fix(config): use three-state atomic for rebootstrap to prevent data race

Replace boolean rebootstrap flag with a three-state atomic integer to
prevent concurrent SetupTransport calls during rebootstrap. The atomic
state machine ensures only one goroutine can proceed from "started" to
"in progress", eliminating the need for a mutex while maintaining
thread safety.

States: NotStarted -> Started -> InProgress -> NotStarted

Note that the race condition is still acceptable because any additional
transports created during the race are functional. Once the connection
is established, the unused transports are safely handled by the garbage
collector.

config.go

@@ -82,6 +82,10 @@ const (
 	endpointPrefixQUIC  = "quic://"
 	endpointPrefixH3    = "h3://"
 	endpointPrefixSdns  = "sdns://"
+
+	rebootstrapNotStarted = 0
+	rebootstrapStarted    = 1
+	rebootstrapInProgress = 2
 )
 
 var (
@@ -270,7 +274,7 @@ type UpstreamConfig struct {
 	Discoverable *bool `mapstructure:"discoverable" toml:"discoverable"`
 
 	g                  singleflight.Group
-	rebootstrap        atomic.Bool
+	rebootstrap        atomic.Int64
 	bootstrapIPs       []string
 	bootstrapIPs4      []string
 	bootstrapIPs6      []string
@@ -511,7 +515,7 @@ func (uc *UpstreamConfig) ReBootstrap(ctx context.Context) {
 		return
 	}
 	_, _, _ = uc.g.Do("ReBootstrap", func() (any, error) {
-		if uc.rebootstrap.CompareAndSwap(false, true) {
+		if uc.rebootstrap.CompareAndSwap(rebootstrapNotStarted, rebootstrapStarted) {
 			logger := LoggerFromCtx(ctx)
 			Log(ctx, logger.Debug(), "Re-bootstrapping upstream: %s", uc.Name)
 		}
@@ -557,8 +561,10 @@ func (uc *UpstreamConfig) ensureSetupTransport(ctx context.Context) {
 	uc.transportOnce.Do(func() {
 		uc.SetupTransport(ctx)
 	})
-	if uc.rebootstrap.CompareAndSwap(true, false) {
+
+	if uc.rebootstrap.CompareAndSwap(rebootstrapStarted, rebootstrapInProgress) {
 		uc.SetupTransport(ctx)
+		uc.rebootstrap.Store(rebootstrapNotStarted)
 	}
 }
 

config_internal_test.go

@@ -3,6 +3,7 @@ package ctrld
 import (
 	"context"
 	"net/url"
+	"sync"
 	"testing"
 
 	"github.com/stretchr/testify/assert"
@@ -506,6 +507,52 @@ func TestUpstreamConfig_IsDiscoverable(t *testing.T) {
 	}
 }
 
+func TestRebootstrapRace(t *testing.T) {
+	uc := &UpstreamConfig{
+		Name:         "test-doh",
+		Type:         ResolverTypeDOH,
+		Endpoint:     "https://example.com/dns-query",
+		Domain:       "example.com",
+		bootstrapIPs: []string{"1.1.1.1", "1.0.0.1"},
+	}
+
+	ctx := LoggerCtx(context.Background(), NopLogger)
+
+	uc.SetupTransport(ctx)
+
+	if uc.transport == nil {
+		t.Fatal("initial transport should be set")
+	}
+
+	const goroutines = 100
+
+	uc.ReBootstrap(ctx)
+
+	started := make(chan struct{})
+	go func() {
+		close(started)
+		for {
+			switch uc.rebootstrap.Load() {
+			case rebootstrapStarted, rebootstrapInProgress:
+				uc.ReBootstrap(ctx)
+			default:
+				return
+			}
+		}
+	}()
+
+	<-started
+
+	var wg sync.WaitGroup
+	for range goroutines {
+		wg.Go(func() {
+			uc.ensureSetupTransport(ctx)
+		})
+	}
+
+	wg.Wait()
+}
+
 func ptrBool(b bool) *bool {
 	return &b
 }