//! Interface for the `NOTIFY_SOCKET` readiness protocol
//!
//! The entry point of this module is the [`Notifier`] struct.
//!
//! # Example
//!
//! ```no_run
//! # main() -> Result<(), Box<dyn std::error::Error>> {
//! // Do whatever you need for your service to be ready.
//! if let Some(notifier) = Notifier::new(true)? {
//!     notifier.notify(&[NotifyState::Ready])?
//! }
//! # Ok(())
//! # }
//! ```

use core::fmt::Display;
use std::env;
use std::ffi::OsString;
use std::io::IoSlice;
use std::os::unix::net::UnixDatagram;

use rustix::{
    fd::{AsFd, BorrowedFd},
    pipe::PipeFlags,
};

use self::error::{BarrierError, NewNotifierError, NotifyError};

pub mod error;
pub mod types;

/// A wrapper around the socket specified by `$NOTIFY_SOCKET`
#[derive(Debug)]
pub struct Notifier {
    sock_addr: rustix::net::SocketAddrUnix,
    socket: UnixDatagram,
}

impl Notifier {
    /// Create a new [`Notifier`].
    ///
    /// If `unset_env` is set to `true` this will unset `NOTIFY_SOCKET` resulting in further call to this function to return `Ok(None)`.
    ///
    /// Returns `Ok(None)` if notification isn't supported (`NOTIFY_SOCKET` isn't defined).
    ///
    /// # Errors
    ///
    /// This function error out if the socket couldn't be opened.
    pub fn new(unset_env: bool) -> Result<Option<Self>, NewNotifierError> {
        let span = tracing::info_span!("new", f_unset_env = ?unset_env);
        let _enter = span.enter();

        tracing::info!("Opening NOTIFY_SOCKET if available.");
        let env_sock = match env::var_os("NOTIFY_SOCKET") {
            None => return Ok(None),
            Some(v) => v,
        };
        tracing::debug!("NOTIFY_SOCKET = {env_sock:?}");

        if unset_env {
            tracing::trace!("Removing NOTIFY_SOCKET from environment.");
            env::remove_var("NOTIFY_SOCKET");
        }

        // False positive
        #[allow(clippy::single_match_else)]
        // If the first character of `$NOTIFY_SOCKET` is '@', the string
        // is understood as Linux abstract namespace socket.
        let socket_addr = match env_sock.as_encoded_bytes().strip_prefix(b"@").map(|v| {
            // SAFETY:
            // - Only strip ASCII '@' which is a non-empty UTF-8 substring
            unsafe { OsString::from_encoded_bytes_unchecked(v.to_vec()) }
        }) {
            Some(stripped_addr) => {
                tracing::trace!("Opening abstract socket {stripped_addr:?}.");
                rustix::net::SocketAddrUnix::new_abstract_name(stripped_addr.as_encoded_bytes())
                    .map_err(NewNotifierError::InvalidAbstractSocket)?
            }
            None => {
                tracing::trace!("Opening named socket {env_sock:?}.");
                rustix::net::SocketAddrUnix::new(env_sock)
                    .map_err(NewNotifierError::InvalidSocketPath)?
            }
        };

        let socket = UnixDatagram::unbound().map_err(NewNotifierError::CouldntOpenSocket)?;
        let ret = Self {
            sock_addr: socket_addr,
            socket,
        };
        Ok(Some(ret))
    }

    /// Notify service manager about status change and send file descriptors.
    ///
    /// Use this together with [`NotifyState::FdStore`]. Otherwise works like [`Notifier::notify()`].
    ///
    /// # Errors
    ///
    /// This function will error out if the passed [`NotifyState`] do not follow the rules set by systemd or if they couldn't be fully sent.
    pub fn notify_with_fds(
        &self,
        state: &[NotifyState<'_>],
        fds: &[BorrowedFd],
    ) -> Result<(), NotifyError> {
        let span =
            tracing::info_span!("notify_with_fds", f_self = ?self, f_state = ?state, f_fds = ?fds);
        let _enter = span.enter();

        let msg = state
            .iter()
            .fold(String::new(), |acc, state| format!("{acc}{state}\n"))
            .into_bytes();
        let msg_len = msg.len();
        let msg_iov = IoSlice::new(&msg);

        let mut ancillary = if fds.is_empty() {
            tracing::trace!("No file descriptors provided, not sending ancillary messages.");
            rustix::net::SendAncillaryBuffer::default()
        } else {
            tracing::trace!(
                "{} file descriptors provided, sending through ancillary messages",
                fds.len()
            );
            let mut ancillary = rustix::net::SendAncillaryBuffer::default();
            let tmp = rustix::net::SendAncillaryMessage::ScmRights(fds);
            if !ancillary.push(tmp) {
                return Err(NotifyError::PushAncillaryMessage);
            }
            ancillary
        };

        span.record("expected_length", msg_len);
        tracing::debug!("Sending notification messages.");
        let sent_len = rustix::net::sendmsg_unix(
            self.socket.as_fd(),
            &self.sock_addr,
            &[msg_iov],
            &mut ancillary,
            rustix::net::SendFlags::empty(),
        )
        .map_err(NotifyError::SendMsg)?;
        span.record("sent_length", sent_len);
        tracing::debug!("Notification message sent. {sent_len} bytes sent.");

        if sent_len != msg_len {
            tracing::error!("The notification message couldn't be completely sent!");
            return Err(NotifyError::PartialSend);
        }

