feat: socks5 proxy server (#3336)

- `SOCKS5_ENABLED=off`
- `SOCKS5_LISTENING_ADDRESS=":1080"`
- `SOCKS5_USER=`
- `SOCKS5_PASSWORD=`

internal/socks5/loop.go

@@ -0,0 +1,106 @@
+package socks5
+
+import (
+	"context"
+	"sync"
+	"time"
+
+	"github.com/qdm12/goservices"
+)
+
+type Loop struct {
+	settings Settings
+
+	mutex     sync.Mutex
+	runCancel context.CancelFunc
+	runDone   <-chan error
+}
+
+func NewLoop(settings Settings) *Loop {
+	return &Loop{
+		settings: settings,
+	}
+}
+
+func (l *Loop) String() string {
+	return "SOCKS5 server loop"
+}
+
+func (l *Loop) Start(_ context.Context) (runError <-chan error, err error) {
+	l.mutex.Lock()
+	defer l.mutex.Unlock()
+
+	var runCtx context.Context
+	runCtx, l.runCancel = context.WithCancel(context.Background())
+
+	runDone := make(chan error)
+	l.runDone = runDone
+
+	go run(runCtx, runDone, l.settings)
+
+	return nil, nil //nolint:nilnil
+}
+
+func run(ctx context.Context, done chan<- error, settings Settings) {
+	defer close(done)
+	logger := settings.Logger
+
+	for ctx.Err() == nil {
+		var server goservices.Service
+		if settings.Enabled {
+			server = newServer(settings)
+		} else {
+			server = new(noopService)
+		}
+
+		errorCh, err := server.Start(ctx)
+		if err != nil {
+			logger.Warnf("failed starting SOCKS5 server: %s", err)
+			waitBeforeRetry(ctx)
+			continue
+		}
+
+		select {
+		case <-ctx.Done():
+			done <- server.Stop()
+			return
+		case err := <-errorCh:
+			if ctx.Err() != nil {
+				return
+			}
+			logger.Warnf("SOCKS5 server crashed: %s", err)
+			waitBeforeRetry(ctx)
+		}
+	}
+}
+
+func (l *Loop) Stop() (err error) {
+	l.mutex.Lock()
+	defer l.mutex.Unlock()
+
+	l.runCancel()
+	return <-l.runDone
+}
+
+func waitBeforeRetry(ctx context.Context) {
+	const retryDelay = 10 * time.Second
+	timer := time.NewTimer(retryDelay)
+	select {
+	case <-timer.C:
+	case <-ctx.Done():
+	}
+}
+
+type noopService struct{}
+
+func (s noopService) Start(_ context.Context) (runErr <-chan error, err error) {
+	return nil, nil //nolint:nilnil
+}
+
+func (s noopService) Stop() error {
+	return nil
+}
+
+func (s noopService) String() string {
+	return "noop service"
+}