refactor(link): bound every teardown and handler wait behind one helper [CI]

backend/fleet/src/Fleet.Link/Transports.cs

@@ -118,6 +118,7 @@ public sealed class WsTransport : IRpcTransport
 public sealed class TlsFrameTransport : IRpcTransport
 {
     private const int HeaderBytes = 5;
+    private static readonly TimeSpan s_sendTimeout = TimeSpan.FromSeconds(20);
 
     private readonly Stream _stream;
     private readonly int _maxFrameBytes;
@@ -139,16 +140,29 @@ public sealed class TlsFrameTransport : IRpcTransport
         byte[] header = new byte[HeaderBytes];
         BinaryPrimitives.WriteUInt32LittleEndian(header, (uint)frame.Data.Length);
         header[4] = (byte)frame.Kind;
-        await _sendLock.WaitAsync(ct);
+        // 对端黑洞时写会一直挂着并占着发送锁，心跳、时钟同步和重连都跟着卡住：超时就掐掉这条连接
+        using var timeout = new CancellationTokenSource(s_sendTimeout);
+        using CancellationTokenSource linked = CancellationTokenSource.CreateLinkedTokenSource(ct, timeout.Token);
+        bool lockTaken = false;
         try
         {
-            await _stream.WriteAsync(header, ct);
-            await _stream.WriteAsync(frame.Data, ct);
-            await _stream.FlushAsync(ct);
+            await _sendLock.WaitAsync(linked.Token);
+            lockTaken = true;
+            await _stream.WriteAsync(header, linked.Token);
+            await _stream.WriteAsync(frame.Data, linked.Token);
+            await _stream.FlushAsync(linked.Token);
+        }
+        catch (OperationCanceledException) when (timeout.IsCancellationRequested && !ct.IsCancellationRequested)
+        {
+            await _stream.DisposeAsync();
+            throw new TimeoutException($"peer send timed out after {s_sendTimeout.TotalSeconds:0} seconds");
         }
         finally
         {
-            _sendLock.Release();
+            if (lockTaken)
+            {
+                _sendLock.Release();
+            }
         }
     }