) = client.recv().await {
                match msg {
                    libffmpeg::libcmd::CommandMonitorMessage::Stdout { line } => {
                        println!("[OUT] {}", line);
                    }
                    libffmpeg::libcmd::CommandMonitorMessage::Stderr { line } => {
                        eprintln!("[ERR] {}", line);
                    }
                }
            }
        })
    };
    
    println!("Starting transcode. Press Ctrl+C to cancel gracefully...");
    
    // Run with graceful shutdown
    let result = ffmpeg_graceful(
        cancellation_token,
        &monitor.client,
        &monitor.server,
        |cmd| {
            cmd.arg("-i").arg(&input);
            cmd.arg("-c:v").arg("libx264");
            cmd.arg("-preset").arg("medium");
            cmd.arg("-crf").arg("23");
            cmd.arg("-c:a").arg("aac");
            cmd.arg("-b:a").arg("128k");
            cmd.arg("-y");
            cmd.arg(&output);
        },
    )
    .await?;
    
    // Wait for monitor task to finish
    let _ = monitor_task.await;
    
    if result.exit_code.as_ref().map(|e| e.success).unwrap_or_default() {
        println!("Transcoding completed successfully!");
    } else {
        println!("Transcoding was cancelled or failed.");
        println!("Output file is valid up to the point of cancellation.");
    }
    
    Ok(())
}

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How It Works

Let’s look at the implementation of ffmpeg_graceful() from the libffmpeg source: 
pub async fn ffmpeg_graceful<Prepare>(
    cancellation_token: CancellationToken,
    client: &CommandMonitorClient,
    server: &CommandMonitorServer,
    prepare: Prepare,
) -> Result<CommandExit, FfmpegError>
where
    Prepare: FnOnce(&mut Command),
{
    // Find ffmpeg binary
    let ffmpeg_path = find_ffmpeg().ok_or(FfmpegError::NotFound)?;
    
    // Create separate token for the process
    let process_token = CancellationToken::new();
    
    // Token cancelled after process exits
    let exit_token = CancellationToken::new();
    
    // Spawn shutdown handler
    let shutdown_handle = {
        let client = client.clone();
        let process_token = process_token.clone();
        let exit_token = exit_token.clone();
        let kill_token = cancellation_token.child_token();
        
        tokio::spawn(async move {
            // Wait for cancellation request or process exit
            tokio::select! {
                () = exit_token.cancelled() => {
                    // Process exited naturally, nothing to do
                    return
                },
                () = kill_token.cancelled() => {
                    // User requested cancellation, continue
                }
            }
            
            // Send 'q' command to ffmpeg's stdin
            client.send("q").await;
            
            // Wait up to 5 seconds for graceful exit
            match tokio::time::timeout(
                Duration::from_secs(5),
                exit_token.cancelled()
            ).await {
                Ok(()) => {
                    // Process exited gracefully
                }
                Err(_timeout) => {
                    // Timeout: force kill
                    tracing::warn!(
                        "ffmpeg did not respond to quit command, sending SIGKILL"
                    );
                    process_token.cancel();
                }
            }
        })
    };
    
    // Run ffmpeg
    let result = libcmd::run(
        ffmpeg_path,
        Some(server.clone()),
        process_token.child_token(),
        prepare,
    )
    .await?;
    
    // Signal that process has exited
    exit_token.cancel();
    
    // Wait for shutdown handler to complete
    let _ = shutdown_handle.await;
    
    result
}

​
The Shutdown Flow

When you cancel a ffmpeg_graceful() operation, here’s what happens:
  1. User triggers cancellation (e.g., presses Ctrl+C) 
    cancellation_token.cancel();
  2. Shutdown handler wakes up and sends the quit command 
    client.send("q").await;
  3. FFmpeg receives ‘q’ on stdin and begins graceful shutdown:
    • Stops reading input
    • Flushes encoder buffers
    • Writes file trailer/metadata
    • Closes output file
    • Exits with status code
  4. Two possible outcomes: a. Success (within 5 seconds):
    • FFmpeg exits cleanly
    • exit_token is cancelled
    • Output file is properly finalized
    b. Timeout (after 5 seconds):
    • process_token.cancel() is called
    • SIGKILL sent to FFmpeg
    • Process terminates immediately
    • Output file may be incomplete

