//! Per-instance game-server process supervision.
//!
//! One `ProcessSupervisor` per process-managed instance. Lifecycle mirrors the
//! proven Go agent behavior — graceful SIGTERM with a 30s budget before force
//! kill, a monitor task that reaps the child and records crash-vs-stop — with
//! two fixes the Go version needed: args are a proper list (no naive space
//! splitting), and every state change is observable through a watch channel
//! so the panel gets push events instead of waiting for the next heartbeat.

use anyhow::{bail, Context, Result};
use serde::Serialize;
use std::path::PathBuf;
use std::process::Stdio;
use std::sync::Arc;
use std::time::{Duration, Instant};
use tokio::process::{Child, Command};
use tokio::sync::{watch, Mutex};

use crate::config::InstanceConfig;

const GRACEFUL_STOP_BUDGET: Duration = Duration::from_secs(30);
const RESTART_PAUSE: Duration = Duration::from_secs(2);

#[derive(Debug, Clone, PartialEq, Serialize)]
#[serde(rename_all = "snake_case", tag = "state")]
pub enum InstanceState {
    /// Not process-managed (no executable configured).
    Unmanaged,
    Stopped,
    Starting,
    Running,
    Stopping,
    /// Process exited without a stop request.
    Crashed {
        #[serde(skip_serializing_if = "Option::is_none")]
        exit_code: Option<i32>,
    },
}

impl InstanceState {
    pub fn as_label(&self) -> &'static str {
        match self {
            InstanceState::Unmanaged => "unmanaged",
            InstanceState::Stopped => "stopped",
            InstanceState::Starting => "starting",
            InstanceState::Running => "running",
            InstanceState::Stopping => "stopping",
            InstanceState::Crashed { .. } => "crashed",
        }
    }
}

struct Inner {
    child: Option<Child>,
    started_at: Option<Instant>,
    /// True while a stop was requested — the monitor uses it to distinguish
    /// an ordered shutdown from a crash.
    stop_requested: bool,
}

pub struct ProcessSupervisor {
    pub instance_id: String,
    executable: Option<PathBuf>,
    args: Vec<String>,
    working_dir: Option<PathBuf>,
    inner: Mutex<Inner>,
    state_tx: watch::Sender<InstanceState>,
}

impl ProcessSupervisor {
    pub fn new(cfg: &InstanceConfig) -> Arc<Self> {
        let executable = cfg.resolved_executable();
        let initial = if executable.is_some() {
            InstanceState::Stopped
        } else {
            InstanceState::Unmanaged
        };
        let (state_tx, _) = watch::channel(initial);
        Arc::new(Self {
            instance_id: cfg.id.clone(),
            executable,
            args: cfg.args.clone(),
            working_dir: cfg.working_dir.clone(),
            inner: Mutex::new(Inner {
                child: None,
                started_at: None,
                stop_requested: false,
            }),
            state_tx,
        })
    }

    pub fn state(&self) -> InstanceState {
        self.state_tx.borrow().clone()
    }

    pub fn watch_state(&self) -> watch::Receiver<InstanceState> {
        self.state_tx.subscribe()
    }

    pub async fn uptime_seconds(&self) -> u64 {
        let inner = self.inner.lock().await;
        match (&*self.state_tx.borrow(), inner.started_at) {
            (InstanceState::Running, Some(t)) => t.elapsed().as_secs(),
            _ => 0,
        }
    }

    pub async fn start(self: &Arc<Self>) -> Result<()> {
        let Some(exe) = self.executable.clone() else {
            bail!("instance '{}' has no executable configured", self.instance_id);
        };
        if !exe.exists() {
            bail!("executable not found: {}", exe.display());
        }

        let mut inner = self.inner.lock().await;
        if matches!(*self.state_tx.borrow(), InstanceState::Running | InstanceState::Starting) {
            bail!("instance '{}' is already running", self.instance_id);
        }

        self.set_state(InstanceState::Starting);

        let workdir = self
            .working_dir
            .clone()
            .or_else(|| exe.parent().map(|p| p.to_path_buf()))
            .unwrap_or_else(|| PathBuf::from("."));

        let child = Command::new(&exe)
            .args(&self.args)
            .current_dir(&workdir)
            .stdin(Stdio::null())
            .stdout(Stdio::inherit())
            .stderr(Stdio::inherit())
            .spawn()
            .with_context(|| format!("spawning {}", exe.display()))?;

        let pid = child.id();
        inner.child = Some(child);
        inner.started_at = Some(Instant::now());
        inner.stop_requested = false;
        drop(inner);

        self.set_state(InstanceState::Running);
        tracing::info!(
            "instance '{}' started: {} (pid {:?})",
            self.instance_id,
            exe.display(),
            pid
        );

