zesdex/src/model/session_lock.rs

//! PID-file based advisory lock preventing two processes from operating on
//! the same session directory concurrently.

use std::path::{Path, PathBuf};
use std::fs;
use std::io::Write;

/// A PID-file lock (`<session_dir>/.lock`) tied to the current process,
/// auto-removed on drop.
pub struct SessionLock {
    path: PathBuf,
    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 `is_alive`:
    /// if that process is still running, fail to acquire; otherwise the
    /// lock is stale — overwrite it with our own PID and succeed.
    ///
    /// Why: `create_new(true)` is atomic on POSIX (unlike the previous
    /// read-then-write pattern which had a TOCTOU race between checking
    /// `path.exists()` and writing). The stale-lock recovery path reads
    /// the stale PID and verifies liveness via `kill(pid, 0)`.
    ///
    /// 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()?;
                return Ok(true);
            }
            Err(ref e) if e.kind() == std::io::ErrorKind::AlreadyExists => {
                // 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) {
                return Ok(false);
            }
        }

        // 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)
                .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 and
    /// is actually a zesdex process (not a recycled PID from a different
    /// program).
    fn is_alive(&self, pid: u32) -> bool {
        // 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 already validated by `try_lock`.
        if unsafe { libc::kill(pid as i32, 0) != 0 } {
            return false;
        }
        // Extra check: verify the PID belongs to a zesdex process via
        // /proc/<pid>/exe to mitigate the PID-reuse race (a recycled PID
        // from a different program would answer kill but shouldn't hold
        // our lock). This is best-effort — /proc may not be available
        // on all platforms.
        let proc_exe = std::path::PathBuf::from(format!("/proc/{}/exe", pid));
        match std::fs::read_link(&proc_exe) {
            Ok(target) => match std::env::current_exe() {
                Ok(exe) => {
                    if target != exe {
                        return false;
                    }
                }
                Err(_) => { /* cannot resolve own exe, trust kill check */ }
            },
            Err(_) => { /* /proc unavailable, trust kill check */ }
        }
        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);
    }
}