​
Key Differences from Standard ffmpeg()

Featureffmpeg()ffmpeg_graceful()
CancellationImmediate SIGKILLSends ‘q’, waits 5s, then SIGKILL
Output finalizationNo guaranteeOutput finalized if process responds
Cancellation latencyInstantUp to 5 seconds
Output validityMay be corruptedValid up to cancellation point

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Using with Progress Monitoring

You can combine graceful shutdown with progress monitoring: 
use libffmpeg::ffmpeg::progress::PartialProgress;
use tokio_util::future::FutureExt;

let monitor = CommandMonitor::with_capacity(100);
let mut client = monitor.client.clone();

let monitor_task = tokio::spawn(async move {
    let mut progress = PartialProgress::default();
    
    while let Some(Some(msg)) = client.recv().await {
        match msg {
            CommandMonitorMessage::Stdout { line } => {
                if progress.with_line(&line) {
                    if let Some(update) = progress.finish() {
                        println!(
                            "Progress: {:.1}% @ {}x speed",
                            update.out_time.as_secs_f64() / total * 100.0,
                            update.speed
                        );
                    }
                }
            }
            CommandMonitorMessage::Stderr { line } => {
                eprintln!("[ERR] {}", line);
            }
        }
    }
});

let result = ffmpeg_graceful(
    cancellation_token,
    &monitor.client,
    &monitor.server,
    |cmd| {
        cmd.arg("-i").arg(input);
        cmd.arg("-progress").arg("pipe:1");
        // ... other args
        cmd.arg(output);
    }
).await?;

monitor_task.await?;

​
Cancellation Token Hierarchy

A common pattern is to use child tokens for different parts of the operation: 
let root_token = CancellationToken::new();
let transcode_token = root_token.child_token();
let monitor_token = root_token.child_token();

// Handle Ctrl+C
libsignal::cancel_after_signal(root_token.clone());

// Start monitoring
let monitor_task = tokio::spawn(async move {
    tokio::select! {
        () = monitor_token.cancelled() => {
            // Cleanup
        }
        _ = async {
            // Monitoring logic
        } => {}
    }
});

// Run ffmpeg
let result = ffmpeg_graceful(
    transcode_token,
    &monitor.client,
    &monitor.server,
    |cmd| { /* ... */ }
).await?;

// Cancel monitoring
monitor_token.cancel();
monitor_task.await?;
When root_token.cancel() is called:
  • Both transcode_token and monitor_token are automatically cancelled
  • FFmpeg begins graceful shutdown
  • Monitor task stops receiving messages
  • Everything cleans up in order

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Testing Graceful Shutdown

To test that graceful shutdown is working: 
# Start a long transcode
cargo run --example graceful_shutdown_example

# After a few seconds, press Ctrl+C

# Check that output file is valid:
ffprobe output.mp4

# Should show valid metadata and be playable
ffplay output.mp4
Compare this to immediate kill: 
# Start transcode
ffmpeg -i input.mp4 output_killed.mp4 &
PID=$!

# Kill immediately
kill -9 $PID

# Output is often corrupted
ffprobe output_killed.mp4  # May show errors

​
When to Use Each Function

  • ffmpeg_slim(): Quick operations where cancellation isn’t expected or output validity doesn’t matter
  • ffmpeg(): When you need progress monitoring but not graceful shutdown (e.g., short operations)
  • ffmpeg_graceful(): Long-running operations where cancelled output should still be valid and playable

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Best Practices

  1. Always use graceful shutdown for user-facing applications where Ctrl+C cancellation is expected
  2. Set reasonable timeouts if 5 seconds is too long for your use case: 
    // You would need to implement a custom version with configurable timeout
  3. Handle both success and cancellation in your UI: 
    match result.exit_code {
    Some(exit) if exit.success => {
        println!("Completed successfully!");
    }
    _ => {
        println!("Cancelled - partial output saved");
    }
}
  4. Test cancellation behavior to ensure output files are valid
  5. Combine with progress monitoring to give users feedback during long operations

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See Also

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