Add the datacenter name validation if provided

Fail to connect if DC name is different in the topology
than the one the user entered.

policies.go

@@ -319,6 +319,8 @@ type HostSelectionPolicy interface {
 	// so it's safe to have internal state without additional synchronization as long as every call to Pick returns
 	// a different instance of NextHost.
 	Pick(ExecutableQuery) NextHost
+	// Checks if datacenter is valid if local aware policy is used (explicitly or as a fallback)
+	IsDatacenterValid(datacenters []string) bool
 }
 
 // SelectedHost is an interface returned when picking a host from a host
@@ -363,6 +365,7 @@ func (r *roundRobinHostPolicy) KeyspaceChanged(KeyspaceUpdateEvent) {}
 func (r *roundRobinHostPolicy) SetPartitioner(partitioner string)   {}
 func (r *roundRobinHostPolicy) Init(*Session)                       {}
 func (r *roundRobinHostPolicy) Reset()                              {}
+func (r *roundRobinHostPolicy) IsDatacenterValid([]string) bool     { return true }
 
 // Experimental, this interface and use may change
 func (r *roundRobinHostPolicy) SetTablets(tablets []*TabletInfo) {}
@@ -489,6 +492,10 @@ func (t *tokenAwareHostPolicy) Reset() {
 	t.logger = nil
 }
 
+func (t *tokenAwareHostPolicy) IsDatacenterValid(datacenters []string) bool {
+	return t.fallback.IsDatacenterValid(datacenters)
+}
+
 func (t *tokenAwareHostPolicy) IsLocal(host *HostInfo) bool {
 	return t.fallback.IsLocal(host)
 }
@@ -823,6 +830,7 @@ type hostPoolHostPolicy struct {
 
 func (r *hostPoolHostPolicy) Init(*Session)                       {}
 func (r *hostPoolHostPolicy) Reset()                              {}
+func (r *hostPoolHostPolicy) IsDatacenterValid([]string) bool     { return true }
 func (r *hostPoolHostPolicy) KeyspaceChanged(KeyspaceUpdateEvent) {}
 func (r *hostPoolHostPolicy) SetPartitioner(string)               {}
 func (r *hostPoolHostPolicy) IsLocal(*HostInfo) bool              { return true }
@@ -984,6 +992,19 @@ func (d *dcAwareRR) Reset()                              {}
 func (d *dcAwareRR) KeyspaceChanged(KeyspaceUpdateEvent) {}
 func (d *dcAwareRR) SetPartitioner(p string)             {}
 
+func (d *dcAwareRR) IsDatacenterValid(datacenters []string) bool {
+	found := false
+
+	for _, dc := range datacenters {
+		if dc == d.local {
+			found = true
+			break
+		}
+	}
+
+	return found
+}
+
 func (d *dcAwareRR) IsLocal(host *HostInfo) bool {
 	return host.DataCenter() == d.local
 }
@@ -1090,6 +1111,19 @@ func (d *rackAwareRR) Reset()                              {}
 func (d *rackAwareRR) KeyspaceChanged(KeyspaceUpdateEvent) {}
 func (d *rackAwareRR) SetPartitioner(p string)             {}
 
+func (d *rackAwareRR) IsDatacenterValid(datacenters []string) bool {
+	found := false
+
+	for _, dc := range datacenters {
+		if dc == d.localDC {
+			found = true
+			break
+		}
+	}
+
+	return found
+}
+
 func (d *rackAwareRR) MaxHostTier() uint {
 	return 2
 }

policies_integration_test.go

@@ -0,0 +1,41 @@
+//go:build integration && scylla
+// +build integration,scylla
+
+package gocql
+
+import (
+	"testing"
+)
+
+// Check if session fail to start if DC name provided in the policy is wrong
+func TestDCValidationTokenAware(t *testing.T) {
+	cluster := createCluster()
+
+	fallback := DCAwareRoundRobinPolicy("WRONG_DC")
+	cluster.PoolConfig.HostSelectionPolicy = TokenAwareHostPolicy(fallback)
+
+	_, err := cluster.CreateSession()
+	if err == nil {
+		t.Fatal("createSession was expected to fail with wrong DC name provided.")
+	}
+}
+
+func TestDCValidationDCAware(t *testing.T) {
+	cluster := createCluster()
+	cluster.PoolConfig.HostSelectionPolicy = DCAwareRoundRobinPolicy("WRONG_DC")
+
+	_, err := cluster.CreateSession()
+	if err == nil {
+		t.Fatal("createSession was expected to fail with wrong DC name provided.")
+	}
+}
+
+func TestDCValidationRackAware(t *testing.T) {
+	cluster := createCluster()
+	cluster.PoolConfig.HostSelectionPolicy = RackAwareRoundRobinPolicy("WRONG_DC", "RACK")
+
+	_, err := cluster.CreateSession()
+	if err == nil {
+		t.Fatal("createSession was expected to fail with wrong DC name provided.")
+	}
+}

policies_test.go

@@ -601,8 +601,8 @@ func TestHostPolicy_DCAwareRR(t *testing.T) {
 
 }
 
-func TestHostPolicy_DCAwareRR_wrongDc(t *testing.T) {
-	p := DCAwareRoundRobinPolicy("wrong_dc", HostPolicyOptionDisableDCFailover)
+func TestHostPolicy_DCAwareRR_disableDCFailover(t *testing.T) {
+	p := DCAwareRoundRobinPolicy("local", HostPolicyOptionDisableDCFailover)
 
 	hosts := [...]*HostInfo{
 		{hostId: "0", connectAddress: net.ParseIP("10.0.0.1"), dataCenter: "local"},
@@ -616,19 +616,28 @@ func TestHostPolicy_DCAwareRR_wrongDc(t *testing.T) {
 	}
 
 	got := make(map[string]bool, len(hosts))
+	var dcs []string
 
 	it := p.Pick(nil)
 	for h := it(); h != nil; h = it() {
 		id := h.Info().hostId
+		dc := h.Info().dataCenter
 
 		if got[id] {
 			t.Fatalf("got duplicate host %s", id)
 		}
 		got[id] = true
+		dcs = append(dcs, dc)
 	}
 
-	if len(got) != 0 {
-		t.Fatalf("expected %d hosts got %d", 0, len(got))
+	if len(got) != 2 {
+		t.Fatalf("expected %d hosts got %d", 2, len(got))
+	}
+
+	for _, dc := range dcs {
+		if dc == "remote" {
+			t.Fatalf("got remote dc but failover was diabled")
+		}
 	}
 }
 

session.go

@@ -197,6 +197,14 @@ func NewSession(cfg ClusterConfig) (*Session, error) {
 		}
 	}
 
+	var datacenters []string
+	for _, host := range s.ring.allHosts() {
+		datacenters = append(datacenters, host.DataCenter())
+	}
+	if !s.policy.IsDatacenterValid(datacenters) {
+		return nil, fmt.Errorf("gocql: unable to create session: datacenter provided in the policy is not valid")
+	}
+
 	return s, nil
 }