SENTINEL: defensive check around ROLE

Author: mgravell

docs/ReleaseNotes.md

@@ -8,7 +8,7 @@ Current package versions:
 
 ## Unreleased
 
-- (none)
+- Avoid sentinel issues if `ROLE` unavailable; fix #3064
 
 ## 2.13.10
 

src/StackExchange.Redis/ConnectionMultiplexer.Sentinel.cs

@@ -240,10 +240,28 @@ public ConnectionMultiplexer GetSentinelMasterConnection(ConfigurationOptions co
 
             // verify role is primary according to:
             // https://redis.io/topics/sentinel-clients
-            if (connection.GetServer(newPrimaryEndPoint)?.Role()?.Value == RedisLiterals.master)
+            bool isPrimary;
+            var server = connection.GetServer(newPrimaryEndPoint);
+            // ReSharper disable once ConditionIsAlwaysTrueOrFalseAccordingToNullableAPIContract
+            if (server is { })
             {
-                success = true;
-                break;
+                try
+                {
+                    isPrimary = connection.CommandMap.IsAvailable(RedisCommand.ROLE)
+                        ? server.Role()?.Value == RedisLiterals.master
+                        : !server.IsReplica;
+                }
+                catch
+                {
+                    // fallback if ROLE unavailable but not declared; see #3064
+                    isPrimary = !server.IsReplica;
+                }
+
+                if (isPrimary)
+                {
+                    success = true;
+                    break;
+                }
             }
 
             Thread.Sleep(100);