Sentinel Support Derived from pr-692 (#1067)

This PR is derived from PR-692 and have been merged with the latest master commit.
Things that have been done:
1. review code for PR-692
2. fixed potential infinite loop in the code
3. Adapt code to success build with the latest master commit
4. Manual testing on 3 Sentinel nodes and 3 Redis nodes (connection and failover)

Usage:
```C#
                ConfigurationOptions sentinelConfig = new ConfigurationOptions();
                sentinelConfig.ServiceName = "mymaster";
                sentinelConfig.EndPoints.Add("192.168.99.102", 26379);
                sentinelConfig.EndPoints.Add("192.168.99.102", 26380);
                sentinelConfig.EndPoints.Add("192.168.99.102", 26381);
                sentinelConfig.TieBreaker = "";
                sentinelConfig.DefaultVersion = new Version(4, 0, 11);                 
                // its important to set the Sentinel commands supported
                sentinelConfig.CommandMap = CommandMap.Sentinel;

                // Get sentinel connection
                ConnectionMultiplexer sentinelConnection = ConnectionMultiplexer.Connect(sentinelConfig, Console.Out);
                // Create master service configuration
                ConfigurationOptions masterConfig = new ConfigurationOptions { ServiceName = "mymaster" };
                // Get master Redis connection
                var redisMasterConnection = sentinelConnection.GetSentinelMasterConnection(masterConfig);

                ...
               IDatabase db = redisMasterConnection.GetDatabase();                
               db.StringSet(key, value);
               ...
               string value1 = db.StringGet(key);

```

appveyor.yml

@@ -38,6 +38,8 @@ install:
     redis-server.exe --service-install --service-name "redis-26379" "..\Sentinel\sentinel-26379.conf" --sentinel
 
     redis-server.exe --service-install --service-name "redis-26380" "..\Sentinel\sentinel-26380.conf" --sentinel
+    
+    redis-server.exe --service-install --service-name "redis-26381" "..\Sentinel\sentinel-26381.conf" --sentinel
 
     cd ..\..\..
 - sh: >-

src/StackExchange.Redis/Interfaces/IServer.cs

@@ -752,6 +752,15 @@ public partial interface IServer : IRedis
         /// <remarks>https://redis.io/topics/sentinel</remarks>
         Task<EndPoint> SentinelGetMasterAddressByNameAsync(string serviceName, CommandFlags flags = CommandFlags.None);
 
+        /// <summary>
+        /// Returns the ip and port numbers of all known Sentinels
+        /// for the given service name.
+        /// </summary>
+        /// <param name="serviveName">the sentinel service name</param>
+        /// <param name="flags"></param>
+        /// <returns>a list of the sentinel ips and ports</returns>
+        Task<EndPoint[]> SentinelGetSentinelAddresses(string serviveName, CommandFlags flags = CommandFlags.None);
+
         /// <summary>
         /// Show the state and info of the specified master.
         /// </summary>

src/StackExchange.Redis/RedisServer.cs

@@ -807,6 +807,12 @@ public Task<EndPoint> SentinelGetMasterAddressByNameAsync(string serviceName, Co
             return ExecuteAsync(msg, ResultProcessor.SentinelMasterEndpoint);
         }
 
+        public Task<EndPoint[]> SentinelGetSentinelAddresses(string serviceName, CommandFlags flags = CommandFlags.None)
+        {
+            var msg = Message.Create(-1, flags, RedisCommand.SENTINEL, RedisLiterals.SENTINELS, (RedisValue)serviceName);
+            return ExecuteAsync(msg, ResultProcessor.SentinelAddressesEndPoints);            
+        }
+
         public KeyValuePair<string, string>[] SentinelMaster(string serviceName, CommandFlags flags = CommandFlags.None)
         {
             var msg = Message.Create(-1, flags, RedisCommand.SENTINEL, RedisLiterals.MASTER, (RedisValue)serviceName);

src/StackExchange.Redis/ResultProcessor.cs

@@ -125,6 +125,9 @@ public static readonly StreamPendingMessagesProcessor
         public static readonly ResultProcessor<EndPoint>
             SentinelMasterEndpoint = new SentinelGetMasterAddressByNameProcessor();
 
+        public static readonly ResultProcessor<EndPoint[]>
+            SentinelAddressesEndPoints = new SentinelGetSentinelAddresses();
+
         public static readonly ResultProcessor<KeyValuePair<string, string>[][]>
             SentinelArrayOfArrays = new SentinelArrayOfArraysProcessor();
 
