cluster: use the first PD for standalone tidb-dashboard

pkg/cluster/manager/scale_in.go

@@ -106,6 +106,14 @@ func (m *Manager) ScaleIn(
 		return err
 	}
 
+	deletedNodes := set.NewStringSet(nodes...)
+	scaledInPD := false
+	topo.IterInstance(func(inst spec.Instance) {
+		if deletedNodes.Exist(inst.ID()) && inst.ComponentName() == spec.ComponentPD {
+			scaledInPD = true
+		}
+	})
+
 	b, err := m.sshTaskBuilder(name, topo, base.User, gOpt)
 	if err != nil {
 		return err
@@ -161,6 +169,15 @@ func (m *Manager) ScaleIn(
 	}
 
 	m.logger.Infof("Scaled cluster `%s` in successfully", name)
+	if scaledInPD {
+		if dash := spec.FindComponent(topo, spec.ComponentDashboard); dash != nil && len(dash.Instances()) > 0 {
+			m.logger.Warnf("%s", color.YellowString(
+				"\nSince PD node(s) were scaled in, the standalone tidb-dashboard connects to a single PD endpoint "+
+					"that may have been removed. If it can no longer reach PD, restart it to pick up a new endpoint:\n\t%s",
+				color.GreenString("%s restart %s -R %s", tui.OsArgs0(), name, spec.ComponentDashboard),
+			))
+		}
+	}
 
 	return nil
 }

pkg/cluster/spec/dashboard.go

@@ -18,7 +18,6 @@ import (
 	"crypto/tls"
 	"fmt"
 	"path/filepath"
-	"strings"
 	"time"
 
 	"github.com/pingcap/tiup/pkg/cluster/ctxt"
@@ -198,10 +197,6 @@ func (i *DashboardInstance) InitConfig(
 	enableTLS := topo.GlobalOptions.TLSEnabled
 	spec := i.InstanceSpec.(*DashboardSpec)
 
-	pds := []string{}
-	for _, pdspec := range topo.PDServers {
-		pds = append(pds, pdspec.GetAdvertiseClientURL(enableTLS))
-	}
 	cfg := &scripts.DashboardScript{
 		// -h, --host string              listen host of the Dashboard Server
 		Host:        i.GetListenHost(),
@@ -211,7 +206,7 @@ func (i *DashboardInstance) InitConfig(
 		LogDir:      paths.Log,
 		Port:        spec.Port,
 		NumaNode:    spec.NumaNode,
-		PD:          strings.Join(pds, ","),
+		PD:          dashboardPDEndpoint(topo.PDServers, enableTLS),
 		TLSEnabled:  enableTLS,
 	}
 
@@ -243,3 +238,17 @@ func (i *DashboardInstance) InitConfig(
 func (i *DashboardInstance) setTLSConfig(ctx context.Context, enableTLS bool, configs map[string]any, paths meta.DirPaths) (map[string]any, error) {
 	return nil, nil
 }
+
+// dashboardPDEndpoint returns the PD URL for standalone tidb-dashboard.
+//
+// Dashboard treats --pd as a single endpoint and does not split a
+// comma-separated list, so passing all PD endpoints breaks its PD
+// connection. See https://github.com/pingcap/tidb-dashboard/issues/1920.
+// Pass only the first PD until Dashboard supports multiple PD HTTP API
+// endpoints, then restore joining all PD URLs.
+func dashboardPDEndpoint(pdServers []*PDSpec, enableTLS bool) string {
+	if len(pdServers) == 0 {
+		return ""
+	}
+	return pdServers[0].GetAdvertiseClientURL(enableTLS)
+}