//! Windows SYSTEM service host for the managed GuruConnect agent (SPEC-018).
//!
//! # Phase 1 scope (this module)
//!
//! Phase 1 proves the *managed/persistent* agent can run as **LocalSystem** in
//! the isolated Session 0 across reboots and at the login screen:
//!
//!   1. Register the agent with the Service Control Manager (SCM) and run, when
//!      started, the **existing persistent-agent logic** (`RunMode::PermanentAgent`
//!      path) *as SYSTEM* — i.e. resolve/enroll the per-machine `cak_` (SPEC-016,
//!      now readable because the SYSTEM-ACL'd store is in-context) and hold the
//!      relay WSS connection.
//!   2. Report a correct service lifecycle to the SCM (`StartPending` ->
//!      `Running` -> `StopPending` -> `Stopped`) and handle `Stop`/`Shutdown`
//!      gracefully. The control handler sets a shared shutdown flag; the agent
//!      runtime observes it both between reconnect attempts AND inside the
//!      connected session loop (SPEC-018 finding H), so a stop received while a
//!      session is live breaks out promptly, closes the WS connection cleanly,
//!      and exits — rather than waiting for the SCM to force-kill.
//!   3. Provide install/uninstall of the service (LocalSystem, auto-start, crash
//!      recovery) so managed mode uses the service as its single autostart
//!      instead of the per-user `HKCU\…\Run` entry.
//!
//! # Phase 2 (deliberately NOT built here — see SPEC-018 §Scope)
//!
//! A SYSTEM service lives in Session 0 and **cannot** capture or inject the
//! interactive desktop directly. Phase 1 therefore enrolls and connects but does
//! **NOT** capture a desktop yet. The following are Phase 2 and are intentionally
//! absent; the seams where they attach are called out inline below:
//!
//!   - the **session broker** (`WTSEnumerateSessionsW` /
//!     `WTSGetActiveConsoleSessionId` / `WTSQueryUserToken`),
//!   - the **per-session capture/input worker** spawned via `CreateProcessAsUserW`
//!     into `winsta0\default`,
//!   - **service <-> worker IPC** (the per-session ACL'd named pipe), and
//!   - **`SERVICE_CONTROL_SESSIONCHANGE`** reaction (logon/logoff/console-connect
//!     retarget).
//!
//! Phase 1 registers the control handler for `Stop`/`Shutdown`/`Interrogate`
//! only. When Phase 2 lands, the broker hangs off the same control handler
//! (adding `SESSIONCHANGE`) and off the same agent runtime started here.

#![cfg(windows)]

use std::ffi::OsString;
use std::sync::atomic::{AtomicBool, Ordering};
use std::sync::Arc;
use std::time::Duration;

use anyhow::{Context, Result};
use tracing::{error, info, warn};

use windows_service::{
    define_windows_service,
    service::{
        ServiceAccess, ServiceControl, ServiceControlAccept, ServiceErrorControl, ServiceExitCode,
        ServiceInfo, ServiceStartType, ServiceState, ServiceStatus, ServiceType,
    },
    service_control_handler::{self, ServiceControlHandlerResult},
    service_dispatcher,
    service_manager::{ServiceManager, ServiceManagerAccess},
};

/// Internal service name registered with the SCM (no spaces; used by `sc`,
/// `ServiceManager`, and the control handler).
pub const SERVICE_NAME: &str = "GuruConnectAgent";

/// Human-facing display name shown in `services.msc`.
pub const SERVICE_DISPLAY_NAME: &str = "GuruConnect Managed Agent";

/// Service description shown in `services.msc`.
pub const SERVICE_DESCRIPTION: &str =
    "Runs the managed GuruConnect remote-support agent as LocalSystem so it is \
     reachable at the login screen and across reboots (SPEC-018).";

/// Hidden subcommand the SCM invokes to enter the service control loop. The
/// service is registered with this as its launch argument (see [`install_service`]),
/// and `main.rs` routes it into [`run_dispatcher`].
pub const SERVICE_RUN_ARG: &str = "service-run";

/// Hint we give the SCM for how long start/stop transitions may take before it
/// should consider the service hung.
const TRANSITION_WAIT: Duration = Duration::from_secs(10);

