fix all-down connection scenarios

Author: mgravell

src/StackExchange.Redis/HealthCheck.StringSetProbe.cs

@@ -35,7 +35,6 @@ public override async Task<HealthCheckResult> CheckHealthAsync(HealthCheck healt
             SharedRandom.NextBytes(pooled);
 #endif
             var payload = (RedisValue)pooled.AsMemory(0, LEN);
-            Lease<byte>? lease = null;
             try
             {
                 // write a value to the db
@@ -52,7 +51,6 @@ await database.LockTakeAsync(
             finally
             {
                 ArrayPool<byte>.Shared.Return(pooled);
-                lease?.Dispose();
             }
         }
     }

src/StackExchange.Redis/MultiGroupDatabase.Async.cs

@@ -11,7 +11,7 @@ public Task<RedisValue> DebugObjectAsync(RedisKey key, CommandFlags flags = Comm
         => GetActiveDatabase().DebugObjectAsync(key, flags);
 
     public Task<EndPoint?> IdentifyEndpointAsync(RedisKey key = default, CommandFlags flags = CommandFlags.None)
-        => GetActiveDatabase().IdentifyEndpointAsync(key, flags);
+        => TryGetActiveDatabase()?.IdentifyEndpointAsync(key, flags) ?? NoEndpoint;
 
     public Task KeyMigrateAsync(RedisKey key, EndPoint toServer, int toDatabase = 0, int timeoutMilliseconds = 0, MigrateOptions migrateOptions = MigrateOptions.None, CommandFlags flags = CommandFlags.None)
         => GetActiveDatabase().KeyMigrateAsync(key, toServer, toDatabase, timeoutMilliseconds, migrateOptions, flags);

src/StackExchange.Redis/MultiGroupDatabase.cs

@@ -13,6 +13,14 @@ internal sealed partial class MultiGroupDatabase(MultiGroupMultiplexer parent, i
     // for high DB numbers this might allocate even for null async-state scenarios; unavoidable for now
     private IDatabase GetActiveDatabase() => parent.Active.GetDatabase(database, asyncState);
 
+    // non-throwing twin of GetActiveDatabase: returns null when the group currently has no active
+    // member, for use by members that have an obvious trivial result when disconnected
+    private IDatabase? TryGetActiveDatabase() => parent.TryGetActive()?.GetDatabase(database, asyncState);
+
+    // a completed "no endpoint" result, reused for IdentifyEndpointAsync when the group is fully down
+    internal static readonly System.Threading.Tasks.Task<System.Net.EndPoint?> NoEndpoint
+        = System.Threading.Tasks.Task.FromResult<System.Net.EndPoint?>(null);
+
     // Core methods
     public IBatch CreateBatch(object? asyncState = null)
         => GetActiveDatabase().CreateBatch(asyncState);
@@ -27,10 +35,10 @@ public RedisValue DebugObject(RedisKey key, CommandFlags flags = CommandFlags.No
         => GetActiveDatabase().DebugObject(key, flags);
 
     public System.Net.EndPoint? IdentifyEndpoint(RedisKey key = default, CommandFlags flags = CommandFlags.None)
-        => GetActiveDatabase().IdentifyEndpoint(key, flags);
+        => TryGetActiveDatabase()?.IdentifyEndpoint(key, flags);
 
     public bool IsConnected(RedisKey key, CommandFlags flags = CommandFlags.None)
-        => GetActiveDatabase().IsConnected(key, flags);
+        => TryGetActiveDatabase()?.IsConnected(key, flags) ?? false;
 
     public System.TimeSpan Ping(CommandFlags flags = CommandFlags.None)
         => GetActiveDatabase().Ping(flags);

src/StackExchange.Redis/MultiGroupMultiplexer.cs

@@ -255,6 +255,9 @@ static ConnectionMultiplexer Throw() =>
         }
     }
 
+    // non-throwing twin of Active, for callers that have a trivial answer when the group is fully down
+    internal ConnectionMultiplexer? TryGetActive() => _active;
+
     private ConnectionGroupMember? GetActiveMember()
     {
         var active = _active;