refactor(dot): simplify DoT connection pool implementation

Replace the map-based pool and refCount bookkeeping with a channel-based
pool. Drop the closed state, per-connection address tracking, and
extra mutexes so the pool relies on the channel for concurrency and
lifecycle.

dot.go

@@ -7,7 +7,6 @@ import (
 	"io"
 	"net"
 	"runtime"
-	"sync"
 	"time"
 
 	"github.com/miekg/dns"
@@ -44,23 +43,20 @@ func (r *dotResolver) Resolve(ctx context.Context, msg *dns.Msg) (*dns.Msg, erro
 	return answer, err
 }
 
-// dotConnPool manages a pool of TCP/TLS connections for DoT queries.
+const dotPoolSize = 16
+
+// dotConnPool manages a pool of TCP/TLS connections for DoT queries using a buffered channel.
 type dotConnPool struct {
 	uc        *UpstreamConfig
 	addrs     []string
 	port      string
 	tlsConfig *tls.Config
 	dialer    *net.Dialer
-	mu        sync.RWMutex
-	conns     map[string]*dotConn
-	closed    bool
+	conns     chan *dotConn
 }
 
 type dotConn struct {
-	conn     *tls.Conn
-	lastUsed time.Time
-	refCount int
-	mu       sync.Mutex
+	conn *tls.Conn
 }
 
 func newDOTClientPool(_ context.Context, uc *UpstreamConfig, addrs []string) *dotConnPool {
@@ -90,7 +86,7 @@ func newDOTClientPool(_ context.Context, uc *UpstreamConfig, addrs []string) *do
 		port:      port,
 		tlsConfig: tlsConfig,
 		dialer:    dialer,
-		conns:     make(map[string]*dotConn),
+		conns:     make(chan *dotConn, dotPoolSize),
 	}
 
 	// Use SetFinalizer here because we need to call a method on the pool itself.
@@ -109,7 +105,7 @@ func (p *dotConnPool) Resolve(ctx context.Context, msg *dns.Msg) (*dns.Msg, erro
 		return nil, errors.New("nil DNS message")
 	}
 
-	conn, addr, err := p.getConn(ctx)
+	conn, err := p.getConn(ctx)
 	if err != nil {
 		return nil, wrapCertificateVerificationError(err)
 	}
@@ -117,7 +113,7 @@ func (p *dotConnPool) Resolve(ctx context.Context, msg *dns.Msg) (*dns.Msg, erro
 	client := dns.Client{Net: "tcp-tls"}
 	answer, _, err := client.ExchangeWithConnContext(ctx, msg, &dns.Conn{Conn: conn})
 	isGood := err == nil
-	p.putConn(addr, conn, isGood)
+	p.putConn(conn, isGood)
 
 	if err != nil {
 		return nil, wrapCertificateVerificationError(err)
@@ -127,71 +123,42 @@ func (p *dotConnPool) Resolve(ctx context.Context, msg *dns.Msg) (*dns.Msg, erro
 }
 
 // getConn gets a TCP/TLS connection from the pool or creates a new one.
-func (p *dotConnPool) getConn(ctx context.Context) (net.Conn, string, error) {
-	p.mu.Lock()
-	defer p.mu.Unlock()
-
-	if p.closed {
-		return nil, "", io.EOF
-	}
-
-	// Try to reuse an existing connection
-	for addr, dotConn := range p.conns {
-		dotConn.mu.Lock()
-		if dotConn.refCount == 0 && dotConn.conn != nil && isAlive(dotConn.conn) {
-			dotConn.refCount++
-			dotConn.lastUsed = time.Now()
-			conn := dotConn.conn
-			dotConn.mu.Unlock()
-			return conn, addr, nil
+// A connection is taken from the channel while in use; putConn returns it.
+func (p *dotConnPool) getConn(ctx context.Context) (net.Conn, error) {
+	for {
+		select {
+		case dc := <-p.conns:
+			if dc.conn != nil && isAlive(dc.conn) {
+				return dc.conn, nil
+			}
+			if dc.conn != nil {
+				dc.conn.Close()
+			}
+		default:
+			_, conn, err := p.dialConn(ctx)
+			if err != nil {
+				return nil, err
+			}
+			return conn, nil
 		}
-		dotConn.mu.Unlock()
 	}
-
-	// No available connection, create a new one
-	addr, conn, err := p.dialConn(ctx)
-	if err != nil {
-		return nil, "", err
-	}
-
-	dotConn := &dotConn{
-		conn:     conn,
-		lastUsed: time.Now(),
-		refCount: 1,
-	}
-	p.conns[addr] = dotConn
-
-	return conn, addr, nil
 }
 
-// putConn returns a connection to the pool.
-func (p *dotConnPool) putConn(addr string, conn net.Conn, isGood bool) {
-	p.mu.Lock()
-	defer p.mu.Unlock()
-
-	dotConn, ok := p.conns[addr]
-	if !ok {
-		return
-	}
-
-	dotConn.mu.Lock()
-	defer dotConn.mu.Unlock()
-
-	dotConn.refCount--
-	if dotConn.refCount < 0 {
-		dotConn.refCount = 0
-	}
-
-	// If connection is bad, remove it from pool
-	if !isGood {
-		delete(p.conns, addr)
+// putConn returns a connection to the pool for reuse by other goroutines.
+func (p *dotConnPool) putConn(conn net.Conn, isGood bool) {
+	if !isGood || conn == nil {
 		if conn != nil {
 			conn.Close()
 		}
 		return
 	}
-
-	dotConn.lastUsed = time.Now()
+	dc := &dotConn{conn: conn.(*tls.Conn)}
+	select {
+	case p.conns <- dc:
+	default:
+		// Channel full, close the connection
+		dc.conn.Close()
+	}
 }
 
 // dialConn creates a new TCP/TLS connection.
@@ -293,20 +260,17 @@ func (p *dotConnPool) dialConn(ctx context.Context) (string, *tls.Conn, error) {
 }
 
 // CloseIdleConnections closes all connections in the pool.
+// Connections currently checked out (in use) are not closed.
 func (p *dotConnPool) CloseIdleConnections() {
-	p.mu.Lock()
-	defer p.mu.Unlock()
-	if p.closed {
-		return
-	}
-	p.closed = true
-	for addr, dotConn := range p.conns {
-		dotConn.mu.Lock()
-		if dotConn.conn != nil {
-			dotConn.conn.Close()
+	for {
+		select {
+		case dc := <-p.conns:
+			if dc.conn != nil {
+				dc.conn.Close()
+			}
+		default:
+			return
 		}
-		dotConn.mu.Unlock()
-		delete(p.conns, addr)
 	}
 }