GODRIVER-1659 fail fast during server selection if incompatible WireVersion (#461)

Author: KritiRav

x/mongo/driver/description/topology.go

@@ -17,6 +17,7 @@ type Topology struct {
 	Servers               []Server
 	Kind                  TopologyKind
 	SessionTimeoutMinutes uint32
+	CompatibilityErr      error
 }
 
 // Server returns the server for the given address. Returns false if the server

x/mongo/driver/topology/fsm.go

@@ -99,7 +99,8 @@ func (f *fsm) apply(s description.Server) (description.Topology, description.Ser
 					supportedWireVersions.Min,
 					minSupportedMongoDBVersion,
 				)
-				return description.Topology{}, s, f.compatibilityErr
+				f.Topology.CompatibilityErr = f.compatibilityErr
+				return f.Topology, s, nil
 			}
 
 			if server.WireVersion.Min > supportedWireVersions.Max {
@@ -110,7 +111,8 @@ func (f *fsm) apply(s description.Server) (description.Topology, description.Ser
 					server.WireVersion.Min,
 					supportedWireVersions.Max,
 				)
-				return description.Topology{}, s, f.compatibilityErr
+				f.Topology.CompatibilityErr = f.compatibilityErr
+				return f.Topology, s, nil
 			}
 		}
 	}

x/mongo/driver/topology/topology.go

@@ -455,6 +455,10 @@ func (t *Topology) selectServerFromDescription(desc description.Topology,
 	// Unlike selectServerFromSubscription, this code path does not check ctx.Done or selectionState.timeoutChan because
 	// selecting a server from a description is not a blocking operation.
 
+	if desc.CompatibilityErr != nil {
+		return nil, desc.CompatibilityErr
+	}
+
 	var allowed []description.Server
 	for _, s := range desc.Servers {
 		if s.Kind != description.Unknown {

x/mongo/driver/topology/topology_test.go

@@ -9,6 +9,7 @@ package topology
 import (
 	"context"
 	"errors"
+	"fmt"
 	"sync/atomic"
 	"testing"
 	"time"
@@ -84,6 +85,54 @@ func TestServerSelection(t *testing.T) {
 			t.Errorf("Incorrect sever selected. got %s; want %s", srvs[0].Addr, desc.Servers[0].Addr)
 		}
 	})
+	t.Run("Compatibility Error Min Version Too High", func(t *testing.T) {
+		topo, err := New()
+		noerr(t, err)
+		desc := description.Topology{
+			Kind: description.Single,
+			Servers: []description.Server{
+				{Addr: address.Address("one:27017"), Kind: description.Standalone, WireVersion: &description.VersionRange{Max: 11, Min: 11}},
+				{Addr: address.Address("two:27017"), Kind: description.Standalone, WireVersion: &description.VersionRange{Max: 9, Min: 2}},
+				{Addr: address.Address("three:27017"), Kind: description.Standalone, WireVersion: &description.VersionRange{Max: 9, Min: 2}},
+			},
+		}
+		want := fmt.Errorf(
+			"server at %s requires wire version %d, but this version of the Go driver only supports up to %d",
+			desc.Servers[0].Addr.String(),
+			desc.Servers[0].WireVersion.Min,
+			supportedWireVersions.Max,
+		)
+		desc.CompatibilityErr = want
+		atomic.StoreInt32(&topo.connectionstate, connected)
+		topo.desc.Store(desc)
+		_, err = topo.SelectServer(context.Background(), selectFirst)
+		assert.Equal(t, err, want, "expected %v, got %v", want, err)
+	})
+	t.Run("Compatibility Error Max Version Too Low", func(t *testing.T) {
+		topo, err := New()
+		noerr(t, err)
+		desc := description.Topology{
+			Kind: description.Single,
+			Servers: []description.Server{
+				{Addr: address.Address("one:27017"), Kind: description.Standalone, WireVersion: &description.VersionRange{Max: 1, Min: 1}},
+				{Addr: address.Address("two:27017"), Kind: description.Standalone, WireVersion: &description.VersionRange{Max: 9, Min: 2}},
+				{Addr: address.Address("three:27017"), Kind: description.Standalone, WireVersion: &description.VersionRange{Max: 9, Min: 2}},
+			},
+		}
+		want := fmt.Errorf(
+			"server at %s reports wire version %d, but this version of the Go driver requires "+
+				"at least %d (MongoDB %s)",
+			desc.Servers[0].Addr.String(),
+			desc.Servers[0].WireVersion.Max,
+			supportedWireVersions.Min,
+			minSupportedMongoDBVersion,
+		)
+		desc.CompatibilityErr = want
+		atomic.StoreInt32(&topo.connectionstate, connected)
+		topo.desc.Store(desc)
+		_, err = topo.SelectServer(context.Background(), selectFirst)
+		assert.Equal(t, err, want, "expected %v, got %v", want, err)
+	})
 	t.Run("Updated", func(t *testing.T) {
 		topo, err := New()
 		noerr(t, err)