CSHARP-2628: Fix issue with creating a not required connection in RetryableReads.

Author: DmitryLukyanov

src/MongoDB.Driver.Core/Core/Operations/CommandOperationBase.cs

@@ -13,15 +13,13 @@
 * limitations under the License.
 */
 
-using System;
 using System.Threading;
 using System.Threading.Tasks;
 using MongoDB.Bson;
 using MongoDB.Bson.IO;
 using MongoDB.Bson.Serialization;
 using MongoDB.Driver.Core.Bindings;
 using MongoDB.Driver.Core.Misc;
-using MongoDB.Driver.Core.Servers;
 using MongoDB.Driver.Core.WireProtocol;
 using MongoDB.Driver.Core.WireProtocol.Messages.Encoders;
 
@@ -144,7 +142,17 @@ public IBsonSerializer<TCommandResult> ResultSerializer
         }
 
         // methods
-        private TCommandResult ExecuteProtocol(IChannelHandle channel, ICoreSessionHandle session, ReadPreference readPreference, CancellationToken cancellationToken)
+        /// <summary>
+        /// Executes the protocol.
+        /// </summary>
+        /// <param name="channel">The channel.</param>
+        /// <param name="session">The session.</param>
+        /// <param name="readPreference">The read preference.</param>
+        /// <param name="cancellationToken">The cancellation token.</param>
+        /// <returns>
+        /// A Task whose result is the command result.
+        /// </returns>
+        protected TCommandResult ExecuteProtocol(IChannelHandle channel, ICoreSessionHandle session, ReadPreference readPreference, CancellationToken cancellationToken)
         {
             var additionalOptions = GetEffectiveAdditionalOptions();
 
@@ -185,7 +193,17 @@ protected TCommandResult ExecuteProtocol(
             }
         }
 
-        private Task<TCommandResult> ExecuteProtocolAsync(IChannelHandle channel, ICoreSessionHandle session, ReadPreference readPreference, CancellationToken cancellationToken)
+        /// <summary>
+        /// Executes the protocol.
+        /// </summary>
+        /// <param name="channel">The channel.</param>
+        /// <param name="session">The session.</param>
+        /// <param name="readPreference">The read preference.</param>
+        /// <param name="cancellationToken">The cancellation token.</param>
+        /// <returns>
+        /// A Task whose result is the command result.
+        /// </returns>
+        protected Task<TCommandResult> ExecuteProtocolAsync(IChannelHandle channel, ICoreSessionHandle session, ReadPreference readPreference, CancellationToken cancellationToken)
         {
             var additionalOptions = GetEffectiveAdditionalOptions();
 

src/MongoDB.Driver.Core/Core/Operations/ReadCommandOperation.cs

@@ -13,7 +13,6 @@
 * limitations under the License.
 */
 
-using System;
 using System.Threading;
 using System.Threading.Tasks;
 using MongoDB.Bson;
@@ -51,7 +50,7 @@ public ReadCommandOperation(
         {
         }
 
-        // public properties        
+        // public properties
         /// <summary>
         /// Gets or sets a value indicating whether to retry.
         /// </summary>
@@ -62,7 +61,7 @@ public bool RetryRequested
             set => _retryRequested = value;
         }
 
-        // public methods        
+        // public methods
         /// <inheritdoc/>
         public TCommandResult Execute(IReadBinding binding, CancellationToken cancellationToken)
         {
@@ -110,13 +109,13 @@ public async Task<TCommandResult> ExecuteAsync(RetryableReadContext context, Can
         /// <inheritdoc/>
         public TCommandResult ExecuteAttempt(RetryableReadContext context, int attempt, long? transactionNumber, CancellationToken cancellationToken)
         {
-            return ExecuteProtocol(context.ChannelSource, context.Binding.Session, context.Binding.ReadPreference, cancellationToken);
+            return ExecuteProtocol(context.Channel, context.Binding.Session, context.Binding.ReadPreference, cancellationToken);
         }
 
         /// <inheritdoc/>
         public Task<TCommandResult> ExecuteAttemptAsync(RetryableReadContext context, int attempt, long? transactionNumber, CancellationToken cancellationToken)
         {
-            return ExecuteProtocolAsync(context.ChannelSource, context.Binding.Session, context.Binding.ReadPreference, cancellationToken);
+            return ExecuteProtocolAsync(context.Channel, context.Binding.Session, context.Binding.ReadPreference, cancellationToken);
         }
     }
 }

tests/MongoDB.Driver.Core.Tests/Core/Operations/ReadCommandOperationTests.cs

@@ -312,6 +312,30 @@ public void Execute_should_call_channel_Command_with_wrapped_command_when_readPr
             }
         }
 
+        [Theory]
+        [ParameterAttributeData]
+        public void Execute_should_call_GetChannel_only_once([Values(false, true)] bool async)
+        {
+            var subject = CreateSubject<BsonDocument>();
+            var readPreference = ReadPreference.Primary;
+            var serverDescription = CreateServerDescription(ServerType.Standalone);
+            var mockChannel = CreateMockChannel();
+            var mockChannelSource = CreateMockChannelSource(serverDescription, mockChannel.Object);
+            var binding = CreateMockReadBinding(readPreference, mockChannelSource.Object).Object;
+            var cancellationToken = new CancellationTokenSource().Token;
+
+            if (async)
+            {
+                subject.ExecuteAsync(binding, cancellationToken).GetAwaiter().GetResult();
+                mockChannelSource.Verify(c => c.GetChannelAsync(cancellationToken), Times.Once);
+            }
+            else
+            {
+                subject.Execute(binding, cancellationToken);
+                mockChannelSource.Verify(c => c.GetChannel(cancellationToken), Times.Once);
+            }
+        }
+
         // private methods
         private Mock<IReadBinding> CreateMockReadBinding(ReadPreference readPreference, IChannelSourceHandle channelSource)
         {