Skip to main content

Cancellation, errors, closing

A connection can end in three ways: cancel the operation through a CancellationToken, call Abort with a reason, or call DisposeAsync. All three close the connection, but they behave differently for a call that is waiting for bytes at that moment, and they leave a different trace in CloseReason. What follows covers MinecraftConnection and MinecraftClient. StreamingConnection follows the same rules, with one addition at the end of the page.

Three ways​

The token passes into ReadPacketAsync, WritePacketAsync, and SendAsync. Exactly one case comes cheap: the token is already canceled before the call. Then the check at the entry throws OperationCanceledException, and the connection stays alive. Once the call has started, cancellation closes the whole connection - even if not a single byte has arrived from the socket yet. The next section explains why.

Abort(reason) can be called from any thread and at any moment, with no wait for a response: a read or write that was holding the socket at that moment fails with the given reason, and CloseReason remembers it. This is the tool the application code uses to tell the library that the protocol broke the connection, not the transport - for example, the server sent a packet that should not appear in this phase.

DisposeAsync uses the same Abort, but with no reason, and adds a wait after it. The order of these steps is described in "Packet stream". At the MinecraftClient level, its own send gate joins this: SendAsync and SendRawAsync hold the gate for the duration of the call, and the client's DisposeAsync waits for the gate to free up within the same five-second budget, then closes the connection regardless - even if the gate never freed up.

Why canceling a started read breaks the connection​

A started read already sits inside the frame reader, and from outside it is not visible what exactly it managed to take: part of a frame - the length or a piece of the body - may already be pulled from the stream and sitting in the buffer. There is no way to give it back, and so there is no way to return the connection to a frame boundary for the next call: the frame is broken, the same as after a network break or an encoding error.

So the connection does not tell a "canceled read" apart from a "failed read" - both close the connection. The caller whose token fired gets OperationCanceledException. Any other call, started later or waiting alongside it, gets ConnectionAbortedException with that same exception inside it. For the application code this means: the token passed into ReadPacketsAsync works as a switch for the whole read loop, not as a way to cancel the current call and get the next packet as if nothing happened.

How to learn the reason​

CloseReason is null while the connection is open, and it stays null after a clean end of stream. In every other case it holds an exception: either what was passed into Abort, or the first failure that the connection's own reader or writer caught. Completion is a task that completes at the moment of closing and never faults: the code can wait for it without a try/catch to learn that closing happened, and then read CloseReason to learn why.

The first failure of the stream reaches the code that caused it as its own exception type, and at the same moment it settles into CloseReason. Every later call - of the same method and of any other connection member - does not touch the dead socket again, and instead throws ConnectionAbortedException right away, with the same reason inside it: a second reader or writer that arrives at an already-dead connection sees the real reason, not some unrelated later error. The exception to this rule is InvalidOperationException that does not come from ObjectDisposedException: a concurrent read or a call after ToStreaming is a bug in the calling code that never reached the stream, and the connection does not close because of it.

What happened -> which exception​

What happenedException
The server closed the stream cleanly, between framesEndOfStreamException
The stream broke off in the middle of a frameEndOfStreamException
This side closed while a call was waiting for bytesConnectionAbortedException
A call after DisposeAsync completedObjectDisposedException
A second, concurrent ReadPacketAsync on the connectionInvalidOperationException
A broken frame (length, varint, decompression size)InvalidDataException

The reader does not tell the first two cases apart - both arrive as a plain EndOfStreamException. The row about a broken frame is not the connection closing for an outside reason, but a finding by the reader itself: it also settles into CloseReason, but it is a data error, not a broken connection.

Closing order in application code​

await using var client = new MinecraftClient(options);
await client.ConnectAsync(token);
try
{
    await foreach (var packet in client.ReadPacketsAsync(token))
        Handle(packet);
}
catch (EndOfStreamException)
{
    // a normal end of session, the server closed the stream itself
}
catch (ConnectionAbortedException ex)
{
    Log(ex.InnerException); // the reason is already inside
}

There is no need to call Abort or DisposeAsync separately here - await using takes care of that: the send gate waits out its budget, and the buffers return to the pool. An explicit Abort is needed only when code unrelated to the read loop closes the connection - for example, another task that notices the server is not following the protocol.

The streaming path​

A StreamingConnection obtained through ToStreaming inherits the common behavior: Abort from any thread, CloseReason, Completion, and the same memory of the first error - it settles into CloseReason, and every later call gets it too. The difference sits in three places.

The main difference is cancellation. A ReadBatchAsync canceled by its own token does not close the connection: the buffer is intact, the frame boundary is not lost, and reading can continue. Only canceling FlushAsync closes the connection, and only after bytes have already gone into the stream.

Next to Abort it has CompleteAsync - a clean finish that sends off whatever accumulated in the send buffer, after which CloseReason stays null. And its DisposeAsync does not use the five-second budget: it waits out Completion in full, and it drops any bytes not yet flushed into the stream.

Next​