Update ReadPrefSelector and move feature package

Updates ReadPrefSelector to use MaxStalenessSupported.
Merges the feature package into the description package.

GODRIVER-333

Change-Id: I8910ae417d6eea11515bc1fb4a9c2babfa56a973

Author: skriptble

core/auth/default.go

@@ -11,7 +11,6 @@ import (
 
 	"github.com/mongodb/mongo-go-driver/core/description"
 	"github.com/mongodb/mongo-go-driver/core/wiremessage"
-	"github.com/mongodb/mongo-go-driver/internal/feature"
 )
 
 func newDefaultAuthenticator(cred *Cred) (Authenticator, error) {
@@ -30,7 +29,7 @@ type DefaultAuthenticator struct {
 func (a *DefaultAuthenticator) Auth(ctx context.Context, desc description.Server, rw wiremessage.ReadWriter) error {
 	var actual Authenticator
 	var err error
-	if err = feature.ScramSHA1(desc.Version); err != nil {
+	if err = description.ScramSHA1Supported(desc.Version); err != nil {
 		actual, err = newMongoDBCRAuthenticator(a.Cred)
 	} else {
 		actual, err = newScramSHA1Authenticator(a.Cred)

core/description/feature.go

@@ -0,0 +1,25 @@
+package description
+
+import (
+	"fmt"
+)
+
+// MaxStalenessSupported returns an error if the given server version
+// does not support max staleness.
+func MaxStalenessSupported(serverVersion Version, wireVersion *VersionRange) error {
+	if !serverVersion.AtLeast(3, 4, 0) || (wireVersion != nil && wireVersion.Max < 5) {
+		return fmt.Errorf("max staleness is only supported for servers 3.4 or newer")
+	}
+
+	return nil
+}
+
+// ScramSHA1Supported returns an error if the given server version
+// does not support scram-sha-1.
+func ScramSHA1Supported(version Version) error {
+	if !version.AtLeast(3, 0, 0) {
+		return fmt.Errorf("SCRAM-SHA-1 is only supported for servers 3.0 or newer")
+	}
+
+	return nil
+}

core/description/feature_test.go

@@ -0,0 +1,40 @@
+package description
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestMaxStaleness(t *testing.T) {
+	wireVersionSupported := NewVersionRange(0, 5)
+	wireVersionUnsupported := NewVersionRange(0, 4)
+
+	tests := []struct {
+		version  Version
+		wire     *VersionRange
+		expected bool
+	}{
+		{Version{Parts: []uint8{2, 4, 0}}, nil, false},
+		{Version{Parts: []uint8{3, 3, 99}}, nil, false},
+		{Version{Parts: []uint8{3, 4, 0}}, nil, true},
+		{Version{Parts: []uint8{3, 4, 1}}, nil, true},
+		{Version{Parts: []uint8{2, 4, 0}}, &wireVersionSupported, false},
+		{Version{Parts: []uint8{2, 4, 0}}, &wireVersionUnsupported, false},
+		{Version{Parts: []uint8{3, 4, 1}}, &wireVersionSupported, true},
+		{Version{Parts: []uint8{3, 4, 1}}, &wireVersionUnsupported, false},
+	}
+
+	for _, test := range tests {
+		t.Run(test.version.String(), func(t *testing.T) {
+			t.Parallel()
+
+			err := MaxStalenessSupported(test.version, test.wire)
+			if test.expected {
+				require.NoError(t, err)
+			} else {
+				require.Error(t, err)
+			}
+		})
+	}
+}

core/description/server_selector.go

@@ -1,7 +1,6 @@
 package description
 
 import (
-	"errors"
 	"fmt"
 	"math"
 	"time"
@@ -115,8 +114,8 @@ func ReadPrefSelector(rp *readpref.ReadPref) ServerSelector {
 		if _, set := rp.MaxStaleness(); set {
 			for _, s := range candidates {
 				if s.Kind != Unknown {
-					if !s.Version.AtLeast(3, 4, 0) || (s.WireVersion != nil && s.WireVersion.Max < 5) {
-						return nil, errors.New("max staleness is only supported for servers 3.4 or newer")
+					if err := MaxStalenessSupported(s.Version, s.WireVersion); err != nil {
+						return nil, err
 					}
 				}
 			}