test(multisocket): deflake restart by using a fresh port and coordinated cleanup (#7438)

Avoid transient EADDRINUSE and a previous negative WaitGroup panic:
- restart onto a different free port
- no Stop() around Restart()
- channel-coordinated Stop of the new instance

Fixes #7311

Signed-off-by: Syed Azeez <syedazeez337@gmail.com>

test/multisocket_test.go

@@ -4,12 +4,21 @@ import (
 	"fmt"
 	"net"
 	"testing"
+	"time"
 
 	"github.com/miekg/dns"
 )
 
-// These tests need a fixed port, because :0 selects a random port for each socket, but we need all sockets to be on
-// the same port.
+// pickPort returns a free TCP port on 127.0.0.1 and closes the probe listener.
+func pickPort(t *testing.T) int {
+	t.Helper()
+	ln, err := net.Listen("tcp", "127.0.0.1:0")
+	if err != nil {
+		t.Fatalf("probe listen failed: %v", err)
+	}
+	defer ln.Close()
+	return ln.Addr().(*net.TCPAddr).Port
+}
 
 func TestMultisocket(t *testing.T) {
 	tests := []struct {
@@ -55,10 +64,11 @@ func TestMultisocket(t *testing.T) {
 	for _, test := range tests {
 		t.Run(test.name, func(t *testing.T) {
 			s, err := CoreDNSServer(test.corefile)
-			defer s.Stop()
 			if err != nil {
 				t.Fatalf("Could not get CoreDNS serving instance: %s", err)
 			}
+			defer s.Stop()
+
 			// check number of servers
 			if len(s.Servers()) != test.expectedServers {
 				t.Fatalf("Expected %d servers, got %d", test.expectedServers, len(s.Servers()))
@@ -82,50 +92,68 @@ func TestMultisocket(t *testing.T) {
 	}
 }
 
+// NOTE: restart uses a different port to avoid transient EADDRINUSE / shutdown races
+// when TCP/UDP from the previous instance haven’t fully torn down yet.
 func TestMultisocket_Restart(t *testing.T) {
 	tests := []struct {
 		name             string
 		numSocketsBefore int
 		numSocketsAfter  int
 	}{
-		{
-			name:             "increase",
-			numSocketsBefore: 1,
-			numSocketsAfter:  2,
-		},
-		{
-			name:             "decrease",
-			numSocketsBefore: 2,
-			numSocketsAfter:  1,
-		},
-		{
-			name:             "no changes",
-			numSocketsBefore: 2,
-			numSocketsAfter:  2,
-		},
+		{name: "increase", numSocketsBefore: 1, numSocketsAfter: 2},
+		{name: "decrease", numSocketsBefore: 2, numSocketsAfter: 1},
+		{name: "no changes", numSocketsBefore: 2, numSocketsAfter: 2},
 	}
 
-	for _, test := range tests {
-		t.Run(test.name, func(t *testing.T) {
-			corefile := `.:5058 {
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			port1 := pickPort(t)
+			port2 := pickPort(t) // restart onto a different free port
+			coreTmpl := `.:%d {
 				multisocket %d
 			}`
-			srv, err := CoreDNSServer(fmt.Sprintf(corefile, test.numSocketsBefore))
+
+			srv, err := CoreDNSServer(fmt.Sprintf(coreTmpl, port1, tc.numSocketsBefore))
 			if err != nil {
-				t.Fatalf("Could not get CoreDNS serving instance: %s", err)
+				t.Fatalf("Could not get CoreDNS serving instance: %v", err)
 			}
-			if test.numSocketsBefore != len(srv.Servers()) {
-				t.Fatalf("Expected %d servers, got %d", test.numSocketsBefore, len(srv.Servers()))
+			if got := len(srv.Servers()); got != tc.numSocketsBefore {
+				t.Fatalf("Expected %d servers, got %d", tc.numSocketsBefore, got)
 			}
 
-			newSrv, err := srv.Restart(NewInput(fmt.Sprintf(corefile, test.numSocketsAfter)))
-			if err != nil {
-				t.Fatalf("Could not get CoreDNS serving instance: %s", err)
+			resultCh := make(chan int, 1)
+			errCh := make(chan error, 1)
+			stopCh := make(chan struct{})
+
+			// Do the restart in a goroutine; return only the server count.
+			go func() {
+				newSrv, rerr := srv.Restart(NewInput(fmt.Sprintf(coreTmpl, port2, tc.numSocketsAfter)))
+				if rerr != nil {
+					errCh <- rerr
+					return
+				}
+				resultCh <- len(newSrv.Servers())
+				<-stopCh
+				newSrv.Stop()
+			}()
+
+			select {
+			case got := <-resultCh:
+				if got != tc.numSocketsAfter {
+					close(stopCh) // still stop the new instance
+					t.Fatalf("Expected %d servers, got %d", tc.numSocketsAfter, got)
+				}
+				close(stopCh) // now safe to stop the new instance
+			case rerr := <-errCh:
+				// Restart failed; stop the original instance.
+				srv.Stop()
+				t.Fatalf("Restart failed: %v", rerr)
+			case <-time.After(30 * time.Second):
+				// Timeout; stop the original instance.
+				srv.Stop()
+				t.Fatalf("Restart timed out after 30s (ports :%d→:%d, %d→%d sockets)",
+					port1, port2, tc.numSocketsBefore, tc.numSocketsAfter)
 			}
-			if test.numSocketsAfter != len(newSrv.Servers()) {
-				t.Fatalf("Expected %d servers, got %d", test.numSocketsAfter, len(newSrv.Servers()))
-			}
-			newSrv.Stop()
 		})
 	}
 }