fix read error with reused uncleared packet

fix 0 length read at the start of a packe in plp stream returning 0 when continuing
handle char array sizing better
change existing test to use multiple packet sizes

Author: Wraith2

src/Microsoft.Data.SqlClient/netcore/src/Microsoft/Data/SqlClient/TdsParser.cs

@@ -13149,7 +13149,7 @@ bool writeDataSizeToSnapshot
             if (stateObj._longlen == 0)
             {
                 Debug.Assert(stateObj._longlenleft == 0);
-                totalCharsRead = 0;
+                totalCharsRead = startOffsetByteCount >> 1;
                 return TdsOperationStatus.Done;       // No data
             }
 
@@ -13169,14 +13169,14 @@ bool writeDataSizeToSnapshot
             //  later needing to repeatedly allocate new target buffers and copy data as we discover new data
             if (buff == null && stateObj._longlen != TdsEnums.SQL_PLP_UNKNOWNLEN && stateObj._longlen < (int.MaxValue >> 1))
             {
-                if (supportRentedBuff && stateObj._longlen < 1073741824) // 1 Gib
+                if (supportRentedBuff && stateObj._longlen >> 1 < 1073741824) // 1 Gib
                 {
-                    buff = ArrayPool<char>.Shared.Rent((int)Math.Min((int)stateObj._longlen, len));
+                    buff = ArrayPool<char>.Shared.Rent((int)Math.Min((int)stateObj._longlen >> 1, len));
                     rentedBuff = true;
                 }
                 else
                 {
-                    buff = new char[(int)Math.Min((int)stateObj._longlen, len)];
+                    buff = new char[(int)Math.Min((int)stateObj._longlen >> 1, len)];
                     rentedBuff = false;
                 }
             }

src/Microsoft.Data.SqlClient/netfx/src/Microsoft/Data/SqlClient/TdsParser.cs

@@ -13312,7 +13312,7 @@ bool writeDataSizeToSnapshot
             if (stateObj._longlen == 0)
             {
                 Debug.Assert(stateObj._longlenleft == 0);
-                totalCharsRead = 0;
+                totalCharsRead = startOffsetByteCount >> 1;
                 return TdsOperationStatus.Done;       // No data
             }
 
@@ -13332,14 +13332,14 @@ bool writeDataSizeToSnapshot
             // allocate the whole buffer in one shot instead of realloc'ing and copying over each time
             if (buff == null && stateObj._longlen != TdsEnums.SQL_PLP_UNKNOWNLEN && stateObj._longlen < (int.MaxValue >> 1))
             {
-                if (supportRentedBuff && stateObj._longlen < 1073741824) // 1 Gib
+                if (supportRentedBuff && stateObj._longlen >> 1 < 1073741824) // 1 Gib
                 {
-                    buff = ArrayPool<char>.Shared.Rent((int)Math.Min((int)stateObj._longlen, len));
+                    buff = ArrayPool<char>.Shared.Rent((int)Math.Min((int)stateObj._longlen >> 1, len));
                     rentedBuff = true;
                 }
                 else
                 {
-                    buff = new char[(int)Math.Min((int)stateObj._longlen, len)];
+                    buff = new char[(int)Math.Min((int)stateObj._longlen >> 1, len)];
                     rentedBuff = false;
                 }
             }

src/Microsoft.Data.SqlClient/src/Microsoft/Data/SqlClient/TdsParserStateObject.cs

@@ -3764,8 +3764,9 @@ internal sealed partial class StateSnapshot
         {
             private sealed partial class PacketData
             {
-                public byte[] Buffer;
-                public int Read;
+                public readonly byte[] Buffer;
+                public readonly int Read;
+
                 public PacketData NextPacket;
                 public PacketData PrevPacket;
 
@@ -3775,6 +3776,12 @@ private sealed partial class PacketData
                 /// </summary>
                 public int RunningDataSize;
 
+                public PacketData(byte[] buffer, int read)
+                {
+                    Buffer = buffer;
+                    Read = read;
+                }
+
                 public int PacketID => Packet.GetIDFromHeader(Buffer.AsSpan(0, TdsEnums.HEADER_LEN));
 
                 internal int GetPacketDataOffset()
@@ -4172,10 +4179,7 @@ internal void AppendPacketData(byte[] buffer, int read)
                     }
                 }
 #endif
-                PacketData packetData =  new PacketData();
-
-                packetData.Buffer = buffer;
-                packetData.Read = read;
+                PacketData packetData = new PacketData(buffer, read);
 #if DEBUG
                 packetData.SetDebugStack(_stateObj._lastStack);
                 packetData.SetDebugPacketId(Interlocked.Increment(ref _packetCounter));

src/Microsoft.Data.SqlClient/tests/ManualTests/SQL/DataReaderTest/DataReaderTest.cs

@@ -1035,11 +1035,18 @@ [Value] [nvarchar](max) NULL
                         }
                     }
 
-                    int counter = 0;
-                    while (counter < 10)
+                    SqlConnectionStringBuilder builder = new SqlConnectionStringBuilder(DataTestUtility.TCPConnectionString);
+                    builder.PersistSecurityInfo = true;
+                    builder.Pooling = false;
+                    
+                    for (int packetSize = 512; packetSize<2048; packetSize++)
                     {
-                        using (var cmd = cn.CreateCommand())
+                        builder.PacketSize = packetSize;
+                        using (SqlConnection sizedConnection = new SqlConnection(builder.ToString()))
+                        using (SqlCommand cmd = sizedConnection.CreateCommand())
                         {
+                            sizedConnection.Open();
+
                             cmd.CommandText = $"SELECT [d].[Id], [d].[DocumentIdentificationId], [d].[Name], [d].[Value] FROM [{tableName}] AS [d]";
                             using (var reader = await cmd.ExecuteReaderAsync())
                             {
@@ -1063,7 +1070,6 @@ [Value] [nvarchar](max) NULL
                                 }
                             }
                         }
-                        counter++;
                     }
                 }
                 finally