Logging: fix race in disposal of a passed-in TextWriter

src/StackExchange.Redis/ConnectionMultiplexer.cs

@@ -638,7 +638,7 @@ private static async Task<ConnectionMultiplexer> ConnectImplAsync(ConfigurationO
             {
                 if (connectHandler != null && muxer != null) muxer.ConnectionFailed -= connectHandler;
                 if (killMe != null) try { killMe.Dispose(); } catch { }
-                if (log is TextWriterLogger twLogger) twLogger.Dispose();
+                if (log is TextWriterLogger twLogger) twLogger.Release();
             }
         }
 
@@ -740,7 +740,7 @@ private static ConnectionMultiplexer ConnectImpl(ConfigurationOptions configurat
             {
                 if (connectHandler != null && muxer != null) muxer.ConnectionFailed -= connectHandler;
                 if (killMe != null) try { killMe.Dispose(); } catch { }
-                if (log is TextWriterLogger twLogger) twLogger.Dispose();
+                if (log is TextWriterLogger twLogger) twLogger.Release();
             }
         }
 

src/StackExchange.Redis/TextWriterLogger.cs

@@ -4,10 +4,10 @@
 
 namespace StackExchange.Redis;
 
-internal sealed class TextWriterLogger : ILogger, IDisposable
+internal sealed class TextWriterLogger : ILogger
 {
     private TextWriter? _writer;
-    private ILogger? _wrapped;
+    private readonly ILogger? _wrapped;
 
     internal static Action<string> NullWriter = _ => { };
 
@@ -26,16 +26,27 @@ public void Log<TState>(LogLevel logLevel, EventId eventId, TState state, Except
         {
             lock (writer)
             {
-                writer.Write($"{DateTime.UtcNow:HH:mm:ss.ffff}: ");
-                writer.WriteLine(formatter(state, exception));
+                // We check here again because it's possible we've released below, and never want to write past releasing.
+                if (_writer is TextWriter innerWriter)
+                {
+                    innerWriter.Write($"{DateTime.UtcNow:HH:mm:ss.ffff}: ");
+                    innerWriter.WriteLine(formatter(state, exception));
+                }
             }
         }
     }
 
-    public void Dispose()
+    public void Release()
     {
-        _writer = null;
-        _wrapped = null;
+        // We lock here because we may have piled up on a lock above and still be writing.
+        // We never want a write to go past the Release(), as many TextWriter implementations are not thread safe.
+        if (_writer is TextWriter writer)
+        {
+            lock (writer)
+            {
+                _writer = null;
+            }
+        }
     }
 }