//! Windows startup and shutdown: the SCM handshake.
//!
//! Unlike systemd, the Windows Service Control Manager cannot supervise an arbitrary console
//! program. A binary registered with `sc.exe create` has ~30 seconds to call
//! `StartServiceCtrlDispatcher` and connect back to the SCM; one that never does is killed with
//! **error 1053, "the service did not respond to the start request in a timely fashion"** — even
//! though the process itself started perfectly and is sitting there serving traffic. That is the
//! entire reason this module exists.
//!
//! ## One binary, two ways in
//!
//! The dispatcher is tried first and *failing is expected*: when the process was started from a
//! shell rather than by the SCM, the connect fails with `ERROR_FAILED_SERVICE_CONTROLLER_CONNECT`
//! (1063), and that — and only that — falls through to a normal foreground run. So
//! `uo-link-sidecar.exe --config ...` stays an ordinary console app you can Ctrl-C, `cargo run`
//! still works, and the same binary can be registered as a service with no `--service` flag for an
//! operator to forget. Any other dispatcher error is a real failure and is reported.
//!
//! ## Logging goes to a file, because a service has no stdout
//!
//! Under the SCM there is no console attached, so the normal stdout subscriber writes into the
//! void. In service mode the sidecar logs to a daily-rolled file next to its config instead
//! (`uo-link-sidecar.YYYY-MM-DD.log`, seven kept). A service whose start fails leaves a reason
//! behind rather than only an SCM error code.

use std::ffi::OsString;
use std::path::PathBuf;
use std::sync::Arc;
use std::sync::OnceLock;
use std::time::Duration;

use tokio::sync::Notify;
use tracing_subscriber::EnvFilter;
use windows_service::service::{
    ServiceControl, ServiceControlAccept, ServiceExitCode, ServiceState, ServiceStatus, ServiceType,
};
use windows_service::service_control_handler::{self, ServiceControlHandlerResult};
use windows_service::{define_windows_service, service_dispatcher};

/// Must match the name the installer registers (`installer/src/service.rs::WINDOWS_SERVICE`). For
/// an own-process service the SCM ignores it, but a mismatch would be a trap for whoever converts
/// this to a shared-process service later.
pub const SERVICE_NAME: &str = "RunicGatewayLink";

const SERVICE_TYPE: ServiceType = ServiceType::OWN_PROCESS;

/// `ERROR_FAILED_SERVICE_CONTROLLER_CONNECT` — "this process was not started by the SCM", which is
/// the normal answer when a human runs the binary.
const ERROR_FAILED_SERVICE_CONTROLLER_CONNECT: i32 = 1063;

/// `service_main` is called through an `extern "system"` trampoline and so can capture nothing.
/// The parsed `--config` is handed over here instead of being re-parsed, so the service and a
/// console run resolve their configuration through exactly the same code path.
static CONFIG_PATH: OnceLock<Option<String>> = OnceLock::new();

pub fn run(config_path: Option<&str>) -> anyhow::Result<()> {
    let _ = CONFIG_PATH.set(config_path.map(str::to_string));

    match service_dispatcher::start(SERVICE_NAME, ffi_service_main) {
        Ok(()) => Ok(()),
        // Not started by the SCM: this is a foreground run, which is not an error.
        Err(windows_service::Error::Winapi(e))
            if e.raw_os_error() == Some(ERROR_FAILED_SERVICE_CONTROLLER_CONNECT) =>
        {
            console_run(config_path)
        }
        Err(e) => Err(anyhow::Error::new(e)
            .context("could not connect to the Windows service control manager")),
    }
}

/// A normal foreground run: stdout logging, Ctrl-C to stop.
fn console_run(config_path: Option<&str>) -> anyhow::Result<()> {
    crate::init_console_tracing();
    tokio::runtime::Runtime::new()?.block_on(crate::app::run(config_path, || {}, async {
        let _ = tokio::signal::ctrl_c().await;
    }))
}

define_windows_service!(ffi_service_main, service_main);

fn service_main(_arguments: Vec<OsString>) {
    // Arguments are deliberately ignored: for an own-process service the `binPath=` arguments
    // arrive on the process command line and have already been parsed in `main`. What lands here
    // is whatever was typed after `sc start`, which nothing in this deployment uses.
    if let Err(e) = serve() {
        // Nowhere left to report to but the log: the status handle is gone or was never obtained.
        tracing::error!(error = %e, "service exited with an error");
    }
}

fn serve() -> anyhow::Result<()> {
    let config_path = CONFIG_PATH.get().cloned().flatten();
    // Held for the life of the service: dropping the guard stops the background log writer.
    let _log_guard = init_service_tracing(config_path.as_deref());