        Ok(())
    }

    /// Notify service manager about status changes.
    ///
    /// Send a notification to the manager about service status changes. Also see [`Notifier::notify_with_fds()`] to send file descriptors.
    ///
    /// # Errors
    ///
    /// This function will error out if the passed [`NotifyState`] couldn't be fully sent.
    pub fn notify(&self, state: &[NotifyState<'_>]) -> Result<(), NotifyError> {
        self.notify_with_fds(state, &[])
    }

    /// Ensure that all previous notifications have been treated by the service manager.
    ///
    /// **This is a blocking call. If you are using it in an async function you might want to use an equivalent of [`tokio::task::spawn_blocking`](https://docs.rs/tokio/latest/tokio/task/fn.spawn_blocking.html).**
    ///
    /// # Errors
    ///
    /// This function will error out if the synchronisation mechanism couldn't be created, the synchronising notification failed or the synchronisation timed out.
    pub fn barrier(&self, timeout: types::BarrierTimeout) -> Result<(), BarrierError> {
        let (to_poll, sent) = rustix::pipe::pipe_with(PipeFlags::CLOEXEC)
            .map_err(BarrierError::FailedPipeCreation)?;

        self.notify_with_fds(
            &[NotifyState::Other(types::OtherState::barrier())],
            &[sent.as_fd()],
        )?;

        core::mem::drop(sent);
        let to_poll = rustix::event::PollFd::new(&to_poll, rustix::event::PollFlags::HUP);

        rustix::event::poll(&mut [to_poll], timeout.to_raw())
            .map_err(BarrierError::FailedPolling)
            .and_then(|events| {
                if events == 0_usize {
                    return Err(BarrierError::TimedOut);
                }
                Ok(())
            })
    }

    /// Create a synchronizing RAII guard.
    ///
    /// This create an RAII guard that automatically call [`barrier()`](Self::barrier) with the provided timeout when dropped.
    ///
    /// Do note that this guard's [`Drop`] implementation will block for the provided timeout and ignore all errors returned by [`barrier()`](Self::barrier).
    pub fn guard(&self, timeout: types::BarrierTimeout) -> NotifyBarrierGuard<'_> {
        NotifyBarrierGuard {
            notifier: self,
            timeout,
        }
    }

    /// Create a scope at the end of which all notifications sent inside should have been treated by the service manager.
    pub fn with_guard<F, T>(&self, timeout: types::BarrierTimeout, f: F) -> T
    where
        F: FnOnce(NotifyBarrierGuard) -> T,
    {
        f(self.guard(timeout))
    }
}

/// RAII guard automatically synchronizing the notifications with the service manager.
///
/// This is created by [`Notifier::guard`].
///
/// Do note that this guard's [`Drop`] implementation will block for the provided timeout and ignore all errors returned by [`barrier()`](Self::barrier).
pub struct NotifyBarrierGuard<'a> {
    notifier: &'a Notifier,
    timeout: types::BarrierTimeout,
}

impl<'a> NotifyBarrierGuard<'a> {
    /// Notify service manager about status changes.
    ///
    /// Send a notification to the manager about service status changes. Also see [`notify_with_fds()`](Self::notify_with_fds) to send file descriptors.
    ///
    /// # Errors
    ///
    /// This function will error out if the passed [`NotifyState`] do not follow the rules set by systemd or if they couldn't be fully sent.
    #[inline]
    pub fn notify(&self, state: &[NotifyState<'_>]) -> Result<(), NotifyError> {
        self.notifier.notify(state)
    }

    /// Notify service manager about status change and send file descriptors.
    ///
    /// Use this together with [`NotifyState::FdStore`]. Otherwise works like [`notify()`](Self::notify).
    ///
    /// # Errors
    ///
    /// This function will error out if the passed [`NotifyState`] do not follow the rules set by systemd or if they couldn't be fully sent.
    #[inline]
    pub fn notify_with_fds(
        &self,
        state: &[NotifyState<'_>],
        fds: &[BorrowedFd],
    ) -> Result<(), NotifyError> {
        self.notifier.notify_with_fds(state, fds)
    }

    /// Ensure that all previous notifications have been treated by the service manager.
    ///
    /// **This is a blocking call. If you are using it in an async function you might want to use an equivalent of [`tokio::task::spawn_blocking`](https://docs.rs/tokio/latest/tokio/task/fn.spawn_blocking.html).**
    ///
    /// # Errors
    ///
    /// This function will error out if the synchronisation mechanism couldn't be created, the synchronising notification failed or the synchronisation timed out.
    #[inline]
    pub fn barrier(&self, timeout: types::BarrierTimeout) -> Result<(), BarrierError> {
        self.notifier.barrier(timeout)
    }

