fix: FastBufferReader offset with the default length (#4221)

* fix: FastBufferReader offset with the default length

A FastBufferReader created by copying a source with an offset and length -1 took
the whole source length as its length, so it reported offset bytes too many and
copied them from past the end of the source. Length -1 now means every byte after
the offset when copying; Allocator.None, which reads the source in place starting
at the offset, keeps the whole source length as before.

The ArraySegment<byte> overload that takes an offset now applies it from the start
of the segment instead of the start of the underlying array.

UUM-158591

* chore: CHANGELOG PR number

Author: NoelStephensUnity

com.unity.netcode.gameobjects/CHANGELOG.md

@@ -22,6 +22,7 @@ Additional documentation and release notes are available at [Multiplayer Documen
 
 ### Fixed
 
+- 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)
 - Issue where moving a `NetworkObject` into another scene made the clients that did not observe it log "Trying to synchronize NetworkObjectId but it was not spawned". The scene migration is now only sent to the clients that observe the `NetworkObject`. (#4185)

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

@@ -90,6 +90,16 @@ internal unsafe void CommitBitwiseReads(int amount)
             return readerHandle;
         }
 
+        // A copy starts at offset, so -1 copies what follows it. Allocator.None keeps the whole source and starts reading at offset.
+        private static int ResolveLength(int length, int sourceLength, int offset, Allocator copyAllocator)
+        {
+            if (length != -1)
+            {
+                return length;
+            }
+            return copyAllocator == Allocator.None ? sourceLength : sourceLength - offset;
+        }
+
         /// <summary>
         /// Create a FastBufferReader from a NativeArray.
         ///
@@ -107,12 +117,12 @@ internal unsafe void CommitBitwiseReads(int amount)
         /// </summary>
         /// <param name="buffer">The NativeArray to create the reader from</param>
         /// <param name="copyAllocator">The allocator type used for internal data when copying an existing buffer if other than Allocator.None is specified, that memory will be owned by this FastBufferReader instance</param>
-        /// <param name="length">The number of bytes to read from the buffer. If set to -1, the entire buffer length will be used</param>
+        /// <param name="length">The number of bytes to read from the buffer. If set to -1, every byte from offset to the end of the buffer will be used</param>
         /// <param name="offset">The offset into the buffer to start reading from</param>
         /// <param name="internalAllocator">The allocator type used for internal data when this reader points directly at a buffer owned by someone else</param>
         public unsafe FastBufferReader(NativeArray<byte> buffer, Allocator copyAllocator, int length = -1, int offset = 0, Allocator internalAllocator = Allocator.Temp)
         {
-            Handle = CreateHandle((byte*)buffer.GetUnsafePtr(), length == -1 ? buffer.Length : length, offset, copyAllocator, internalAllocator);
+            Handle = CreateHandle((byte*)buffer.GetUnsafePtr(), ResolveLength(length, buffer.Length, offset, copyAllocator), offset, copyAllocator, internalAllocator);
         }
 
         /// <summary>
@@ -127,7 +137,7 @@ public unsafe FastBufferReader(NativeArray<byte> buffer, Allocator copyAllocator
         /// <param name="buffer">The buffer to copy from</param>
         /// <param name="copyAllocator">The allocator type used for internal data when copying an existing buffer if other than Allocator.None is specified, that memory will be owned by this FastBufferReader instance</param>
         /// <param name="length">The number of bytes to copy (all if this is -1)</param>
-        /// <param name="offset">The offset of the buffer to start copying from</param>
+        /// <param name="offset">The offset into the segment to start copying from</param>
         public unsafe FastBufferReader(ArraySegment<byte> buffer, Allocator copyAllocator, int length = -1, int offset = 0)
         {
             if (copyAllocator == Allocator.None)
@@ -136,7 +146,7 @@ public unsafe FastBufferReader(ArraySegment<byte> buffer, Allocator copyAllocato
             }
             fixed (byte* data = buffer.Array)
             {
-                Handle = CreateHandle(data, length == -1 ? buffer.Count : length, offset, copyAllocator, Allocator.Temp);
+                Handle = CreateHandle(data + buffer.Offset, ResolveLength(length, buffer.Count, offset, copyAllocator), offset, copyAllocator, Allocator.Temp);
             }
         }
 
@@ -209,7 +219,7 @@ public unsafe FastBufferReader(byte[] buffer, Allocator copyAllocator, int lengt
             }
             fixed (byte* data = buffer)
             {
-                Handle = CreateHandle(data, length == -1 ? buffer.Length : length, offset, copyAllocator, Allocator.Temp);
+                Handle = CreateHandle(data, ResolveLength(length, buffer.Length, offset, copyAllocator), offset, copyAllocator, Allocator.Temp);
             }
         }
 
@@ -260,7 +270,7 @@ public unsafe FastBufferReader(byte* buffer, Allocator copyAllocator, int length
         /// <param name="internalAllocator">The allocator type used for internal data when this reader points directly at a buffer owned by someone else</param>
         public unsafe FastBufferReader(FastBufferWriter writer, Allocator copyAllocator, int length = -1, int offset = 0, Allocator internalAllocator = Allocator.Temp)
         {
-            Handle = CreateHandle(writer.GetUnsafePtr(), length == -1 ? writer.Length : length, offset, copyAllocator, internalAllocator);
+            Handle = CreateHandle(writer.GetUnsafePtr(), ResolveLength(length, writer.Length, offset, copyAllocator), offset, copyAllocator, internalAllocator);
         }
 
