Add diagnostic information to server selection errors.

GODRIVER-733

Change-Id: Ib5fe3784dbc75287b60aa89045687d7249e111ae

Author: Divjot Arora

x/mongo/driver/topology/server.go

@@ -14,6 +14,7 @@ import (
 	"sync/atomic"
 	"time"
 
+	"fmt"
 	"github.com/mongodb/mongo-go-driver/bson"
 	"github.com/mongodb/mongo-go-driver/event"
 	"github.com/mongodb/mongo-go-driver/x/bsonx"
@@ -61,6 +62,21 @@ const (
 	connecting
 )
 
+func connectionStateString(state int32) string {
+	switch state {
+	case 0:
+		return "Disconnected"
+	case 1:
+		return "Disconnecting"
+	case 2:
+		return "Connected"
+	case 3:
+		return "Connecting"
+	}
+
+	return ""
+}
+
 // Server is a single server within a topology.
 type Server struct {
 	cfg     *serverConfig
@@ -456,6 +472,24 @@ func (s *Server) BuildLegacyCursor(ns command.Namespace, cursorID int64, batch [
 	return newLegacyCursor(ns, cursorID, batch, limit, batchSize, s)
 }
 
+// String implements the Stringer interface.
+func (s *Server) String() string {
+	desc := s.Description()
+	str := fmt.Sprintf("Addr: %s, Type: %s, State: %s",
+		s.address, desc.Kind, connectionStateString(s.connectionstate))
+	if len(desc.Tags) != 0 {
+		str += fmt.Sprintf(", Tag sets: %s", desc.Tags)
+	}
+	if s.connectionstate == connected {
+		str += fmt.Sprintf(", Avergage RTT: %d", s.averageRTT)
+	}
+	if desc.LastError != nil {
+		str += fmt.Sprintf(", Last error: %s", desc.LastError)
+	}
+
+	return str
+}
+
 // ServerSubscription represents a subscription to the description.Server updates for
 // a specific server.
 type ServerSubscription struct {

x/mongo/driver/topology/topology.go

@@ -18,6 +18,7 @@ import (
 	"sync/atomic"
 	"time"
 
+	"fmt"
 	"github.com/mongodb/mongo-go-driver/bson/bsoncodec"
 	"github.com/mongodb/mongo-go-driver/x/mongo/driver/session"
 	"github.com/mongodb/mongo-go-driver/x/network/address"
@@ -289,6 +290,10 @@ func (t *Topology) FindServer(selected description.Server) (*SelectedServer, err
 	}, nil
 }
 
+func wrapServerSelectionError(err error, t *Topology) error {
+	return fmt.Errorf("server selection error: %v\ncurrent topology: %s", err, t.String())
+}
+
 // selectServer is the core piece of server selection. It handles getting
 // topology descriptions and running sever selection on those descriptions.
 func (t *Topology) selectServer(ctx context.Context, subscriptionCh <-chan description.Topology, ss description.ServerSelector, timeoutCh <-chan time.Time) ([]description.Server, error) {
@@ -298,7 +303,7 @@ func (t *Topology) selectServer(ctx context.Context, subscriptionCh <-chan descr
 		case <-ctx.Done():
 			return nil, ctx.Err()
 		case <-timeoutCh:
-			return nil, ErrServerSelectionTimeout
+			return nil, wrapServerSelectionError(ErrServerSelectionTimeout, t)
 		case current = <-subscriptionCh:
 		}
 
@@ -311,7 +316,7 @@ func (t *Topology) selectServer(ctx context.Context, subscriptionCh <-chan descr
 
 		suitable, err := ss.SelectServer(current, allowed)
 		if err != nil {
-			return nil, err
+			return nil, wrapServerSelectionError(err, t)
 		}
 
 		if len(suitable) > 0 {
@@ -426,6 +431,16 @@ func (t *Topology) removeServer(ctx context.Context, addr address.Address, serve
 	delete(t.servers, addr)
 }
 
+// String implements the Stringer interface
+func (t *Topology) String() string {
+	desc := t.Description()
+	str := fmt.Sprintf("Type: %s\nServers:\n", desc.Kind)
+	for _, s := range t.servers {
+		str += s.String() + "\n"
+	}
+	return str
+}
+
 // Subscription is a subscription to updates to the description of the Topology that created this
 // Subscription.
 type Subscription struct {

x/mongo/driver/topology/topology_test.go

@@ -172,8 +172,8 @@ func TestServerSelection(t *testing.T) {
 			t.Errorf("Timed out while trying to retrieve selected servers")
 		}
 
-		if err != ErrServerSelectionTimeout {
-			t.Errorf("Incorrect error received. got %v; want %v", err, ErrServerSelectionTimeout)
+		if err == nil {
+			t.Fatalf("did not receive error from server selection")
 		}
 	})
 	t.Run("Error", func(t *testing.T) {
@@ -201,8 +201,8 @@ func TestServerSelection(t *testing.T) {
 			t.Errorf("Timed out while trying to retrieve selected servers")
 		}
 
-		if err != errSelectionError {
-			t.Errorf("Incorrect error received. got %v; want %v", err, errSelectionError)
+		if err == nil {
+			t.Fatalf("did not receive error from server selection")
 		}
 	})
 	t.Run("findServer returns topology kind", func(t *testing.T) {