CSHARP-600: added support only sending read preference when using one other than primary and secondary preferred or when tags are present.

Author: craiggwilson

Driver/Core/MongoCursorEnumerator.cs

@@ -46,6 +46,8 @@ public class MongoCursorEnumerator<TDocument> : IEnumerator<TDocument>
         private int _replyIndex;
         private ResponseFlags _responseFlags;
         private long _openCursorId;
+        private ReadPreference _readPreference;
+        private QueryFlags _queryFlags;
 
         // constructors
         /// <summary>
@@ -206,8 +208,24 @@ private MongoConnection AcquireConnection()
         {
             if (_serverInstance == null)
             {
+                _readPreference = _cursor.ReadPreference;
+                _queryFlags = _cursor.Flags;
+                if (_readPreference.ReadPreferenceMode != ReadPreferenceMode.Primary && _cursor.Collection.Name == "$cmd")
+                {
+                    var queryDocument = _cursor.Query.ToBsonDocument();
+                    var isSecondaryOk = MongoDefaults.CanCommandBeSentToSecondary(queryDocument);
+                    if (!isSecondaryOk)
+                    {
+                        // if the command can't be sent to a secondary, then we use primary here
+                        // regardless of the user's choice.
+                        _readPreference = ReadPreference.Primary;
+                        // remove the slaveOk bit from the flags
+                        _queryFlags &= ~QueryFlags.SlaveOk;
+                    }
+                }
+
                 // first time we need a connection let Server.AcquireConnection pick the server instance
-                var connection = _cursor.Server.AcquireConnection(_cursor.Database, _cursor.ReadPreference);
+                var connection = _cursor.Server.AcquireConnection(_cursor.Database, _readPreference);
                 _serverInstance = connection.ServerInstance;
                 return connection;
             }
@@ -248,7 +266,7 @@ private MongoReplyMessage<TDocument> GetFirst()
                 }
 
                 var writerSettings = _cursor.Collection.GetWriterSettings(connection);
-                using (var message = new MongoQueryMessage(writerSettings, _cursor.Collection.FullName, _cursor.Flags, _cursor.Skip, numberToReturn, WrapQuery(), _cursor.Fields))
+                using (var message = new MongoQueryMessage(writerSettings, _cursor.Collection.FullName, _queryFlags, _cursor.Skip, numberToReturn, WrapQuery(), _cursor.Fields))
                 {
                     return GetReply(connection, message);
                 }
@@ -331,15 +349,14 @@ private void KillCursor()
         private IMongoQuery WrapQuery()
         {
             BsonDocument formattedReadPreference = null;
-            if (_serverInstance.InstanceType == MongoServerInstanceType.ShardRouter)
+            if (_serverInstance.InstanceType == MongoServerInstanceType.ShardRouter 
+                && _readPreference.ReadPreferenceMode != ReadPreferenceMode.Primary)
             {
-                var readPreference = _cursor.ReadPreference;
-
                 BsonArray tagSetsArray = null;
-                if (readPreference.ReadPreferenceMode != ReadPreferenceMode.Primary && readPreference.TagSets != null)
+                if (_readPreference.TagSets != null)
                 {
                     tagSetsArray = new BsonArray();
-                    foreach (var tagSet in readPreference.TagSets)
+                    foreach (var tagSet in _readPreference.TagSets)
                     {
                         var tagSetDocument = new BsonDocument();
                         foreach (var tag in tagSet)
@@ -350,11 +367,14 @@ private IMongoQuery WrapQuery()
                     }
                 }
 
-                formattedReadPreference = new BsonDocument
+                if (tagSetsArray != null || _readPreference.ReadPreferenceMode != ReadPreferenceMode.SecondaryPreferred)
                 {
-                    { "mode", MongoUtils.ToCamelCase(readPreference.ReadPreferenceMode.ToString()) },
-                    { "tags", tagSetsArray, tagSetsArray != null } // optional
-                };
+                    formattedReadPreference = new BsonDocument
+                    {
+                        { "mode", MongoUtils.ToCamelCase(_readPreference.ReadPreferenceMode.ToString()) },
+                        { "tags", tagSetsArray, tagSetsArray != null } // optional
+                    };
+                }
             }
 
             if (_cursor.Options == null && formattedReadPreference == null)

Driver/MongoDefaults.cs

@@ -265,26 +265,17 @@ private static bool CanCommandBeSendToSecondaryDefault(BsonDocument document)
                 return false;
             }
 
-            var command = document.GetElement(0);
+            var commandName = document.GetElement(0).Name;
 
-            if (__secondaryOkCommands.Contains(command.Name))
+            if (__secondaryOkCommands.Contains(commandName))
             {
                 return true;
             }
 
-            if (!command.Value.IsBsonDocument)
+            BsonValue outValue;
+            if (document.TryGetValue("out", out outValue) && outValue.IsBsonDocument)
             {
-                return false;
-            }
-
-            if (command.Name.Equals("mapreduce", StringComparison.InvariantCultureIgnoreCase))
-            {
-                var options = command.Value.AsBsonDocument;
-                BsonValue outValue;
-                if (options.TryGetValue("out", out outValue) && outValue.IsBsonDocument)
-                {
-                    return outValue.AsBsonDocument.Contains("inline");
-                }
+                return outValue.AsBsonDocument.Contains("inline");
             }
 
             return false;

DriverUnitTests/MongoDefaultsTests.cs

@@ -29,13 +29,13 @@ public void TestCanSendCommandToSecondary(string command, bool expectedResult)
         [Test]
         public void TestCanSendInlineMapReduceToSecondary()
         {
-            var doc = new BsonDocument("mapreduce",
-                new BsonDocument
-                {
-                    { "map", "emit()" },
-                    { "reduce", "return 1" },
-                    { "out", new BsonDocument("inline", 1) }
-                });
+            var doc = new BsonDocument
+            {
+                { "mapreduce", "col" },
+                { "map", "emit()" },
+                { "reduce", "return 1" },
+                { "out", new BsonDocument("inline", 1) }
+            };
 
             var result = MongoDefaults.CanCommandBeSentToSecondary(doc);
 
@@ -45,12 +45,12 @@ public void TestCanSendInlineMapReduceToSecondary()
         [Test]
         public void TestCannotSendNonInlineMapReduceToSecondary()
         {
-            var doc = new BsonDocument("mapreduce",
-                new BsonDocument
-                {
-                    { "map", "emit()" },
-                    { "reduce", "return 1" }
-                });
+            var doc = new BsonDocument            
+            {
+                { "mapreduce", "col" },
+                { "map", "emit()" },
+                { "reduce", "return 1" }
+            };
 
             var result = MongoDefaults.CanCommandBeSentToSecondary(doc);
 
@@ -60,13 +60,13 @@ public void TestCannotSendNonInlineMapReduceToSecondary()
         [Test]
         public void TestCannotSendNonInlineMapReduceToSecondary2()
         {
-            var doc = new BsonDocument("mapreduce",
-                new BsonDocument
-                {
-                    { "map", "emit()" },
-                    { "reduce", "return 1" },
-                    { "out", new BsonDocument("merge", "foo") }
-                });
+            var doc = new BsonDocument            
+            {
+                { "mapreduce", "col" },
+                { "map", "emit()" },
+                { "reduce", "return 1" },
+                { "out", new BsonDocument("merge", "funny") }
+            };
 
             var result = MongoDefaults.CanCommandBeSentToSecondary(doc);