dnsserver: Rely on dns.Server.ShutdownContext to gracefully stop (#7517)

core/dnsserver/server.go

@@ -6,7 +6,6 @@ import (
 	"fmt"
 	"maps"
 	"net"
-	"runtime"
 	"runtime/debug"
 	"strings"
 	"sync"
@@ -42,7 +41,6 @@ type Server struct {
 	m      sync.Mutex     // protects the servers
 
 	zones        map[string][]*Config // zones keyed by their address
-	dnsWg        sync.WaitGroup       // used to wait on outstanding connections
 	graceTimeout time.Duration        // the maximum duration of a graceful shutdown
 	trace        trace.Trace          // the trace plugin for the server
 	debug        bool                 // disable recover()
@@ -53,7 +51,7 @@ type Server struct {
 
 	// Ensure Stop is idempotent when invoked concurrently (e.g., during reload and SIGTERM).
 	stopOnce sync.Once
-	wgDoneOnce sync.Once
+	stopErr  error
 }
 
 // MetadataCollector is a plugin that can retrieve metadata functions from all metadata providing plugins
@@ -74,14 +72,6 @@ func NewServer(addr string, group []*Config) (*Server, error) {
 		tsigSecret:   make(map[string]string),
 	}
 
-	// We have to bound our wg with one increment
-	// to prevent a "race condition" that is hard-coded
-	// into sync.WaitGroup.Wait() - basically, an add
-	// with a positive delta must be guaranteed to
-	// occur before Wait() is called on the wg.
-	// In a way, this kind of acts as a safety barrier.
-	s.dnsWg.Add(1)
-
 	for _, site := range group {
 		if site.Debug {
 			s.debug = true
@@ -209,44 +199,36 @@ func (s *Server) ListenPacket() (net.PacketConn, error) {
 	return p, nil
 }
 
-// Stop stops the server. It blocks until the server is
+// Stop attempts to gracefully stop the server.
-// totally stopped. On POSIX systems, it will wait for
+// It waits until the server is stopped and its connections are closed,
-// connections to close (up to a max timeout of a few
+// up to a max timeout of a few seconds. If unsuccessful, an error is returned.
-// seconds); on Windows it will close the listener
+//
-// immediately.
 // This implements Caddy.Stopper interface.
-func (s *Server) Stop() (err error) {
+func (s *Server) Stop() error {
-	var onceErr error
 	s.stopOnce.Do(func() {
-		if runtime.GOOS != "windows" {
+		ctx, cancelCtx := context.WithTimeout(context.Background(), s.graceTimeout)
-			// force connections to close after timeout
+		defer cancelCtx()
-			done := make(chan struct{})
-			go func() {
-				// decrement our initial increment used as a barrier, but only once
-				s.wgDoneOnce.Do(func() { s.dnsWg.Done() })
-				s.dnsWg.Wait()
-				close(done)
-			}()
 
-			// Wait for remaining connections to finish or
+		var wg sync.WaitGroup
-			// force them all to close after timeout
-			select {
-			case <-time.After(s.graceTimeout):
-			case <-done:
-			}
-		}
-
-		// Close the listener now; this stops the server without delay
 		s.m.Lock()
 		for _, s1 := range s.server {
 			// We might not have started and initialized the full set of servers
-			if s1 != nil {
+			if s1 == nil {
-				onceErr = s1.Shutdown()
+				continue
 			}
+
+			wg.Add(1)
+			go func() {
+				s1.ShutdownContext(ctx)
+				wg.Done()
+			}()
 		}
 		s.m.Unlock()
+		wg.Wait()
+
+		s.stopErr = ctx.Err()
 	})
-	return onceErr
+	return s.stopErr
 }
 
 // Address together with Stop() implement caddy.GracefulServer.

core/dnsserver/server_test.go

@@ -2,8 +2,11 @@ package dnsserver
 
 import (
 	"context"
+	"errors"
+	"net"
 	"sync"
 	"testing"
+	"time"
 
 	"github.com/coredns/coredns/plugin"
 	"github.com/coredns/coredns/plugin/pkg/log"
@@ -20,6 +23,24 @@ func (tp testPlugin) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.
 
 func (tp testPlugin) Name() string { return "local" }
 
+// blockingPlugin uses sync.Mutex to simulate extended processing.
+type blockingPlugin struct {
+	sync.Mutex
+}
+
+func (b *blockingPlugin) Name() string { return "blocking" }
+
+func (b *blockingPlugin) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg) (int, error) {
+	// Respond immediately to avoid waiting in dns.Exchange
+	m := new(dns.Msg)
+	m.SetRcodeFormatError(r)
+	w.WriteMsg(m)
+
+	b.Lock()
+	defer b.Unlock()
+	return dns.RcodeSuccess, nil
+}
+
 func testConfig(transport string, p plugin.Handler) *Config {
 	c := &Config{
 		Zone:        "example.com.",
@@ -104,6 +125,44 @@ func TestStacktrace(t *testing.T) {
 	}
 }
 
+func TestGracefulStopTimeout_Internal(t *testing.T) {
+	p := new(blockingPlugin)
+	cfg := testConfig("dns", p)
+
+	s, err := NewServer("127.0.0.1:0", []*Config{cfg})
+	if err != nil {
+		t.Fatalf("NewServer failed: %v", err)
+	}
+
+	// Shorten the graceful timeout
+	s.graceTimeout = 500 * time.Millisecond
+
+	pc, err := net.ListenPacket("udp", "127.0.0.1:0")
+	if err != nil {
+		t.Fatalf("ListenPacket failed: %v", err)
+	}
+	defer pc.Close()
+
+	go s.ServePacket(pc)
+	udp := pc.LocalAddr().String()
+
+	// Block the handler
+	p.Lock()
+	defer p.Unlock()
+
+	m := new(dns.Msg)
+	m.SetQuestion("example.com.", dns.TypeA)
+	_, err = dns.Exchange(m, udp)
+	if err != nil {
+		t.Fatalf("dns.Exchange failed: %v", err)
+	}
+
+	err = s.Stop()
+	if !errors.Is(err, context.DeadlineExceeded) {
+		t.Fatalf("expected context.DeadlineExceeded, got %v", err)
+	}
+}
+
 func BenchmarkCoreServeDNS(b *testing.B) {
 	s, err := NewServer("127.0.0.1:53", []*Config{testConfig("dns", testPlugin{})})
 	if err != nil {
@@ -121,22 +180,3 @@ func BenchmarkCoreServeDNS(b *testing.B) {
 		s.ServeDNS(ctx, w, m)
 	}
 }
-
-// Validates Stop is idempotent and safe under concurrent calls.
-func TestStopIsIdempotent(t *testing.T) {
-	t.Parallel()
-
-	s := &Server{}
-	s.dnsWg.Add(1)
-
-	const n = 10
-	var wg sync.WaitGroup
-	wg.Add(n)
-	for range n {
-		go func() {
-			defer wg.Done()
-			_ = s.Stop()
-		}()
-	}
-	wg.Wait()
-}