// The `windows-service` dispatcher requires a `extern "system"` entry point with
// a fixed ABI; this macro generates `ffi_service_main`, which trampolines into
// our safe `service_main`.
define_windows_service!(ffi_service_main, service_main);

/// Enter the SCM dispatcher (called from `main.rs` for the `service-run`
/// subcommand). Blocks until the service stops. This must be invoked by the SCM,
/// not interactively — `service_dispatcher::start` fails with
/// `ERROR_FAILED_SERVICE_CONTROLLER_CONNECT` (1063) if there is no controlling
/// SCM, which is the expected outcome of running `guruconnect service-run` by hand.
pub fn run_dispatcher() -> Result<()> {
    service_dispatcher::start(SERVICE_NAME, ffi_service_main)
        .context("failed to connect to the service control dispatcher (must be started by the SCM)")
}

/// SCM-invoked service body. Any error is logged; the function cannot return an
/// error to the SCM directly, so [`run_service`] reports a failed exit code on the
/// status handle before returning.
fn service_main(_arguments: Vec<OsString>) {
    if let Err(e) = run_service() {
        error!("service exited with error: {e:#}");
    }
}

/// Drive the full service lifecycle: register the control handler, report
/// `Running`, run the persistent agent until a stop is requested, then report
/// `Stopped`.
fn run_service() -> Result<()> {
    info!("GuruConnect managed agent service starting (running as SYSTEM in session 0)");

    // Cooperative shutdown flag flipped by the SCM control handler and observed by
    // the agent runtime. `AtomicBool` keeps the handler closure trivially `Send`
    // and avoids holding a lock inside an SCM callback.
    let shutdown = Arc::new(AtomicBool::new(false));
    let shutdown_for_handler = shutdown.clone();

    let event_handler = move |control_event| -> ServiceControlHandlerResult {
        match control_event {
            // SPEC-018 Phase 1: graceful stop. Phase 2 adds
            // `ServiceControl::SessionChange(_)` here to drive the session broker
            // (retarget the capture/input worker on logon/logoff/console-connect);
            // we intentionally do not accept SESSIONCHANGE yet.
            ServiceControl::Stop | ServiceControl::Shutdown => {
                info!("received {control_event:?}; signalling agent to shut down");
                // Set the cooperative-stop flag. The agent runtime observes it on
                // every idle tick of the connected session loop and between
                // reconnect attempts (SPEC-018 finding H), so it breaks out and
                // closes the WebSocket cleanly within ~100ms even if a session is
                // currently connected.
                shutdown_for_handler.store(true, Ordering::SeqCst);
                ServiceControlHandlerResult::NoError
            }
            ServiceControl::Interrogate => ServiceControlHandlerResult::NoError,
            _ => ServiceControlHandlerResult::NotImplemented,
        }
    };

    let status_handle = service_control_handler::register(SERVICE_NAME, event_handler)
        .context("failed to register the service control handler")?;

    // Report StartPending while we spin up the runtime and connect.
    set_status(
        &status_handle,
        ServiceState::StartPending,
        ServiceControlAccept::empty(),
        TRANSITION_WAIT,
    );

    // Report Running and accept Stop + Shutdown. We report Running before the
    // first connect attempt completes because the agent loop reconnects forever;
    // "the service is up and trying" is the correct steady state, and blocking the
    // SCM on the first relay handshake would risk a start timeout on a slow boot.
    set_status(
        &status_handle,
        ServiceState::Running,
        ServiceControlAccept::STOP | ServiceControlAccept::SHUTDOWN,
        Duration::default(),
    );
    info!("service reported Running; entering managed-agent control loop");

    // Run the existing persistent-agent logic as SYSTEM. This is the Phase 1
    // payload: resolve/enroll the cak_ (SPEC-016) and hold the relay connection.
    let run_result = crate::run_managed_agent_service(shutdown.clone());

    if let Err(e) = &run_result {
        // The agent loop only returns Err on an unrecoverable LOCAL fault (e.g. no
        // usable credential and nothing to enroll with). Network errors are
        // retried inside the loop and never surface here. Report the failure to
        // the SCM so recovery actions (restart) engage.
        error!("managed-agent control loop terminated with error: {e:#}");
    } else {
        info!("managed-agent control loop exited cleanly on stop request");
    }