         /// <summary>
@@ -285,7 +295,7 @@ public unsafe FastBufferReader(FastBufferWriter writer, Allocator copyAllocator,
         /// <param name="internalAllocator">The allocator type used for internal data when this reader points directly at a buffer owned by someone else</param>
         public unsafe FastBufferReader(FastBufferReader reader, Allocator copyAllocator, int length = -1, int offset = 0, Allocator internalAllocator = Allocator.Temp)
         {
-            Handle = CreateHandle(reader.GetUnsafePtr(), length == -1 ? reader.Length : length, offset, copyAllocator, internalAllocator);
+            Handle = CreateHandle(reader.GetUnsafePtr(), ResolveLength(length, reader.Length, offset, copyAllocator), offset, copyAllocator, internalAllocator);
         }
 
         /// <summary>

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

@@ -1118,6 +1118,78 @@ public void WhenCreatingAReaderFromAnEmptyBuffer_LengthIsZero()
             }
         }
 
+        private const int k_SourceLength = 10;
+        private const int k_ReaderOffset = 2;
+
+        private static byte[] CreateSequentialBytes()
+        {
+            var bytes = new byte[k_SourceLength];
+            for (var i = 0; i < bytes.Length; ++i)
+            {
+                bytes[i] = (byte)i;
+            }
+            return bytes;
+        }
+
+        private static void AssertReadsToEndFrom(FastBufferReader reader, int firstValue)
+        {
+            var expectedCount = k_SourceLength - firstValue;
+            Assert.AreEqual(expectedCount, reader.Length - reader.Position);
+            for (var i = 0; i < expectedCount; ++i)
+            {
+                reader.ReadByteSafe(out byte value);
+                Assert.AreEqual((byte)(firstValue + i), value);
+            }
+        }
+
+        [Test]
+        public void WhenCreatingAReaderFromAnArrayWithAnOffsetAndNoLength_TheRestOfTheArrayIsCopied()
+        {
+            using var reader = new FastBufferReader(CreateSequentialBytes(), Allocator.Temp, -1, k_ReaderOffset);
+            AssertReadsToEndFrom(reader, k_ReaderOffset);
+        }
+
+        [Test]
+        public void WhenCreatingAReaderFromAnArraySegmentWithAnOffsetAndNoLength_TheRestOfTheSegmentIsCopied()
+        {
+            using var reader = new FastBufferReader(new ArraySegment<byte>(CreateSequentialBytes()), Allocator.Temp, -1, k_ReaderOffset);
+            AssertReadsToEndFrom(reader, k_ReaderOffset);
+        }
+
+        [Test]
+        public void WhenCreatingAReaderFromAnOffsetArraySegmentWithAnOffset_TheOffsetIsRelativeToTheSegment()
+        {
+            var segmentOffset = 3;
+            var segment = new ArraySegment<byte>(CreateSequentialBytes(), segmentOffset, k_SourceLength - segmentOffset);
+            using var reader = new FastBufferReader(segment, Allocator.Temp, -1, k_ReaderOffset);
+            AssertReadsToEndFrom(reader, segmentOffset + k_ReaderOffset);
+        }
+
+        [Test]
+        public void WhenCreatingAReaderFromANativeArrayWithAnOffsetAndNoLength_TheRestOfTheArrayIsRead([Values(Allocator.Temp, Allocator.None)] Allocator copyAllocator)
+        {
+            using var source = new NativeArray<byte>(CreateSequentialBytes(), Allocator.Temp);
+            using var reader = new FastBufferReader(source, copyAllocator, -1, k_ReaderOffset);
+            AssertReadsToEndFrom(reader, k_ReaderOffset);
+        }
+
+        [Test]
+        public void WhenCreatingAReaderFromAWriterWithAnOffsetAndNoLength_TheRestOfTheWriterIsRead([Values(Allocator.Temp, Allocator.None)] Allocator copyAllocator)
+        {
+            using var writer = new FastBufferWriter(k_SourceLength, Allocator.Temp);
+            writer.WriteBytesSafe(CreateSequentialBytes());
+            using var reader = new FastBufferReader(writer, copyAllocator, -1, k_ReaderOffset);
+            AssertReadsToEndFrom(reader, k_ReaderOffset);
+        }
+
+        [Test]
+        public void WhenCreatingAReaderFromAReaderWithAnOffsetAndNoLength_TheRestOfTheReaderIsRead([Values(Allocator.Temp, Allocator.None)] Allocator copyAllocator)
+        {
+            using var source = new FastBufferReader(CreateSequentialBytes(), Allocator.Temp);
+            using var reader = new FastBufferReader(source, copyAllocator, -1, k_ReaderOffset);
+            AssertReadsToEndFrom(reader, k_ReaderOffset);
+        }
+
         [Test]
         public void WhenCreatingNewFastBufferReader_IsInitializedIsTrue()
         {