make LBSelector interface and implement all the current methods plus roundrobin

app/main.go

@@ -6,7 +6,6 @@ import (
 	"fmt"
 	"io"
 	"math"
-	"math/rand"
 	"net/http"
 	"net/rpc"
 	"os"
@@ -36,7 +35,7 @@ var opts struct {
 	DropHeaders         []string `long:"drop-header" env:"DROP_HEADERS" description:"incoming headers to drop" env-delim:","`
 	AuthBasicHtpasswd   string   `long:"basic-htpasswd" env:"BASIC_HTPASSWD" description:"htpasswd file for basic auth"`
 	RemoteLookupHeaders bool     `long:"remote-lookup-headers" env:"REMOTE_LOOKUP_HEADERS" description:"enable remote lookup headers"`
-	LBType              string   `long:"lb-type" env:"LB_TYPE" description:"load balancer type" choice:"random" choice:"failover" default:"random"` // nolint
+	LBType              string   `long:"lb-type" env:"LB_TYPE" description:"load balancer type" choice:"random" choice:"failover" choice:"roundrobin" default:"random"` // nolint
 
 	SSL struct {
 		Type          string   `long:"type" env:"TYPE" description:"ssl (auto) support" choice:"none" choice:"static" choice:"auto" default:"none"` // nolint
@@ -414,14 +413,16 @@ func makeSSLConfig() (config proxy.SSLConfig, err error) {
 	return config, err
 }
 
-func makeLBSelector() func(len int) int {
+func makeLBSelector() proxy.LBSelector {
 	switch opts.LBType {
 	case "random":
-		return rand.Intn
+		return &proxy.RandomSelector{}
 	case "failover":
-		return func(int) int { return 0 } // dead server won't be in the list, we can safely pick the first one
+		return &proxy.FailoverSelector{}
+	case "roundrobin":
+		return &proxy.RoundRobinSelector{}
 	default:
-		return func(int) int { return 0 }
+		return &proxy.FailoverSelector{}
 	}
 }
 

app/proxy/lb_selector.go

@@ -0,0 +1,45 @@
+package proxy
+
+import (
+	"math/rand"
+	"sync"
+)
+
+// RoundRobinSelector is a simple round-robin selector, thread-safe
+type RoundRobinSelector struct {
+	lastSelected int
+	mu           sync.Mutex
+}
+
+// Select returns next backend index
+func (r *RoundRobinSelector) Select(n int) int {
+	r.mu.Lock()
+	defer r.mu.Unlock()
+	selected := r.lastSelected
+	r.lastSelected = (r.lastSelected + 1) % n
+	return selected
+}
+
+// RandomSelector is a random selector, thread-safe
+type RandomSelector struct{}
+
+// Select returns random backend index
+func (r *RandomSelector) Select(n int) int {
+	return rand.Intn(n) //nolint:gosec // no need for crypto/rand here
+}
+
+// FailoverSelector is a selector with failover, thread-safe
+type FailoverSelector struct{}
+
+// Select returns next backend index
+func (r *FailoverSelector) Select(_ int) int {
+	return 0 // dead server won't be in the list, we can safely pick the first one
+}
+
+// LBSelectorFunc is a functional adapted for LBSelector to select backend from the list
+type LBSelectorFunc func(n int) int
+
+// Select returns backend index
+func (f LBSelectorFunc) Select(n int) int {
+	return f(n)
+}