    // Transition StopPending -> Stopped.
    set_status(
        &status_handle,
        ServiceState::StopPending,
        ServiceControlAccept::empty(),
        TRANSITION_WAIT,
    );

    let exit_code = match run_result {
        Ok(()) => ServiceExitCode::Win32(0),
        // ERROR_SERVICE_SPECIFIC_ERROR-style: surface a non-zero service-specific
        // code so the SCM treats the exit as a failure and applies recovery.
        Err(_) => ServiceExitCode::ServiceSpecific(1),
    };

    set_status_with_exit(
        &status_handle,
        ServiceState::Stopped,
        ServiceControlAccept::empty(),
        Duration::default(),
        exit_code,
    );
    info!("service reported Stopped");

    Ok(())
}

/// Report a status with a zero (success) exit code.
fn set_status(
    handle: &service_control_handler::ServiceStatusHandle,
    state: ServiceState,
    accepted: ServiceControlAccept,
    wait_hint: Duration,
) {
    set_status_with_exit(
        handle,
        state,
        accepted,
        wait_hint,
        ServiceExitCode::Win32(0),
    );
}

/// Report a status to the SCM. A failure to report is logged (best-effort) — we
/// cannot do anything actionable about it and must not panic inside the service.
fn set_status_with_exit(
    handle: &service_control_handler::ServiceStatusHandle,
    state: ServiceState,
    accepted: ServiceControlAccept,
    wait_hint: Duration,
    exit_code: ServiceExitCode,
) {
    let status = ServiceStatus {
        service_type: ServiceType::OWN_PROCESS,
        current_state: state,
        controls_accepted: accepted,
        exit_code,
        checkpoint: 0,
        wait_hint,
        process_id: None,
    };
    if let Err(e) = handle.set_service_status(status) {
        warn!("failed to report service status {state:?} to the SCM: {e}");
    }
}

// ---------------------------------------------------------------------------
// Install / uninstall (used by install.rs for managed mode)
// ---------------------------------------------------------------------------

/// Install (or reinstall) the managed agent as a LocalSystem auto-start service
/// pointing at `exe_path` with the [`SERVICE_RUN_ARG`] launch argument.
///
/// Idempotent: if the service already exists it is stopped and deleted first,
/// then recreated, so an upgrade picks up a new binary path / config. Configures
/// crash recovery (restart on failure) via `sc failure`.
///
/// Requires Administrator (SCM `CREATE_SERVICE`). Returns an error otherwise.
pub fn install_service(exe_path: &std::path::Path) -> Result<()> {
    let manager = ServiceManager::local_computer(
        None::<&str>,
        ServiceManagerAccess::CONNECT | ServiceManagerAccess::CREATE_SERVICE,
    )
    .context("failed to connect to the Service Control Manager (run as Administrator)")?;

    // Remove any prior installation so the binary path / args are refreshed.
    let mut deleted_existing = false;
    if let Ok(existing) = manager.open_service(
        SERVICE_NAME,
        ServiceAccess::QUERY_STATUS | ServiceAccess::STOP | ServiceAccess::DELETE,
    ) {
        info!("existing {SERVICE_NAME} service found; removing before reinstall");
        stop_if_running(&existing);
        existing
            .delete()
            .context("failed to delete the existing service before reinstall")?;
        drop(existing);
        deleted_existing = true;
    }

    let service_info = ServiceInfo {
        name: OsString::from(SERVICE_NAME),
        display_name: OsString::from(SERVICE_DISPLAY_NAME),
        service_type: ServiceType::OWN_PROCESS,
        start_type: ServiceStartType::AutoStart,
        error_control: ServiceErrorControl::Normal,
        executable_path: exe_path.to_path_buf(),
        launch_arguments: vec![OsString::from(SERVICE_RUN_ARG)],
        dependencies: vec![],
        // account_name: None => LocalSystem (the SPEC-018 requirement).
        account_name: None,
        account_password: None,
    };

    let service = create_service_with_retry(&manager, &service_info, deleted_existing)
        .context("failed to create the GuruConnect managed agent service")?;

    service
        .set_description(SERVICE_DESCRIPTION)
        .context("failed to set the service description")?;

    configure_recovery();

    info!(
        "installed {SERVICE_NAME} (LocalSystem, auto-start) -> {} {}",
        exe_path.display(),
        SERVICE_RUN_ARG
    );
    Ok(())
}

