basic implementation of server selection feature

dialer/selection.go

@@ -0,0 +1,131 @@
+package dialer
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"io"
+	"math/rand/v2"
+	"net/http"
+	"strings"
+	"time"
+
+	"github.com/hashicorp/go-multierror"
+)
+
+type ServerSelection int
+
+const (
+	_ = iota
+	ServerSelectionFirst
+	ServerSelectionRandom
+	ServerSelectionFastest
+)
+
+func (ss ServerSelection) String() string {
+	switch ss {
+	case ServerSelectionFirst:
+		return "first"
+	case ServerSelectionRandom:
+		return "random"
+	case ServerSelectionFastest:
+		return "fastest"
+	default:
+		return fmt.Sprintf("ServerSelection(%d)", int(ss))
+	}
+}
+
+func ParseServerSelection(s string) (ServerSelection, error) {
+	switch strings.ToLower(s) {
+	case "first":
+		return ServerSelectionFirst, nil
+	case "random":
+		return ServerSelectionRandom, nil
+	case "fastest":
+		return ServerSelectionFastest, nil
+	}
+	return 0, errors.New("unknown server selection strategy")
+}
+
+type SelectionFunc = func(ctx context.Context, dialers []ContextDialer) (ContextDialer, error)
+
+func SelectFirst(_ context.Context, dialers []ContextDialer) (ContextDialer, error) {
+	if len(dialers) == 0 {
+		return nil, errors.New("empty dialers list")
+	}
+	return dialers[0], nil
+}
+
+func SelectRandom(_ context.Context, dialers []ContextDialer) (ContextDialer, error) {
+	if len(dialers) == 0 {
+		return nil, errors.New("empty dialers list")
+	}
+	return dialers[rand.IntN(len(dialers))], nil
+}
+
+func probeDialer(ctx context.Context, dialer ContextDialer, url string, dlLimit int64) error {
+	httpClient := http.Client{
+		Transport: &http.Transport{
+			MaxIdleConns:          100,
+			IdleConnTimeout:       90 * time.Second,
+			TLSHandshakeTimeout:   10 * time.Second,
+			ExpectContinueTimeout: 1 * time.Second,
+			DialContext:           dialer.DialContext,
+		},
+	}
+	req, err := http.NewRequestWithContext(ctx, "GET", url, nil)
+	if err != nil {
+		return err
+	}
+	resp, err := httpClient.Do(req)
+	if err != nil {
+		return err
+	}
+	defer resp.Body.Close()
+	if resp.StatusCode != http.StatusOK {
+		return fmt.Errorf("bad status code %d for URL %q", resp.StatusCode, url)
+	}
+	var rd io.Reader = resp.Body
+	if dlLimit > 0 {
+		rd = io.LimitReader(rd, dlLimit)
+	}
+	_, err = io.Copy(io.Discard, rd)
+	return err
+}
+
+func NewFastestServerSelectionFunc(url string, dlLimit int64) SelectionFunc {
+	return func(ctx context.Context, dialers []ContextDialer) (ContextDialer, error) {
+		var resErr error
+		masterNotInterested := make(chan struct{})
+		defer close(masterNotInterested)
+		errors := make(chan error)
+		success := make(chan ContextDialer)
+		for _, dialer := range dialers {
+			go func(dialer ContextDialer) {
+				err := probeDialer(ctx, dialer, url, dlLimit)
+				if err == nil {
+					select {
+					case success <- dialer:
+					case <-masterNotInterested:
+					}
+				} else {
+					select {
+					case errors <- err:
+					case <-masterNotInterested:
+					}
+				}
+			}(dialer)
+		}
+		for _ = range dialers {
+			select {
+			case <-ctx.Done():
+				return nil, ctx.Err()
+			case d := <-success:
+				return d, nil
+			case err := <-errors:
+				resErr = multierror.Append(resErr, err)
+			}
+		}
+		return nil, resErr
+	}
+}