//go:build linux

package control

import (
	"context"
	"log/slog"
	"net"
	"os"
	"time"
)

// sockPath lives in the unit's RuntimeDirectory; mode 0666 lets every
// local user ask, nothing can reach it from off the machine.
const sockPath = "/run/gpu-turnstile/control.sock"

// Serve starts the socket listener in the background and returns; only a
// setup failure is reported. Each client connection is answered in its own
// goroutine.
func Serve(ctx context.Context, h Handler, log *slog.Logger) error {
	os.Remove(sockPath) // stale socket from a previous run
	ln, err := net.Listen("unix", sockPath)
	if err != nil {
		return err
	}
	if err := os.Chmod(sockPath, 0o666); err != nil {
		ln.Close()
		return err
	}
	go func() {
		<-ctx.Done()
		ln.Close()
	}()
	go func() {
		for {
			c, err := ln.Accept()
			if err != nil {
				return // shutting down
			}
			uc := unixConn{c}
			if !serve(uc, h) {
				uc.ForceClose()
			}
		}
	}()
	return nil
}

// unixConn adds an abortive ForceClose to net.Conn: a deadline in the
// past fails pending and future I/O immediately.
type unixConn struct{ net.Conn }

func (c unixConn) ForceClose() error {
	c.SetDeadline(time.Now().Add(-time.Second)) //nolint:errcheck // best effort
	return c.Conn.Close()
}

// Ask sends one command to the running service and returns its reply.
func Ask(cmd string) (string, error) {
	c, err := net.DialTimeout("unix", sockPath, 2*time.Second)
	if err != nil {
		return "", ErrUnavailable
	}
	defer c.Close()
	return readReply(c, cmd)
}