fix: FastBufferWriter growth from a size of 0

com.unity.netcode.gameobjects/CHANGELOG.md

@@ -22,6 +22,7 @@ Additional documentation and release notes are available at [Multiplayer Documen
 
 ### Fixed
 
+- Issue where a growable `FastBufferWriter` created with a size of 0 never returned from its first write or `TryBeginWrite` call that needed the buffer to grow. (#4223)
 - Issue where a `FastBufferReader` created with an offset and the default length of -1 read past the end of its source. The `ArraySegment<byte>` overload that takes an offset now applies it from the start of the segment. (#4221)
 - Issue where a client synchronized while a `NetworkList<T>` still had unsent changes applied those changes twice, resulting in duplicate entries and index-based events landing on the wrong elements on that client. (#4215)
 - Issue where showing a `NetworkObject` with `NetworkShow` while its `NetworkVariable` updates were held back by `NetworkVariableUpdateTraits` stopped those pending changes from being sent until the variable was modified again. (#4215)

com.unity.netcode.gameobjects/Runtime/Serialization/FastBufferWriter.cs

@@ -193,7 +193,7 @@ public unsafe BitWriter EnterBitwiseContext()
 
         internal unsafe void Grow(int additionalSizeRequired)
         {
-            var desiredSize = Handle->Capacity * 2;
+            var desiredSize = Math.Max(Handle->Capacity * 2, 1);
             while (desiredSize < Position + additionalSizeRequired)
             {
                 desiredSize *= 2;

com.unity.netcode.gameobjects/Tests/Editor/Serialization/FastBufferWriterTests.cs

@@ -1524,6 +1524,17 @@ public void WhenBufferGrowthRequiredIsMoreThanDouble_BufferGrowsEnoughToContainR
             }
         }
 
+        [Test]
+        public void WhenGrowingWriterStartsWithZeroSize_BufferGrows()
+        {
+            using var growingWriter = new FastBufferWriter(0, Allocator.Temp, 64);
+            Assert.IsTrue(growingWriter.TryBeginWrite(4));
+            Assert.AreEqual(4, growingWriter.Capacity);
+
+            growingWriter.WriteByteSafe(1);
+            Assert.AreEqual(new byte[] { 1 }, growingWriter.ToArray());
+        }
+
         [Test]
         public void WhenTryingToWritePastMaxCapacity_GrowthDoesNotOccurAndTryBeginWriteReturnsFalse()
         {