mindfork 0.18.1

//! Running the application as a single instance.
//! See spec §1.4 (single instance) and plan M0.
//!
//! The lock is OS-level: a named mutex on Windows, an abstract unix socket on
//! Linux, an `flock` on a file on macOS (a detail of the `single-instance`
//! crate). The unprefixed name lands in the current login session's namespace
//! — enough to keep one user from launching the app twice; the binary's path
//! doesn't affect the lock (two copies in different directories still conflict
//! by name). On macOS the name is turned into a path first ([`lock_name`]).

use single_instance::SingleInstance;
use thiserror::Error;

/// Unique lock identifier. Keeps the historical `mindfork-rs` id after the
/// binary's rename to `mindfork`: a pre-rename build and a current one are the
/// same app and must still exclude each other (docs/research/binary-rename.md §3).
const INSTANCE_ID: &str = "mindfork-rs-single-instance";

/// Holder of the single-instance lock.
///
/// Must live for the process's whole run: on its `drop` the lock is released
/// and a new instance can be launched.
pub struct InstanceGuard {
    _inner: SingleInstance,
}

/// Error acquiring the single-instance lock.
#[derive(Debug, Error)]
pub enum InstanceError {
    /// The application is already running as another instance — not a
    /// failure, but a launch refusal.
    #[error("application is already running")]
    AlreadyRunning,
    /// Failed to initialize the lock (a system error from the crate).
    #[error("failed to initialize the single-instance lock: {0}")]
    Init(String),
}

/// Attempts to acquire the single-instance lock.
///
/// `Err(InstanceError::AlreadyRunning)` — the application is already running
/// (the caller should show a message and exit); `Err(InstanceError::Init)` —
/// a genuine initialization failure.
pub fn acquire() -> Result<InstanceGuard, InstanceError> {
    acquire_named(INSTANCE_ID)
}

/// Implementation of `acquire` with an explicit name — for tests (so as not
/// to conflict with the real lock of a running app instance on the same
/// machine).
fn acquire_named(name: &str) -> Result<InstanceGuard, InstanceError> {
    let inner =
        SingleInstance::new(&lock_name(name)).map_err(|e| InstanceError::Init(e.to_string()))?;
    if !inner.is_single() {
        return Err(InstanceError::AlreadyRunning);
    }
    Ok(InstanceGuard { _inner: inner })
}

/// What the crate is given for `name`. On macOS `single-instance` takes the
/// name as a **file path** (`File::create` + `flock`), so a bare name was a file
/// in the working directory: it littered it, two instances started from two
/// directories both got "the" lock and ran on one data root, and a start from
/// `/` failed (all three measured, docs/research/macos.md §3.2). There it is a
/// path in the per-user temporary directory (`/var/folders/…/T/`), so the lock
/// is one per user, as the other two OSes' are.
fn lock_name(name: &str) -> String {
    if cfg!(target_os = "macos") {
        std::env::temp_dir()
            .join(name)
            .to_string_lossy()
            .into_owned()
    } else {
        name.to_string()
    }
}

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

    #[test]
    fn second_acquire_reports_already_running() {
        // A per-test unique name — doesn't touch the app's real lock.
        let name = "mindfork-rs-test-second-acquire-reports-already-running";

        let first = acquire_named(name).expect("first acquire should succeed");
        match acquire_named(name) {
            Err(InstanceError::AlreadyRunning) => {}
            Err(other) => panic!("expected AlreadyRunning, got: {other:?}"),
            Ok(_) => panic!("second acquire should not succeed while the first is alive"),
        }

        // After the first lock is released, acquiring again is possible.
        drop(first);
        let _again = acquire_named(name).expect("after drop, acquiring again is possible");
    }

    /// The lock does not depend on where the app was started: nothing is
    /// written to the working directory, and on macOS the lock is a file in
    /// the per-user temporary directory.
    #[test]
    fn the_lock_leaves_the_working_directory_alone() {
        let name = "mindfork-rs-test-the-lock-leaves-the-working-directory-alone";
        let _guard = acquire_named(name).expect("acquire");
        assert!(
            !std::path::Path::new(name).exists(),
            "a lock file in the cwd"
        );
        if cfg!(target_os = "macos") {
            assert!(std::env::temp_dir().join(name).is_file());
        } else {
            assert_eq!(lock_name(name), name);
        }
    }
}