@@ -714,6 +717,11 @@ protected override bool SetResultCore(PhysicalConnection connection, Message mes
                                 }
                             }
                         }
+                        else if (message?.Command == RedisCommand.SENTINEL)
+                        {
+                            server.ServerType = ServerType.Sentinel;
+                            server.Multiplexer.Trace("Auto-configured server-type: sentinel");
+                        }
                         SetResult(message, true);
                         return true;
                     case ResultType.MultiBulk:
@@ -765,6 +773,11 @@ protected override bool SetResultCore(PhysicalConnection connection, Message mes
                                 }
                             }
                         }
+                        else if (message?.Command == RedisCommand.SENTINEL)
+                        {
+                            server.ServerType = ServerType.Sentinel;
+                            server.Multiplexer.Trace("Auto-configured server-type: sentinel");
+                        }
                         SetResult(message, true);
                         return true;
                 }
@@ -1970,12 +1983,62 @@ protected override bool SetResultCore(PhysicalConnection connection, Message mes
                             return true;
                         }
                         break;
+                }
+                return false;
+            }
+        }
+
+        private sealed class SentinelGetSentinelAddresses : ResultProcessor<EndPoint[]>
+        {
+            protected override bool SetResultCore(PhysicalConnection connection, Message message, in RawResult result)
+            {
+                List<EndPoint> endPoints = new List<EndPoint>();
+
+                switch (result.Type)
+                {
+                    case ResultType.MultiBulk:
+                        foreach (RawResult item in result.GetItems())
+                        {
+                            var arr = item.GetItemsAsValues();
+                            string ip = null;
+                            string portStr = null;
+
+                            for (int i = 0; i < arr.Length && (ip == null || portStr == null); i += 2)
+                            {
+                                string name = arr[i];
+                                string value = arr[i + 1];
+
+                                switch (name)
+                                {
+                                    case "ip":
+                                        ip = value;
+                                        break;
+                                    case "port":
+                                        portStr = value;
+                                        break;
+                                }
+                            }
+
+                            if (ip != null && portStr != null && int.TryParse(portStr, out int port))
+                            {
+                                endPoints.Add(Format.ParseEndPoint(ip, port));
+                            }
+                        }
+                        break;
+
                     case ResultType.SimpleString:
                         //We don't want to blow up if the master is not found
                         if (result.IsNull)
                             return true;
                         break;
                 }
+
+                if (endPoints.Count > 0)
+                {
+                    SetResult(message, endPoints.ToArray());
+                    return true;
+                }
+
                 return false;
             }
         }

src/StackExchange.Redis/ServerEndPoint.cs

@@ -284,6 +284,12 @@ internal void AutoConfigure(PhysicalConnection connection)
                 msg.SetInternalCall();
                 WriteDirectOrQueueFireAndForgetSync(connection, msg, ResultProcessor.AutoConfigure);
             }
+            if (commandMap.IsAvailable(RedisCommand.SENTINEL))
+            {
+                msg = Message.Create(-1, flags, RedisCommand.SENTINEL, RedisLiterals.MASTERS);
+                msg.SetInternalCall();
+                WriteDirectOrQueueFireAndForgetSync(connection, msg, ResultProcessor.AutoConfigure);
+            }
             if (commandMap.IsAvailable(RedisCommand.INFO))
             {
                 lastInfoReplicationCheckTicks = Environment.TickCount;

tests/RedisConfigs/Sentinel/sentinel-26381.conf

 port 26381
-sentinel monitor mymaster 127.0.0.1 7011 2
+sentinel monitor mymaster 127.0.0.1 7010 1
 sentinel down-after-milliseconds mymaster 1000
 sentinel failover-timeout mymaster 1000
 sentinel config-epoch mymaster 0

tests/RedisConfigs/start-all.sh

@@ -33,7 +33,7 @@ redis-server cluster-7005.conf &>/dev/null &
 popd > /dev/null
 
 #Sentinel Servers
-echo Starting Sentinel: 7010-7011,26379-26380
+echo Starting Sentinel: 7010-7011,26379-26381
 pushd Sentinel > /dev/null
 echo "${INDENT}Targets: 7010-7011"
 redis-server redis-7010.conf &>/dev/null &

tests/StackExchange.Redis.Tests/Helpers/TestConfig.cs

@@ -79,7 +79,9 @@ public class Config
             public string RemoteServerAndPort => RemoteServer + ":" + RemotePort.ToString();
 
             public string SentinelServer { get; set; } = "127.0.0.1";
-            public int SentinelPort { get; set; } = 26379;
+            public int SentinelPortA { get; set; } = 26379;
+            public int SentinelPortB { get; set; } = 26380;
+            public int SentinelPortC { get; set; } = 26381;
             public string SentinelSeviceName { get; set; } = "mymaster";
 
             public string ClusterServer { get; set; } = "127.0.0.1";