//! Sample-level locking for pipeline execution.
//!
//! Uses atomic directory creation for reliable locking on distributed filesystems (BeegFS, NFS, etc.).

use anyhow::Context;
use log::{debug, info, warn};
use std::{
    fs, io,
    path::{Path, PathBuf},
    process::Command,
    time::Duration,
};

/// Atomic filesystem lock for preventing concurrent pipeline execution on the same sample.
///
/// Uses directory creation (atomic on POSIX) rather than file locks, which are unreliable
/// on distributed filesystems like BeegFS.
///
/// # Example
///
/// ```ignore
/// let _lock = SampleLock::acquire("/data/locks", "34528", "clairs")?;
/// // Lock held until _lock is dropped
/// ```
#[derive(Debug)]
pub struct SampleLock {
    path: PathBuf,
}

impl SampleLock {
    /// Maximum age before a lock is considered stale (24 hours).
    const STALE_TIMEOUT: Duration = Duration::from_secs(24 * 3600);

    /// Attempts to acquire an exclusive lock for a sample/pipeline combination.
    ///
    /// # Arguments
    ///
    /// * `lock_dir` - Base directory for lock files
    /// * `id` - Sample identifier
    /// * `pipeline` - Pipeline name (e.g., "clairs", "deepvariant")
    ///
    /// # Errors
    ///
    /// Returns an error if:
    /// - Lock directory cannot be created
    /// - Lock is already held by another process (and not stale)
    /// - Metadata cannot be written
    pub fn acquire(lock_dir: &str, id: &str, pipeline: &str) -> anyhow::Result<Self> {
        fs::create_dir_all(lock_dir)
            .with_context(|| format!("Failed to create lock directory: {lock_dir}"))?;

        let path = PathBuf::from(format!("{}/{}.{}.lock", lock_dir, id, pipeline));

        match fs::create_dir(&path) {
            Ok(_) => {
                let lock = Self { path };
                lock.write_metadata()?;
                info!("Acquired lock: {}", lock.path.display());
                Ok(lock)
            }
            Err(e) if e.kind() == io::ErrorKind::AlreadyExists => {
                // Check if stale
                if Self::is_stale(&path) {
                    warn!("Removing stale lock: {}", path.display());
                    fs::remove_dir_all(&path)?;
                    // Retry once
                    fs::create_dir(&path)?;
                    let lock = Self { path };
                    lock.write_metadata()?;
                    info!(
                        "Acquired lock (after stale removal): {}",
                        lock.path.display()
                    );
                    Ok(lock)
                } else {
                    let holder = Self::read_holder_info(&path);
                    anyhow::bail!(
                        "Sample '{}' is locked for '{}' pipeline.\nLock: {}\n{}",
                        id,
                        pipeline,
                        path.display(),
                        holder
                    )
                }
            }
            Err(e) => Err(e).with_context(|| format!("Failed to create lock: {}", path.display())),
        }
    }

    /// Writes metadata to the lock directory for debugging and stale detection.
    fn write_metadata(&self) -> anyhow::Result<()> {
        let meta_path = self.path.join("meta.txt");
        let hostname = hostname::get()
            .map(|h| h.to_string_lossy().into_owned())
            .unwrap_or_else(|_| "unknown".into());

        let slurm_job = std::env::var("SLURM_JOB_ID").unwrap_or_default();
        let slurm_node = std::env::var("SLURMD_NODENAME").unwrap_or_default();

        let content = format!(
            "pid={}\nhost={}\nslurm_job_id={}\nslurm_node={}\ntime={}\n",
            std::process::id(),
            hostname,
            slurm_job,
            slurm_node,
            chrono::Utc::now().to_rfc3339()
        );

        fs::write(&meta_path, content)
            .with_context(|| format!("Failed to write lock metadata: {}", meta_path.display()))?;

        Ok(())
    }

    /// Reads holder information for error messages.
    fn read_holder_info(path: &Path) -> String {
        let meta = path.join("meta.txt");
        fs::read_to_string(&meta).unwrap_or_else(|_| "Holder info unavailable".into())
    }

    /// Checks if a lock is stale (process/job no longer running).
    fn is_stale(path: &Path) -> bool {
        let meta = path.join("meta.txt");

        if let Ok(content) = fs::read_to_string(&meta) {
            // Check SLURM job first (most reliable for cluster jobs)
            if let Some(job_id) = Self::parse_field(&content, "slurm_job_id") {
                if !job_id.is_empty() {
                    return !Self::slurm_job_exists(job_id);
                }
            }

            // Check local PID (only valid on same host)
            if let Some(lock_host) = Self::parse_field(&content, "host") {
                let current_host = hostname::get()
                    .map(|h| h.to_string_lossy().into_owned())
                    .unwrap_or_default();

                if lock_host == current_host {
                    if let Some(pid) =
                        Self::parse_field(&content, "pid").and_then(|p| p.parse::<u32>().ok())
                    {
                        return !Path::new(&format!("/proc/{}", pid)).exists();
                    }
                }
            }
        }

        // Fall back to timeout-based staleness
        if let Ok(meta) = fs::metadata(path) {
            if let Ok(modified) = meta.modified() {
                if let Ok(age) = modified.elapsed() {
                    let is_stale = age > Self::STALE_TIMEOUT;
                    if is_stale {
                        debug!(
                            "Lock {} is stale (age: {:?} > {:?})",
                            path.display(),
                            age,
                            Self::STALE_TIMEOUT
                        );
                    }
                    return is_stale;
                }
            }
        }

        false
    }

    /// Parses a field from the metadata content.
    fn parse_field<'a>(content: &'a str, field: &str) -> Option<&'a str> {
        content
            .lines()
            .find(|l| l.starts_with(&format!("{}=", field)))
            .and_then(|l| l.split('=').nth(1))
    }

    /// Checks if a SLURM job is still running.
    fn slurm_job_exists(job_id: &str) -> bool {
        Command::new("squeue")
            .args(["-j", job_id, "-h", "-o", "%i"])
            .output()
            .map(|o| o.status.success() && !o.stdout.is_empty())
            .unwrap_or(false)
    }
}

impl Drop for SampleLock {
    fn drop(&mut self) {
        debug!("Releasing lock: {}", self.path.display());
        if let Err(e) = fs::remove_dir_all(&self.path) {
            warn!("Failed to release lock {}: {}", self.path.display(), e);
        }
    }
}