CSHARP-1694: Add support for linearizable read concern.

Author: craiggwilson

src/MongoDB.Driver.Core/ReadConcern.cs

@@ -26,6 +26,7 @@ namespace MongoDB.Driver
     public sealed class ReadConcern : IEquatable<ReadConcern>, IConvertibleToBsonDocument
     {
         private static readonly ReadConcern __default = new ReadConcern();
+        private static readonly ReadConcern __linearizable = new ReadConcern(ReadConcernLevel.Linearizable);
         private static readonly ReadConcern __local = new ReadConcern(ReadConcernLevel.Local);
         private static readonly ReadConcern __majority = new ReadConcern(ReadConcernLevel.Majority);
 
@@ -34,13 +35,18 @@ public sealed class ReadConcern : IEquatable<ReadConcern>, IConvertibleToBsonDoc
         /// </summary>
         public static ReadConcern Default => __default;
 
+        /// <summary>
+        /// Gets a linearizable read concern.
+        /// </summary>
+        public static ReadConcern Linearizable => __linearizable;
+
         /// <summary>
         /// Gets a local read concern.
         /// </summary>
         public static ReadConcern Local => __local;
 
         /// <summary>
-        /// Gets a majority read concern
+        /// Gets a majority read concern.
         /// </summary>
         public static ReadConcern Majority => __majority;
 
@@ -59,6 +65,8 @@ public static ReadConcern FromBsonDocument(BsonDocument document)
                 var level = (ReadConcernLevel)Enum.Parse(typeof(ReadConcernLevel), (string)levelValue, true);
                 switch (level)
                 {
+                    case ReadConcernLevel.Linearizable:
+                        return ReadConcern.Linearizable;
                     case ReadConcernLevel.Local:
                         return ReadConcern.Local;
                     case ReadConcernLevel.Majority:
@@ -140,6 +148,9 @@ public BsonDocument ToBsonDocument()
             {
                 switch (_level.Value)
                 {
+                    case ReadConcernLevel.Linearizable:
+                        level = "linearizable";
+                        break;
                     case ReadConcernLevel.Local:
                         level = "local";
                         break;

src/MongoDB.Driver.Core/ReadConcernLevel.cs

@@ -33,6 +33,14 @@ public enum ReadConcernLevel
         /// <summary>
         /// Reads data committed to a majority of nodes.
         /// </summary>
-        Majority
+        Majority,
+        /// <summary>
+        /// Avoids returning data from a "stale" primary 
+        /// (one that has already been superseded by a new primary but doesn't know it yet). 
+        /// It is important to note that readConcern level linearizable does not by itself 
+        /// produce linearizable reads; they must be issued in conjunction with w:majority 
+        /// writes to the same document(s) in order to be linearizable.
+        /// </summary>
+        Linearizable
     }
 }

tests/MongoDB.Driver.Core.Tests/ReadConcernTests.cs

@@ -54,13 +54,17 @@ public void Equals_should_return_false_when_level_is_not_equal()
             ReadConcern.Default.Should().NotBe(ReadConcern.Local);
             ReadConcern.Default.Should().NotBe(ReadConcern.Majority);
             ReadConcern.Local.Should().NotBe(ReadConcern.Majority);
+            ReadConcern.Linearizable.Should().NotBe(ReadConcern.Local);
+            ReadConcern.Linearizable.Should().NotBe(ReadConcern.Majority);
         }
 
         [Fact]
         public void Equals_should_return_true_when_level_is_equal()
         {
             ReadConcern.Default.Should().Be(ReadConcern.Default);
             new ReadConcern().Should().Be(ReadConcern.Default);
+            ReadConcern.Linearizable.Should().Be(ReadConcern.Linearizable);
+            new ReadConcern(ReadConcernLevel.Linearizable).Should().Be(ReadConcern.Linearizable);
             ReadConcern.Local.Should().Be(ReadConcern.Local);
             new ReadConcern(ReadConcernLevel.Local).Should().Be(ReadConcern.Local);
             ReadConcern.Majority.Should().Be(ReadConcern.Majority);
@@ -101,6 +105,7 @@ public void ThrowIfNotSupported_should_throw_when_the_serverVersion_is_less_than
         }
 
         [Theory]
+        [InlineData(ReadConcernLevel.Linearizable, "{ level: 'linearizable' }")]
         [InlineData(ReadConcernLevel.Local, "{ level: 'local' }")]
         [InlineData(ReadConcernLevel.Majority, "{ level: 'majority' }")]
         [InlineData(null, "{ }")]