//! PID-file based advisory lock preventing two processes from operating on
//! the same session directory concurrently.
//!
//! # Flow
//!
//! [`SessionLock::new`] creates a handle → [`SessionLock::try_lock`] attempts
//! atomic `O_CREAT|O_EXCL` creation. If the lock file already exists, the
//! owning PID is checked via liveness verification. Stale locks are
//! overwritten atomically (temp-file + rename + fsync). On [`Drop`],
//! the lock file is removed automatically.
//!
//! # Components
//!
//! - `SessionLock` — RAII guard wrapping a lock file path and PID
//! - `try_lock` — three-phase atomic acquire with stale-lock recovery
//! - `unlock` / `Drop` — explicit and implicit release
use std::fs;
use std::io::Write;
use std::path::{Path, PathBuf};
use tracing;

/// A PID-file lock (`<session_dir>/.lock`) tied to the current process,
/// auto-removed on drop.
#[derive(Debug)]
pub struct SessionLock {
    /// Path to the `.lock` file inside the session directory.
    pub(crate) path: PathBuf,
    /// Process ID that holds (or will hold) this lock.
    pub(crate) pid: u32,
}

impl SessionLock {
    /// Construct a lock handle for a session directory (does not acquire
    /// the lock yet — call `try_lock`).
    pub fn new(session_dir: &Path) -> Self {
        SessionLock {
            path: session_dir.join(".lock"),
            pid: std::process::id(),
        }
    }

    /// Attempt to acquire the session lock using an atomic file creation.
    ///
    /// Flow: try `O_CREAT | O_EXCL` via `create_new(true)` → if that
    /// succeeds, the lock is ours — write our PID and return ok. If the
    /// file already exists, read the PID inside it and check whether that
    /// PID is still alive: if the process is still running, fail to acquire;
    /// otherwise the lock is stale — overwrite it with our own PID and succeed.
    ///
    /// Return: `Ok(true)` if acquired, `Ok(false)` if another live
    /// process holds it, `Err` on I/O failure.
    pub fn try_lock(&self) -> std::io::Result<bool> {
        // Phase 1: try atomic create. If it succeeds, the lock is ours.
        match fs::OpenOptions::new()
            .create_new(true)
            .write(true)
            .open(&self.path)
        {
            Ok(mut file) => {
                write!(file, "{}", self.pid)?;
                file.sync_all()?;
                tracing::debug!(path = %self.path.display(), pid = self.pid, "session lock acquired");
                return Ok(true);
            }
            Err(ref e) if e.kind() == std::io::ErrorKind::AlreadyExists => {
                tracing::debug!(path = %self.path.display(), "session lock already exists, checking staleness");
                // Lock file exists — check if it's stale.
            }
            Err(e) => return Err(e),
        }

        // Phase 2: lock file exists — check liveness of the owning process.
        let content = fs::read_to_string(&self.path).unwrap_or_default();
        if let Ok(pid) = content.trim().parse::<u32>() {
            if Self::is_alive(pid) {
                tracing::warn!(stale = pid, path = %self.path.display(), "session lock held by live process");
                return Ok(false);
            }
            tracing::debug!(stale = pid, "stale lock detected, overwriting");
        }

        // Phase 3: stale lock — overwrite it atomically (best-effort).
        // Use a temp file + rename to avoid partial writes corrupting the lock.
        let tmp = self.path.with_extension("lock.tmp");
        {
            let mut tmp_file = fs::OpenOptions::new()
                .create(true)
                .truncate(true)
                .write(true)
                .open(&tmp)?;
            write!(tmp_file, "{}", self.pid)?;
            tmp_file.sync_all()?;
        }
        fs::rename(&tmp, &self.path)?;
        // Sync the parent directory so the rename survives a crash.
        if let Some(parent) = self.path.parent() {
            let _ = fs::File::open(parent).and_then(|d| d.sync_all());
        }
        Ok(true)
    }

    /// Explicitly release the lock by removing the lock file.
    pub fn unlock(&self) {
        let _ = fs::remove_file(&self.path);
    }

    /// Check whether a process with the given PID is currently alive.
    ///
    /// Uses `kill(pid, 0)` on Unix via the `nix` or `libc` crate in production;
    /// here we provide a best-effort check using the process table.
    /// On non-Unix platforms this always returns `true` (conservative).
    fn is_alive(pid: u32) -> bool {
        // On Unix, signal 0 checks process existence without sending a signal.
        #[cfg(unix)]
        {
            // SAFETY: `libc::kill(pid, 0)` does not send a signal; it only checks
            // whether the process exists and the caller has permission to signal it.
            // The integer argument is a PID validated by `try_lock`.
            let pid_signed: i32 = match pid.try_into() {
                Ok(p) => p,
                Err(_) => return false,
            };
            if unsafe { libc::kill(pid_signed, 0) != 0 } {
                return false;
            }
            // Extra check: verify the PID belongs to a zesdex process via
            // /proc/<pid>/exe to mitigate the PID-reuse race.
            let proc_exe = std::path::PathBuf::from(format!("/proc/{pid}/exe"));
            if let Ok(target) = std::fs::read_link(&proc_exe) {
                if let Ok(exe) = std::env::current_exe() {
                    if target != exe {
                        return false;
                    }
                }
            }
            true
        }

        #[cfg(not(unix))]
        {
            // Fallback: always assume alive (conservative).
            let _ = pid;
            true
        }
    }
}

impl Drop for SessionLock {
    /// Release the lock automatically when the guard goes out of scope,
    /// so an ungracefully-exited process doesn't leave a dangling lock.
    fn drop(&mut self) {
        let _ = fs::remove_file(&self.path);
    }
}