/// Create the service, retrying briefly if the SCM still has the prior instance
/// "marked for deletion" (SPEC-018 finding L1).
///
/// When a service is deleted, the SCM only removes it from its database once every
/// open handle to it closes; until then a fresh `CreateService` fails with
/// `ERROR_SERVICE_MARKED_FOR_DELETE` (1072). The previous implementation papered
/// over this with a fixed 2s sleep after `delete()`, which is both slower than
/// necessary in the common case and still racy on a busy box. Instead we attempt
/// the create immediately and, only if we just deleted an existing instance and
/// hit 1072, retry a few times with short backoff — succeeding as soon as the SCM
/// finishes the removal, and giving up with the real error if it never does.
///
/// The retry is gated on `deleted_existing`: on a clean first install there was no
/// prior instance, so a 1072 there is unexpected and is surfaced immediately
/// rather than masked by retries.
fn create_service_with_retry(
    manager: &ServiceManager,
    service_info: &ServiceInfo,
    deleted_existing: bool,
) -> Result<windows_service::service::Service, windows_service::Error> {
    // ERROR_SERVICE_MARKED_FOR_DELETE (winerror.h). The service is gone from the
    // caller's perspective but the SCM has not finished reaping it.
    const ERROR_SERVICE_MARKED_FOR_DELETE: i32 = 1072;
    // Bounded: ~5 attempts over ~2s total worst case (matches the old fixed sleep
    // ceiling) but returns the instant the SCM is ready.
    const MAX_ATTEMPTS: u32 = 5;
    const BACKOFF: Duration = Duration::from_millis(400);

    let mut attempt = 0;
    loop {
        attempt += 1;
        match manager.create_service(service_info, ServiceAccess::CHANGE_CONFIG) {
            Ok(service) => return Ok(service),
            Err(windows_service::Error::Winapi(ref io_err))
                if deleted_existing
                    && io_err.raw_os_error() == Some(ERROR_SERVICE_MARKED_FOR_DELETE)
                    && attempt < MAX_ATTEMPTS =>
            {
                warn!(
                    "{SERVICE_NAME} still marked for deletion by the SCM \
                     (attempt {attempt}/{MAX_ATTEMPTS}); retrying in {}ms",
                    BACKOFF.as_millis()
                );
                std::thread::sleep(BACKOFF);
            }
            Err(e) => return Err(e),
        }
    }
}

/// Configure SCM crash-recovery so the service restarts on unexpected exit.
///
/// `windows-service` 0.7 does not expose `ChangeServiceConfig2` recovery actions
/// in a stable, ergonomic form, so we mirror the established pattern used by the
/// SAS service binary and shell out to `sc failure`. `reset=86400` clears the
/// failure count after a day; three `restart/5000` actions retry after 5s each.
fn configure_recovery() {
    use std::os::windows::process::CommandExt;
    const CREATE_NO_WINDOW: u32 = 0x0800_0000;

    match std::process::Command::new("sc")
        .args([
            "failure",
            SERVICE_NAME,
            "reset=86400",
            "actions=restart/5000/restart/5000/restart/5000",
        ])
        .creation_flags(CREATE_NO_WINDOW)
        .output()
    {
        Ok(out) if out.status.success() => {
            info!("configured crash-recovery (restart) for {SERVICE_NAME}");
        }
        Ok(out) => {
            warn!(
                "could not configure crash-recovery for {SERVICE_NAME} (sc failure exit {:?}); \
                 the service will still run but will not auto-restart on crash",
                out.status.code()
            );
        }
        Err(e) => {
            warn!("could not invoke `sc failure` to set crash-recovery for {SERVICE_NAME}: {e}");
        }
    }
}