        // Monitor: reap the child and classify the exit.
        let sup = Arc::clone(self);
        tokio::spawn(async move { sup.monitor().await });
        Ok(())
    }

    async fn monitor(self: Arc<Self>) {
        // Take a waiter without holding the lock across the whole child
        // lifetime: Child::wait needs &mut, so the child stays in inner and
        // we poll it.
        loop {
            let status = {
                let mut inner = self.inner.lock().await;
                let Some(child) = inner.child.as_mut() else { return };
                match child.try_wait() {
                    Ok(Some(status)) => Some(status),
                    Ok(None) => None,
                    Err(e) => {
                        tracing::error!("instance '{}' wait failed: {e}", self.instance_id);
                        return;
                    }
                }
            };

            match status {
                Some(status) => {
                    let mut inner = self.inner.lock().await;
                    inner.child = None;
                    inner.started_at = None;
                    let ordered = inner.stop_requested;
                    inner.stop_requested = false;
                    drop(inner);

                    if ordered {
                        self.set_state(InstanceState::Stopped);
                        tracing::info!("instance '{}' stopped ({status})", self.instance_id);
                    } else {
                        let exit_code = status.code();
                        self.set_state(InstanceState::Crashed { exit_code });
                        tracing::warn!(
                            "instance '{}' exited unexpectedly ({status}) — marked crashed",
                            self.instance_id
                        );
                    }
                    return;
                }
                None => tokio::time::sleep(Duration::from_millis(500)).await,
            }
        }
    }

    pub async fn stop(self: &Arc<Self>) -> Result<()> {
        let mut inner = self.inner.lock().await;
        if inner.child.is_none() {
            bail!("instance '{}' is not running", self.instance_id);
        }
        inner.stop_requested = true;
        self.set_state(InstanceState::Stopping);
        let child = inner.child.as_mut().expect("checked above");

        // Graceful first: SIGTERM on unix; Windows has no SIGTERM equivalent
        // for console processes, so it goes straight to kill there.
        #[cfg(unix)]
        if let Some(pid) = child.id() {
            unsafe {
                libc::kill(pid as i32, libc::SIGTERM);
            }
        }
        #[cfg(not(unix))]
        {
            let _ = child.start_kill();
        }
        drop(inner);

        // Wait for the monitor to observe the exit; force kill on budget.
        let mut rx = self.watch_state();
        let deadline = tokio::time::timeout(GRACEFUL_STOP_BUDGET, async {
            loop {
                if matches!(*rx.borrow(), InstanceState::Stopped) {
                    return;
                }
                if rx.changed().await.is_err() {
                    return;
                }
            }
        })
        .await;

        if deadline.is_err() {
            tracing::warn!(
                "instance '{}' ignored SIGTERM for {}s — force killing",
                self.instance_id,
                GRACEFUL_STOP_BUDGET.as_secs()
            );
            let mut inner = self.inner.lock().await;
            if let Some(child) = inner.child.as_mut() {
                let _ = child.start_kill();
            }
            drop(inner);

            let mut rx = self.watch_state();
            let _ = tokio::time::timeout(Duration::from_secs(5), async {
                while !matches!(*rx.borrow(), InstanceState::Stopped) {
                    if rx.changed().await.is_err() {
                        break;
                    }
                }
            })
            .await;
        }
        Ok(())
    }

    pub async fn restart(self: &Arc<Self>) -> Result<()> {
        if !matches!(*self.state_tx.borrow(), InstanceState::Stopped | InstanceState::Crashed { .. } | InstanceState::Unmanaged) {
            self.stop().await?;
        }
        tokio::time::sleep(RESTART_PAUSE).await;
        self.start().await
    }

    fn set_state(&self, state: InstanceState) {
        // send_replace never fails even with zero receivers.
        let _ = self.state_tx.send_replace(state);
    }
}