    /// Create a scope at the end of which all notifications sent inside should have been treated by the service manager.
    #[inline]
    pub fn with_guard<F, T>(&self, timeout: types::BarrierTimeout, f: F) -> T
    where
        F: FnOnce(Self) -> T,
    {
        f(self.notifier.guard(timeout))
    }
}

impl Drop for NotifyBarrierGuard<'_> {
    fn drop(&mut self) {
        self.barrier(self.timeout).unwrap_or_default();
    }
}

/// Check for watchdog support at runtime
///
/// If `unset_env` is true, the environment variables related to watchdog support will be cleared.
///
/// # Return
///
/// * [`None`] if watchdog support is not enabled.
/// * The timeout before which the watchdog expects a response from the process otherwise.
pub fn is_watchdog_enabled(unset_env: bool) -> Option<std::time::Duration> {
    let timeout = std::env::var("WATCHDOG_USEC").ok();
    let watchdog_pid = std::env::var("WATCHDOG_PID").ok();

    if unset_env {
        std::env::remove_var("WATCHDOG_USEC");
        std::env::remove_var("WATCHDOG_PID");
    }

    let timeout = timeout
        .and_then(|timeout| timeout.parse::<u64>().ok())
        .map(std::time::Duration::from_micros)?;

    let watchdog_pid = if let Some(pid) = watchdog_pid {
        pid.parse::<u32>().ok()?
    } else {
        return Some(timeout);
    };

    if watchdog_pid == std::process::id() {
        Some(timeout)
    } else {
        None
    }
}

/// Status changes, see `sd_notify(3)`.
#[allow(clippy::module_name_repetitions)]
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
pub enum NotifyState<'a> {
    /// D-Bus error-style error code.
    BusError(types::BusError<'a>),
    /// errno-style error code.
    Errno(u8),
    /// A name for the submitted file descriptors.
    FdName(types::FdName<'a>),
    /// Stores additional file descriptors in the service manager. Use [`Notifier::notify_with_fds()`] with this.
    FdStore,
    /// Remove stored file descriptors. Must be used together with [`NotifyState::FdName`].
    FdStoreRemove,
    /// Tell the service manager to not poll the filedescriptors for errors. This causes
    /// systemd to hold on to broken file descriptors which must be removed manually.
    /// Must be used together with [`NotifyState::FdStore`].
    FdpollDisable,
    /// The main process ID of the service, in case of forking applications.
    Mainpid(libc::pid_t),
    /// Custom state change, as a `KEY=VALUE` string.
    Other(types::OtherState<'a>),
    /// Service startup is finished.
    Ready,
    /// Service is reloading.
    Reloading,
    /// Custom status change.
    Status(types::StatusLine<'a>),
    /// Service is beginning to shutdown.
    Stopping,
    /// Tell the service manager to update the watchdog timestamp.
    Watchdog,
    /// Tell the service manager to execute the configured watchdog option.
    WatchdogTrigger,
    /// Reset watchdog timeout value during runtime.
    /// Minimal precision is microseconds, not nanoseconds.
    WatchdogUsec(types::Microseconds),
    /// Tells the service manager to extend the startup, runtime or shutdown service timeout corresponding the current state.
    /// Minimal precision is microseconds, not nanoseconds.
    ExtendTimeoutUsec(types::Microseconds),
}

impl<'a> Display for NotifyState<'a> {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        match *self {
            NotifyState::BusError(s) => write!(f, "BUSERROR={s}"),
            NotifyState::Errno(e) => write!(f, "ERRNO={e}"),
            NotifyState::FdName(name) => write!(f, "FDNAME={name}"),
            NotifyState::FdStore => f.write_str("FDSTORE=1"),
            NotifyState::FdStoreRemove => f.write_str("FDSTOREREMOVE=1"),
            NotifyState::FdpollDisable => f.write_str("FDPOLL=0"),
            NotifyState::Mainpid(pid) => write!(f, "MAINPID={pid}"),
            NotifyState::Other(message) => f.write_str(message.as_ref()),
            NotifyState::Ready => f.write_str("READY=1"),
            NotifyState::Reloading => f.write_str("RELOADING=1"),
            NotifyState::Status(status) => write!(f, "STATUS={status}"),
            NotifyState::Stopping => f.write_str("STOPPING=1"),
            NotifyState::Watchdog => f.write_str("WATCHDOG=1"),
            NotifyState::WatchdogTrigger => f.write_str("WATCHDOG=trigger"),
            NotifyState::WatchdogUsec(duration) => {
                write!(f, "WATCHDOG_USEC={duration}")
            }
            NotifyState::ExtendTimeoutUsec(duration) => {
                write!(f, "EXTEND_TIMEOUT_USEC={duration}")
            }
        }
    }
}