dns: Refactor reordered pool

dns/transport/conn_pool.go

@@ -6,6 +6,8 @@ import (
 	"sync"
 
 	"github.com/sagernet/sing/common/x/list"
+
+	"golang.org/x/sync/semaphore"
 )
 
 type ConnPoolMode int
@@ -17,6 +19,8 @@ const (
 
 type ConnPoolOptions[T comparable] struct {
 	Mode ConnPoolMode
+	// MaxInflight caps concurrent in-progress dials. Only honored in ConnPoolOrdered mode.
+	MaxInflight int
 	IsAlive     func(T) bool
 	Close       func(T, error)
 }
@@ -24,6 +28,8 @@ type ConnPoolOptions[T comparable] struct {
 type ConnPool[T comparable] struct {
 	options ConnPoolOptions[T]
 
+	sem *semaphore.Weighted
+
 	access sync.Mutex
 	closed bool
 	state  *connPoolState[T]
@@ -53,10 +59,14 @@ type connPoolConnect[T comparable] struct {
 }
 
 func NewConnPool[T comparable](options ConnPoolOptions[T]) *ConnPool[T] {
-	return &ConnPool[T]{
+	p := &ConnPool[T]{
 		options: options,
-		state:   newConnPoolState[T](options.Mode),
 	}
+	if options.Mode == ConnPoolOrdered && options.MaxInflight > 0 {
+		p.sem = semaphore.NewWeighted(int64(options.MaxInflight))
+	}
+	p.state = newConnPoolState[T](options.Mode)
+	return p
 }
 
 func newConnPoolState[T comparable](mode ConnPoolMode) *connPoolState[T] {
@@ -113,7 +123,7 @@ func (p *ConnPool[T]) Release(conn T, reuse bool) {
 		return
 	}
 	if p.options.Mode == ConnPoolOrdered {
-		if _, loaded := state.idleElements[conn]; !loaded {
+		if _, idle := state.idleElements[conn]; !idle {
 			state.idleElements[conn] = state.idle.PushBack(conn)
 		}
 	}
@@ -137,6 +147,31 @@ func (p *ConnPool[T]) Invalidate(conn T, cause error) {
 	p.options.Close(conn, cause)
 }
 
+func (p *ConnPool[T]) acquireSlot(ctx context.Context, state *connPoolState[T]) error {
+	if p.sem == nil {
+		return nil
+	}
+	acquireCtx, cancel := context.WithCancel(ctx)
+	stopStateCancel := context.AfterFunc(state.ctx, cancel)
+	err := p.sem.Acquire(acquireCtx, 1)
+	stopStateCancel()
+	cancel()
+	if err == nil {
+		return nil
+	}
+	ctxErr := ctx.Err()
+	if ctxErr != nil {
+		return ctxErr
+	}
+	return context.Cause(state.ctx)
+}
+
+func (p *ConnPool[T]) releaseSlot() {
+	if p.sem != nil {
+		p.sem.Release(1)
+	}
+}
+
 // removeConn must be called with p.access held.
 func (p *ConnPool[T]) removeConn(state *connPoolState[T], conn T, cause error) {
 	delete(state.all, conn)
@@ -199,19 +234,29 @@ func (p *ConnPool[T]) acquireOrdered(ctx context.Context, dial func(context.Cont
 		}
 		current := p.state
 		if element := current.idle.Front(); element != nil {
-			conn := current.idle.Remove(element)
-			delete(current.idleElements, conn)
-			if p.options.IsAlive(conn) {
+			idleConn := current.idle.Remove(element)
+			delete(current.idleElements, idleConn)
+			if p.options.IsAlive(idleConn) {
 				p.access.Unlock()
-				return conn, false, nil
+				return idleConn, false, nil
 			}
-			delete(current.all, conn)
+			delete(current.all, idleConn)
 			p.access.Unlock()
-			p.options.Close(conn, net.ErrClosed)
+			p.options.Close(idleConn, net.ErrClosed)
 			continue
 		}
 		p.access.Unlock()
+		return p.dialAndInstall(ctx, current, dial)
+	}
+}
 
+func (p *ConnPool[T]) dialAndInstall(ctx context.Context, current *connPoolState[T], dial func(context.Context) (T, error)) (T, bool, error) {
+	var zero T
+	err := p.acquireSlot(ctx, current)
+	if err != nil {
+		return zero, false, err
+	}
+	defer p.releaseSlot()
 	dialCtx, dialCancel := context.WithCancelCause(ctx)
 	stopStateCancel := context.AfterFunc(current.ctx, func() {
 		dialCancel(context.Cause(current.ctx))
@@ -249,7 +294,6 @@ func (p *ConnPool[T]) acquireOrdered(ctx context.Context, dial func(context.Cont
 	p.access.Unlock()
 	return conn, true, nil
 }
-}
 
 func (p *ConnPool[T]) acquireShared(ctx context.Context, dial func(context.Context) (T, error)) (T, context.Context, bool, error) {
 	var zero T

dns/transport/tls.go

@@ -22,6 +22,8 @@ import (
 
 var _ adapter.DNSTransport = (*TLSTransport)(nil)
 
+const tlsDNSMaxInflight = 8
+
 func RegisterTLS(registry *dns.TransportRegistry) {
 	dns.RegisterTransport[option.RemoteTLSDNSServerOptions](registry, C.DNSTypeTLS, NewTLS)
 }
@@ -72,6 +74,7 @@ func NewTLSRaw(logger logger.ContextLogger, adapter dns.TransportAdapter, dialer
 		tlsConfig:        tlsConfig,
 		connections: NewConnPool(ConnPoolOptions[*tlsDNSConn]{
 			Mode:        ConnPoolOrdered,
+			MaxInflight: tlsDNSMaxInflight,
 			IsAlive: func(conn *tlsDNSConn) bool {
 				return conn != nil
 			},

go.mod

@@ -59,6 +59,7 @@ require (
 	golang.org/x/exp v0.0.0-20251219203646-944ab1f22d93
 	golang.org/x/mod v0.33.0
 	golang.org/x/net v0.50.0
+	golang.org/x/sync v0.19.0
 	golang.org/x/sys v0.41.0
 	golang.zx2c4.com/wireguard/wgctrl v0.0.0-20241231184526-a9ab2273dd10
 	google.golang.org/grpc v1.79.1
@@ -159,7 +160,6 @@ require (
 	go.uber.org/zap/exp v0.3.0 // indirect
 	go4.org/mem v0.0.0-20240501181205-ae6ca9944745 // indirect
 	golang.org/x/oauth2 v0.34.0 // indirect
-	golang.org/x/sync v0.19.0 // indirect
 	golang.org/x/term v0.40.0 // indirect
 	golang.org/x/text v0.34.0 // indirect
 	golang.org/x/time v0.11.0 // indirect