fix(network): stop silently dropping packets when the send buffer is full

NetState.Send had three failure modes once the send buffer filled, none of
them visible:

  - No writable space: GetSendBuffer returned false and the packet was
    dropped with no log and no disconnect. The client stayed connected
    while quietly missing game state.

  - Some space but not enough: NetworkCompression.Compress hit its guard
    and returned 0, so CommitWrite(0) ran and the packet was dropped the
    same way.

  - One to three bytes writable: Compress computes
    safeOutputLength = (nuint)output.Length - 4, which underflows for
    those sizes. The hot loop's bounds check never trips and it writes
    past the span, corrupting the in-flight region of the ring buffer.
    Reachable because callers only check for non-zero space.

Compress now refuses an output too small to bound, and Send reports
exhaustion instead of dropping: it logs and disconnects with the bytes
needed, writable, awaiting acknowledgement, and total capacity. Those
numbers separate a slow client holding the buffer from a buffer genuinely
too small for the shard, which is the case that warrants raising
network.sendBufferSize.

Tests cover the underflow via sentinel bytes around the output window and
were confirmed to fail without the guard.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Projects/Server.Tests/Tests/Network/NetworkCompressionBoundsTests.cs

@@ -0,0 +1,86 @@
+using System;
+using Server.Network;
+using Xunit;
+
+namespace Server.Tests.Network;
+
+/// <summary>
+/// Bounds behaviour of the Huffman compressor when the destination is too small.
+///
+/// This is reachable in production: NetState only checks that the send buffer has *some* writable
+/// space before handing the remainder to Compress, so a nearly-full buffer can offer a span of one
+/// to three bytes. The internal guard is computed as an unsigned <c>output.Length - 4</c>, which
+/// underflows for those sizes and stops bounding the writes at all.
+/// </summary>
+public class NetworkCompressionBoundsTests
+{
+    [Theory]
+    [InlineData(0)]
+    [InlineData(1)]
+    [InlineData(2)]
+    [InlineData(3)]
+    public void RefusesOutputTooSmallToBound(int outputSize)
+    {
+        var input = new byte[64];
+        Array.Fill(input, (byte)'A');
+
+        // Sentinel-filled backing array; only the middle window is offered to the compressor, so
+        // any write past the span shows up as a modified sentinel rather than silent corruption.
+        var backing = new byte[256];
+        Array.Fill(backing, (byte)0xCC);
+
+        const int windowStart = 64;
+        var output = backing.AsSpan(windowStart, outputSize);
+
+        var written = NetworkCompression.Compress(input, output);
+
+        Assert.Equal(0, written);
+
+        for (var i = 0; i < backing.Length; i++)
+        {
+            Assert.Equal(0xCC, backing[i]);
+        }
+    }
+
+    [Fact]
+    public void StillCompressesWhenOutputIsLargeEnough()
+    {
+        var input = new byte[64];
+        Array.Fill(input, (byte)'A');
+
+        var output = new byte[256];
+
+        var written = NetworkCompression.Compress(input, output);
+
+        Assert.True(written > 0);
+        Assert.True(written <= output.Length);
+    }
+
+    [Fact]
+    public void ReportsFailureRatherThanOverrunningATightOutput()
+    {
+        // Large input against a small-but-bounded output: the guard is well-defined here, so this
+        // must fail cleanly rather than write past the end.
+        var input = new byte[4096];
+        Array.Fill(input, (byte)'A');
+
+        var backing = new byte[256];
+        Array.Fill(backing, (byte)0xCC);
+
+        const int windowStart = 64;
+        const int windowSize = 16;
+        var output = backing.AsSpan(windowStart, windowSize);
+
+        NetworkCompression.Compress(input, output);
+
+        for (var i = 0; i < windowStart; i++)
+        {
+            Assert.Equal(0xCC, backing[i]);
+        }
+
+        for (var i = windowStart + windowSize; i < backing.Length; i++)
+        {
+            Assert.Equal(0xCC, backing[i]);
+        }
+    }
+}

Projects/Server/Network/NetState/NetState.cs

@@ -444,17 +444,36 @@ public partial class NetState : IComparable<NetState>, IValueLinkListNode<NetSta
         }
 
         var length = span.Length;
-        if (length <= 0 || !GetSendBuffer(out var buffer))
+        if (length <= 0)
         {
             return;
         }
 
+        // Never drop silently: the client would stay connected while missing game state.
+        if (!GetSendBuffer(out var buffer))
+        {
+            SendBufferExhausted(length, 0);
+            return;
+        }
+
         try
         {
             // Apply encoding first (e.g., compression from UOContent)
             if (CompressionEnabled)
             {
                 length = NetworkCompression.Compress(span, buffer);
+
+                // 0 means nothing was written, whether it did not fit or the input was too large.
+                if (length <= 0)
+                {
+                    SendBufferExhausted(span.Length, buffer.Length);
+                    return;
+                }
+            }
+            else if (span.Length > buffer.Length)
+            {
+                SendBufferExhausted(span.Length, buffer.Length);
+                return;
             }
             else
             {
@@ -484,6 +503,31 @@ public partial class NetState : IComparable<NetState>, IValueLinkListNode<NetSta
         }
     }
 
+    /// <summary>
+    /// Handles a packet that cannot be placed in the send buffer.
+    /// </summary>
+    /// <remarks>
+    /// High unacked means a slow client holding the buffer; needed approaching capacity means the
+    /// buffer is too small for this shard and network.sendBufferSize should be raised.
+    /// </remarks>
+    private void SendBufferExhausted(int needed, int writable)
+    {
+        var sendBuffer = _socket?.SendBuffer;
+        var unacked = sendBuffer?.InFlightBytes ?? 0;
+        var capacity = sendBuffer?.PhysicalSize ?? 0;
+
+        logger.Warning(
+            "{NetState}: send buffer exhausted - needed {Needed} bytes, {Writable} writable, {Unacked} awaiting acknowledgement, {Capacity} capacity. Raise network.sendBufferSize (power of two) if this recurs on healthy connections.",
+            this,
+            needed,
+            writable,
+            unacked,
+            capacity
+        );
+
+        Disconnect($"Send buffer exhausted (needed {needed}, writable {writable}, unacked {unacked}, capacity {capacity})");
+    }
+
     private void StartPacketLog()
     {
         try

Projects/Server/Network/NetworkCompression.cs

@@ -73,7 +73,9 @@ public static class NetworkCompression
 
     public static int Compress(ReadOnlySpan<byte> input, Span<byte> output)
     {
-        if (input.Length > DefiniteOverflow)
+        // output.Length < 4 underflows safeOutputLength below (nuint), defeating the hot loop's
+        // bounds check. Reachable whenever the send buffer is nearly full.
+        if (input.Length > DefiniteOverflow || output.Length < 4)
         {
             return 0;
         }