dnswatcher/internal/portcheck/portcheck.go

// Package portcheck provides TCP port connectivity checking.
package portcheck

import (
	"context"
	"errors"
	"fmt"
	"log/slog"
	"net"
	"strconv"
	"sync"
	"time"

	"go.uber.org/fx"
	"golang.org/x/sync/errgroup"

	"sneak.berlin/go/dnswatcher/internal/logger"
)

const (
	minPort        = 1
	maxPort        = 65535
	defaultTimeout = 5 * time.Second
)

// ErrInvalidPort is returned when a port number is outside
// the valid TCP range (1–65535).
var ErrInvalidPort = errors.New("invalid port number")

// PortResult holds the outcome of a single TCP port check.
type PortResult struct {
	// Open indicates whether the port accepted a connection.
	Open bool
	// Error contains a description if the connection failed.
	Error string
	// Latency is the time taken for the TCP handshake.
	Latency time.Duration
}

// Params contains dependencies for Checker.
type Params struct {
	fx.In

	Logger *logger.Logger
}

// Checker performs TCP port connectivity checks.
type Checker struct {
	log *slog.Logger
}

// New creates a new port Checker instance.
func New(
	_ fx.Lifecycle,
	params Params,
) (*Checker, error) {
	return &Checker{
		log: params.Logger.Get(),
	}, nil
}

// NewStandalone creates a Checker without fx dependencies.
func NewStandalone() *Checker {
	return &Checker{
		log: slog.Default(),
	}
}

// validatePort checks that a port number is within the valid
// TCP port range (1–65535).
func validatePort(port int) error {
	if port < minPort || port > maxPort {
		return fmt.Errorf(
			"%w: %d (must be between %d and %d)",
			ErrInvalidPort, port, minPort, maxPort,
		)
	}

	return nil
}

// CheckPort tests TCP connectivity to the given address and port.
// It uses a 5-second timeout unless the context has an earlier
// deadline.
func (c *Checker) CheckPort(
	ctx context.Context,
	address string,
	port int,
) (*PortResult, error) {
	err := validatePort(port)
	if err != nil {
		return nil, err
	}

	target := net.JoinHostPort(
		address, strconv.Itoa(port),
	)

	timeout := defaultTimeout

	deadline, hasDeadline := ctx.Deadline()
	if hasDeadline {
		remaining := time.Until(deadline)
		if remaining < timeout {
			timeout = remaining
		}
	}

	return c.checkConnection(ctx, target, timeout), nil
}

// CheckPorts tests TCP connectivity to multiple ports on the
// given address concurrently. It returns a map of port number
// to result.
func (c *Checker) CheckPorts(
	ctx context.Context,
	address string,
	ports []int,
) (map[int]*PortResult, error) {
	for _, port := range ports {
		err := validatePort(port)
		if err != nil {
			return nil, err
		}
	}

	var mu sync.Mutex

	results := make(map[int]*PortResult, len(ports))

	g, ctx := errgroup.WithContext(ctx)

	for _, port := range ports {
		g.Go(func() error {
			result, err := c.CheckPort(ctx, address, port)
			if err != nil {
				return fmt.Errorf(
					"checking port %d: %w", port, err,
				)
			}

			mu.Lock()
			results[port] = result
			mu.Unlock()

			return nil
		})
	}

	err := g.Wait()
	if err != nil {
		return nil, err
	}

	return results, nil
}

// checkConnection performs the TCP dial and returns a result.
func (c *Checker) checkConnection(
	ctx context.Context,
	target string,
	timeout time.Duration,
) *PortResult {
	dialer := &net.Dialer{Timeout: timeout}
	start := time.Now()

	conn, dialErr := dialer.DialContext(ctx, "tcp", target)
	latency := time.Since(start)

	if dialErr != nil {
		c.log.Debug(
			"port check failed",
			"target", target,
			"error", dialErr.Error(),
		)

		return &PortResult{
			Open:    false,
			Error:   dialErr.Error(),
			Latency: latency,
		}
	}

	closeErr := conn.Close()
	if closeErr != nil {
		c.log.Debug(
			"closing connection",
			"target", target,
			"error", closeErr.Error(),
		)
	}

	c.log.Debug(
		"port check succeeded",
		"target", target,
		"latency", latency,
	)

	return &PortResult{
		Open:    true,
		Latency: latency,
	}
}