    // The SCM calls the control handler on its own thread, so the stop signal crosses a thread
    // boundary into the async world. `notify_one` stores a permit if nothing is waiting yet, so a
    // stop that arrives during startup is not lost.
    let stop = Arc::new(Notify::new());
    let handler_stop = stop.clone();
    let status_handle =
        service_control_handler::register(SERVICE_NAME, move |control| match control {
            ServiceControl::Interrogate => ServiceControlHandlerResult::NoError,
            ServiceControl::Stop | ServiceControl::Shutdown => {
                handler_stop.notify_one();
                ServiceControlHandlerResult::NoError
            }
            _ => ServiceControlHandlerResult::NotImplemented,
        })?;

    // Registering the handler is the handshake 1053 was about. Everything after this point gets to
    // take as long as it credibly needs, as long as the state keeps being reported.
    status_handle.set_service_status(ServiceStatus {
        service_type: SERVICE_TYPE,
        current_state: ServiceState::StartPending,
        controls_accepted: ServiceControlAccept::empty(),
        exit_code: ServiceExitCode::Win32(0),
        checkpoint: 0,
        wait_hint: Duration::from_secs(30),
        process_id: None,
    })?;

    let ready_handle = status_handle;
    let result = tokio::runtime::Runtime::new()?.block_on(crate::app::run(
        config_path.as_deref(),
        // Reported only once the shard port is bound and the store is open, so a bad config or a
        // taken port fails the *start* instead of flapping Running → Stopped a moment later.
        move || {
            let _ = ready_handle.set_service_status(ServiceStatus {
                service_type: SERVICE_TYPE,
                current_state: ServiceState::Running,
                controls_accepted: ServiceControlAccept::STOP | ServiceControlAccept::SHUTDOWN,
                exit_code: ServiceExitCode::Win32(0),
                checkpoint: 0,
                wait_hint: Duration::default(),
                process_id: None,
            });
        },
        async move { stop.notified().await },
    ));

    // A failed run must leave a nonzero SERVICE_EXIT_CODE behind: `sc query` reporting STOPPED with
    // exit code 0 is what made the original failure look like a clean stop.
    let exit_code = match &result {
        Ok(()) => ServiceExitCode::Win32(0),
        Err(e) => {
            tracing::error!(error = %e, "sidecar failed");
            ServiceExitCode::ServiceSpecific(1)
        }
    };
    status_handle.set_service_status(ServiceStatus {
        service_type: SERVICE_TYPE,
        current_state: ServiceState::Stopped,
        controls_accepted: ServiceControlAccept::empty(),
        exit_code,
        checkpoint: 0,
        wait_hint: Duration::default(),
        process_id: None,
    })?;

    result
}

/// Where the service writes its log: beside the config it was pointed at, which is the directory
/// the installer already provisions and grants the service account write access to.
fn log_dir(config_path: Option<&str>) -> PathBuf {
    if let Some(parent) = config_path
        .map(PathBuf::from)
        .as_deref()
        .and_then(|p| p.parent())
        .filter(|p| !p.as_os_str().is_empty())
    {
        return parent.to_path_buf();
    }
    match std::env::var_os("ProgramData") {
        Some(program_data) => PathBuf::from(program_data).join("RunicGateway"),
        None => std::env::temp_dir(),
    }
}

/// Returns `None` if the log file could not be opened — a service that cannot write a log is still
/// a service worth running, and the SCM start must not fail over it.
fn init_service_tracing(
    config_path: Option<&str>,
) -> Option<tracing_appender::non_blocking::WorkerGuard> {
    let appender = tracing_appender::rolling::Builder::new()
        .rotation(tracing_appender::rolling::Rotation::DAILY)
        .filename_prefix("uo-link-sidecar")
        .filename_suffix("log")
        .max_log_files(7)
        .build(log_dir(config_path))
        .ok()?;

    let (writer, guard) = tracing_appender::non_blocking(appender);
    tracing_subscriber::fmt()
        .with_env_filter(
            EnvFilter::try_from_default_env().unwrap_or_else(|_| EnvFilter::new("info")),
        )
        .with_ansi(false) // a log file is not a terminal
        .with_writer(writer)
        .init();
    Some(guard)
}

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

    #[test]
    fn log_dir_follows_the_config_file() {
        assert_eq!(
            log_dir(Some(r"C:\ProgramData\RunicGateway\sidecar.toml")),
            PathBuf::from(r"C:\ProgramData\RunicGateway")
        );
    }

    #[test]
    fn a_bare_filename_does_not_become_the_filesystem_root() {
        // `--config sidecar.toml` has a parent of "", which as a path means the root of the current
        // drive — somewhere a service account cannot write. Fall back instead.
        let dir = log_dir(Some("sidecar.toml"));
        assert_ne!(dir, PathBuf::from(""));
        assert!(dir.is_absolute(), "{}", dir.display());
    }

    #[test]
    fn no_config_falls_back_to_program_data() {
        let dir = log_dir(None);
        assert!(dir.is_absolute(), "{}", dir.display());
    }

    #[test]
    fn service_name_matches_the_installer() {
        // installer/src/service.rs::WINDOWS_SERVICE. Kept as a literal on both sides — the two
        // repos are released independently and do not share a crate.
        assert_eq!(SERVICE_NAME, "RunicGatewayLink");
    }
}