GODRIVER-1713 Pass read preference to operations layer for RunCommand (#487)

Author: Divjot Arora

mongo/database.go

@@ -168,10 +168,12 @@ func (db *Database) processRunCommand(ctx context.Context, cmd interface{},
 	return operation.NewCommand(runCmdDoc).
 		Session(sess).CommandMonitor(db.client.monitor).
 		ServerSelector(readSelect).ClusterClock(db.client.clock).
-		Database(db.name).Deployment(db.client.deployment).ReadConcern(db.readConcern).Crypt(db.client.crypt), sess, nil
+		Database(db.name).Deployment(db.client.deployment).ReadConcern(db.readConcern).
+		Crypt(db.client.crypt).ReadPreference(ro.ReadPreference), sess, nil
 }
 
-// RunCommand executes the given command against the database.
+// RunCommand executes the given command against the database. This function does not obey the Database's read
+// preference. To specify a read preference, the RunCmdOptions.ReadPreference option must be used.
 //
 // The runCommand parameter must be a document for the command to be executed. It cannot be nil.
 // This must be an order-preserving type such as bson.D. Map types such as bson.M are not valid.
@@ -198,8 +200,9 @@ func (db *Database) RunCommand(ctx context.Context, runCommand interface{}, opts
 }
 
 // RunCommandCursor executes the given command against the database and parses the response as a cursor. If the command
-// being executed does not return a cursor (e.g. insert), the command will be executed on the server and an error
-// will be returned because the server response cannot be parsed as a cursor.
+// being executed does not return a cursor (e.g. insert), the command will be executed on the server and an error will
+// be returned because the server response cannot be parsed as a cursor. This function does not obey the Database's read
+// preference. To specify a read preference, the RunCmdOptions.ReadPreference option must be used.
 //
 // The runCommand parameter must be a document for the command to be executed. It cannot be nil.
 // This must be an order-preserving type such as bson.D. Map types such as bson.M are not valid.

mongo/integration/database_test.go

@@ -18,6 +18,8 @@ import (
 	"go.mongodb.org/mongo-driver/mongo"
 	"go.mongodb.org/mongo-driver/mongo/integration/mtest"
 	"go.mongodb.org/mongo-driver/mongo/options"
+	"go.mongodb.org/mongo-driver/mongo/readpref"
+	"go.mongodb.org/mongo-driver/x/bsonx/bsoncore"
 )
 
 const (
@@ -60,6 +62,32 @@ func TestDatabase(t *testing.T) {
 			assert.Equal(mt, true, result.IsMaster, "expected isMaster value true, got false")
 			assert.Equal(mt, 1.0, result.Ok, "expected ok value 1.0, got %v", result.Ok)
 		})
+
+		// We set min server version 3.6 because pre-3.6 servers use OP_QUERY, so the command document will look like
+		// {$query: {...}, $readPreference: {...}}. Per the command monitoring spec, the $query subdocument is unwrapped
+		// and the $readPreference is dropped for monitoring purposes, so we can't examine it.
+		readPrefOpts := mtest.NewOptions().
+			Topologies(mtest.Sharded).
+			MinServerVersion("3.6")
+		mt.RunOpts("read pref passed to mongos", readPrefOpts, func(mt *mtest.T) {
+			// When communicating with a mongos, the supplied read preference should be passed down to the operations
+			// layer, which should add a top-level $readPreference field to the command.
+
+			runCmdOpts := options.RunCmd().
+				SetReadPreference(readpref.SecondaryPreferred())
+			err := mt.DB.RunCommand(mtest.Background, bson.D{{"isMaster", 1}}, runCmdOpts).Err()
+			assert.Nil(mt, err, "RunCommand error: %v", err)
+
+			expected := bson.Raw(bsoncore.BuildDocumentFromElements(
+				nil,
+				bsoncore.AppendStringElement(nil, "mode", "secondaryPreferred"),
+			))
+			evt := mt.GetStartedEvent()
+			assert.Equal(mt, "isMaster", evt.CommandName, "expected 'isMaster' command to be sent, got %q", evt.CommandName)
+			actual, ok := evt.Command.Lookup("$readPreference").DocumentOK()
+			assert.True(mt, ok, "expected command %v to contain a $readPreference document", evt.Command)
+			assert.Equal(mt, expected, actual, "expected $readPreference document %v, got %v", expected, actual)
+		})
 	})
 
 	dropOpts := mtest.NewOptions().DatabaseName("dropDb")