experimental spike for SO66453263 - zero padding

src/Examples/Issues/SO66453263.cs

@@ -0,0 +1,55 @@
+using ProtoBuf;
+using System.IO;
+using Xunit;
+
+namespace Examples.Issues
+{
+    public class SO66453263 // handling zero-padded data
+    {
+
+        static MemoryStream GetZeroPaddedData()
+        {
+            var ms = new MemoryStream();
+            Serializer.Serialize(ms, new Foo { Id = 42, Name = "abc" });
+            for (int i = 0; i < 10; i++) ms.WriteByte(0); // add some zero padding
+            ms.Position = 0;
+            return ms;
+        }
+
+        [Fact]
+        public void ZeroPaddingFailsByDefault()
+        {
+            using var ms = GetZeroPaddedData();
+            var ex = Assert.Throws<ProtoException>(() => Serializer.Deserialize<Foo>(ms));
+            Assert.Equal("Invalid field in source data: 0", ex.Message);
+        }
+
+        [Fact]
+        public void ZeroPaddingWorksWithCustomStateContext()
+        {
+            using var ms = GetZeroPaddedData();
+            var obj = Serializer.Deserialize<Foo>(ms, userState: MyCustomState.Instance);
+            Assert.Equal(42, obj.Id);
+            Assert.Equal("abc", obj.Name);
+        }
+
+        class MyCustomState : ISerializationOptions
+        {
+            public static MyCustomState Instance { get; } = new MyCustomState();
+
+            SerializationOptions ISerializationOptions.Options => SerializationOptions.AllowZeroPadding;
+
+            private MyCustomState() { }
+        }
+
+        [ProtoContract]
+        public class Foo
+        {
+            [ProtoMember(1)]
+            public int Id { get; set; }
+
+            [ProtoMember(2)]
+            public string Name { get; set; }
+        }
+    }
+}

src/protobuf-net.Core/ISerializationOptions.cs

@@ -0,0 +1,32 @@
+using System;
+
+namespace ProtoBuf
+{
+    /// <summary>
+    /// Describes additional optional serialization behaviours
+    /// </summary>
+    public interface ISerializationOptions
+    {
+        /// <summary>
+        /// Gets the additional options to consider for this operation
+        /// </summary>
+        SerializationOptions Options { get; }
+    }
+
+    /// <summary>
+    /// Describes additional optional serialization behaviours
+    /// </summary>
+    [Flags]
+    public enum SerializationOptions
+    {
+        /// <summary>
+        /// No additional options
+        /// </summary>
+        None = 0,
+
+        /// <summary>
+        /// Interpret trailing zeros in data as EOF, rather than throwing an error
+        /// </summary>
+        AllowZeroPadding = 1 << 0,
+    }
+}

src/protobuf-net.Core/ProtoReader.State.ReadMethods.cs

@@ -741,7 +741,7 @@ public bool TryReadFieldHeader(int field)
             [MethodImpl(HotPath)]
             internal void CheckFullyConsumed()
             {
-                if (!_reader.IsFullyConsumed(ref this)) ThrowProtoException("Incorrect number of bytes consumed");
+                if (!_reader.IsFullyConsumed(ref this) && !_reader.AllowZeroPadding) ThrowProtoException("Incorrect number of bytes consumed");
             }
 
             /// <summary>

src/protobuf-net.Core/ProtoReader.cs

@@ -305,16 +305,24 @@ public static SubItemToken StartSubItem(ProtoReader reader)
         [MethodImpl(HotPath)]
         private int SetTag(uint tag)
         {
-            if ((_fieldNumber = (int)(tag >> 3)) < 1) ThrowInvalidField(_fieldNumber);
+            if ((_fieldNumber = (int)(tag >> 3)) < 1) ThrowInvalidField();
             if ((WireType = (WireType)(tag & 7)) == WireType.EndGroup)
             {
                 if (_depth > 0) return 0; // spoof an end, but note we still set the field-number
                 ThrowUnexpectedEndGroup();
             }
             return _fieldNumber;
         }
-        private static void ThrowInvalidField(int fieldNumber)
-            => ThrowHelper.ThrowProtoException("Invalid field in source data: " + fieldNumber.ToString());
+
+        private bool AllowZeroPadding => UserState is ISerializationOptions options && (options.Options & SerializationOptions.AllowZeroPadding) != 0;
+
+        private void ThrowInvalidField()
+        {
+            if (!(_fieldNumber == 0 && AllowZeroPadding))
+            {
+                ThrowHelper.ThrowProtoException("Invalid field in source data: " + _fieldNumber.ToString());
+            }
+        }
         private static void ThrowUnexpectedEndGroup()
             => ThrowHelper.ThrowProtoException("Unexpected end-group in source data; this usually means the source data is corrupt");