/// Stop (if running) and delete the managed agent service. Idempotent: succeeds
/// quietly if the service is not installed.
pub fn uninstall_service() -> Result<()> {
    let manager = ServiceManager::local_computer(None::<&str>, ServiceManagerAccess::CONNECT)
        .context("failed to connect to the Service Control Manager (run as Administrator)")?;

    match manager.open_service(
        SERVICE_NAME,
        ServiceAccess::QUERY_STATUS | ServiceAccess::STOP | ServiceAccess::DELETE,
    ) {
        Ok(service) => {
            stop_if_running(&service);
            service
                .delete()
                .context("failed to delete the managed agent service")?;
            info!("uninstalled {SERVICE_NAME} service");
            Ok(())
        }
        Err(_) => {
            // Not installed — nothing to do (idempotent uninstall).
            info!("{SERVICE_NAME} service is not installed; nothing to uninstall");
            Ok(())
        }
    }
}

/// Start the managed agent service now (used right after a first-run install so
/// the agent comes up without waiting for the next boot). Best-effort: logs and
/// returns the SCM error if the start fails, but a failure is not fatal to install
/// because the service is auto-start and will come up on the next boot regardless.
pub fn start_service() -> Result<()> {
    let manager = ServiceManager::local_computer(None::<&str>, ServiceManagerAccess::CONNECT)
        .context("failed to connect to the Service Control Manager")?;
    let service = manager
        .open_service(
            SERVICE_NAME,
            ServiceAccess::START | ServiceAccess::QUERY_STATUS,
        )
        .context("failed to open the managed agent service to start it")?;

    // If it is already running (e.g. reinstall-over-running), there is nothing to do.
    if let Ok(status) = service.query_status() {
        if status.current_state == ServiceState::Running
            || status.current_state == ServiceState::StartPending
        {
            info!("{SERVICE_NAME} is already running/starting");
            return Ok(());
        }
    }

    service
        .start::<String>(&[])
        .context("failed to start the managed agent service")?;
    info!("started {SERVICE_NAME}");
    Ok(())
}

/// Report whether the managed agent service is currently installed.
pub fn is_service_installed() -> bool {
    match ServiceManager::local_computer(None::<&str>, ServiceManagerAccess::CONNECT) {
        Ok(manager) => manager
            .open_service(SERVICE_NAME, ServiceAccess::QUERY_STATUS)
            .is_ok(),
        Err(_) => false,
    }
}

/// Best-effort stop of a service, waiting briefly for it to leave the running
/// state so a subsequent `delete` does not race an in-flight stop.
fn stop_if_running(service: &windows_service::service::Service) {
    if let Ok(status) = service.query_status() {
        if status.current_state != ServiceState::Stopped {
            info!("stopping {SERVICE_NAME} before delete");
            let _ = service.stop();
            for _ in 0..10 {
                std::thread::sleep(Duration::from_millis(500));
                match service.query_status() {
                    Ok(s) if s.current_state == ServiceState::Stopped => break,
                    _ => continue,
                }
            }
        }
    }
}

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

    /// The launch argument the service is registered with MUST equal the hidden
    /// `service-run` subcommand `main.rs` dispatches into [`run_dispatcher`]; a
    /// mismatch would register a service the SCM could start but that would fall
    /// through to normal (non-service) mode and immediately exit.
    ///
    /// This pins the value of the constant itself. The companion test
    /// `tests::service_run_subcommand_matches_scm_launch_arg` in `main.rs` pins the
    /// other half — that the clap `#[command(name = "service-run")]` attribute on
    /// `Commands::ServiceRun` resolves to this same constant — so the two string
    /// literals cannot silently drift apart.
    #[test]
    fn service_run_arg_matches_subcommand_name() {
        assert_eq!(SERVICE_RUN_ARG, "service-run");
    }

    /// Service identifiers are non-empty and the internal name carries no spaces
    /// (the SCM key / `sc` argument must be a single token).
    #[test]
    fn service_identifiers_are_well_formed() {
        assert!(!SERVICE_NAME.is_empty());
        assert!(
            !SERVICE_NAME.contains(char::is_whitespace),
            "the SCM service name must be a single whitespace-free token"
        );
        assert!(!SERVICE_DISPLAY_NAME.is_empty());
        assert!(!SERVICE_DESCRIPTION.is_empty());
    }

    /// `is_service_installed` must never panic regardless of elevation/SCM access;
    /// on a dev workstation without the service installed it returns `false`. (We
    /// do NOT install the service in tests — that is a VM/admin integration step.)
    #[test]
    fn is_service_installed_is_total() {
        let _ = is_service_installed();
    }
}