app/proxy/lb_selector_test.go

@@ -0,0 +1,121 @@
+package proxy
+
+import (
+	"sync"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestRoundRobinSelector_Select(t *testing.T) {
+	selector := &RoundRobinSelector{}
+
+	testCases := []struct {
+		name     string
+		len      int
+		expected int
+	}{
+		{"First call", 3, 0},
+		{"Second call", 3, 1},
+		{"Third call", 3, 2},
+		{"Back to zero", 3, 0},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			result := selector.Select(tc.len)
+			assert.Equal(t, tc.expected, result)
+		})
+	}
+}
+
+func TestRoundRobinSelector_SelectConcurrent(t *testing.T) {
+	selector := &RoundRobinSelector{}
+	l := 3
+	numGoroutines := 1000
+
+	var wg sync.WaitGroup
+	wg.Add(numGoroutines)
+
+	results := &sync.Map{}
+
+	for i := 0; i < numGoroutines; i++ {
+		go func() {
+			defer wg.Done()
+			result := selector.Select(l)
+			results.Store(result, struct{}{})
+		}()
+	}
+
+	wg.Wait()
+
+	// check that all possible results are present in the map.
+	for i := 0; i < l; i++ {
+		_, ok := results.Load(i)
+		assert.True(t, ok, "expected to find %d in the results", i)
+	}
+}
+
+func TestRandomSelector_Select(t *testing.T) {
+	selector := &RandomSelector{}
+
+	testCases := []struct {
+		name string
+		len  int
+	}{
+		{"First call", 5},
+		{"Second call", 5},
+		{"Third call", 5},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			result := selector.Select(tc.len)
+			assert.True(t, result >= 0 && result < tc.len)
+		})
+	}
+}
+
+func TestFailoverSelector_Select(t *testing.T) {
+	selector := &FailoverSelector{}
+
+	testCases := []struct {
+		name     string
+		len      int
+		expected int
+	}{
+		{"First call", 5, 0},
+		{"Second call", 5, 0},
+		{"Third call", 5, 0},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			result := selector.Select(tc.len)
+			assert.Equal(t, tc.expected, result)
+		})
+	}
+}
+
+func TestLBSelectorFunc_Select(t *testing.T) {
+	selector := LBSelectorFunc(func(n int) int {
+		return n - 1 // simple selection logic for testing
+	})
+
+	testCases := []struct {
+		name     string
+		len      int
+		expected int
+	}{
+		{"First call", 5, 4},
+		{"Second call", 3, 2},
+		{"Third call", 1, 0},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			result := selector.Select(tc.len)
+			assert.Equal(t, tc.expected, result)
+		})
+	}
+}

app/proxy/proxy.go

@@ -5,7 +5,6 @@ import (
 	"context"
 	"fmt"
 	"io"
-	"math/rand"
 	"net"
 	"net/http"
 	"net/http/httputil"
@@ -47,7 +46,7 @@ type Http struct { // nolint golint
 	Metrics          MiddlewareProvider
 	PluginConductor  MiddlewareProvider
 	Reporter         Reporter
-	LBSelector       func(len int) int
+	LBSelector       LBSelector
 	OnlyFrom         *OnlyFrom
 	BasicAuthEnabled bool
 	BasicAuthAllowed []string
@@ -75,6 +74,11 @@ type Reporter interface {
 	Report(w http.ResponseWriter, code int)
 }
 
+// LBSelector defines load balancer strategy
+type LBSelector interface {
+	Select(len int) int // return index of picked server
+}
+
 // Timeouts consolidate timeouts for both server and transport
 type Timeouts struct {
 	// server timeouts
@@ -101,7 +105,7 @@ func (h *Http) Run(ctx context.Context) error {
 	}
 
 	if h.LBSelector == nil {
-		h.LBSelector = rand.Intn
+		h.LBSelector = &RandomSelector{}
 	}
 
 	var httpServer, httpsServer *http.Server
@@ -277,7 +281,7 @@ func (h *Http) proxyHandler() http.HandlerFunc {
 // and if match found sets it to the request context. Context used by proxy handler as well as by plugin conductor
 func (h *Http) matchHandler(next http.Handler) http.Handler {
 
-	getMatch := func(mm discovery.Matches, picker func(len int) int) (m discovery.MatchedRoute, ok bool) {
+	getMatch := func(mm discovery.Matches, picker LBSelector) (m discovery.MatchedRoute, ok bool) {
 		if len(mm.Routes) == 0 {
 			return m, false
 		}
@@ -294,7 +298,7 @@ func (h *Http) matchHandler(next http.Handler) http.Handler {
 		case 1:
 			return matches[0], true
 		default:
-			return matches[picker(len(matches))], true
+			return matches[picker.Select(len(matches))], true
 		}
 	}
 

app/proxy/proxy_test.go

@@ -874,7 +874,7 @@ func TestHttp_matchHandler(t *testing.T) {
 	client := http.Client{}
 	for _, tt := range tbl {
 		t.Run(tt.name, func(t *testing.T) {
-			h := Http{Matcher: matcherMock, LBSelector: func(len int) int { return 0 }}
+			h := Http{Matcher: matcherMock, LBSelector: &FailoverSelector{}}
 			handler := h.matchHandler(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 				t.Logf("req: %+v", r)
 				t.Logf("dst: %v", r.Context().Value(ctxURL))