4. Merging "Buffer read" methods of TdsParserStateObject, port dotnet…

…#285 to netfx.

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

@@ -2445,8 +2445,6 @@ partial void SetStackInternal(string value)
             private PacketData _snapshotInBuffList;
             private PacketData _sparePacket;
 
-            internal byte[] _plpBuffer;
-
             private int _snapshotInBuffCount;
 
 #if DEBUG

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

@@ -393,7 +393,7 @@ internal bool TryReadPlpBytes(ref byte[] buff, int offset, int len, out int tota
                 Debug.Assert(_longlenleft == 0);
                 if (buff == null)
                 {
-                    buff = new byte[0];
+                    buff = Array.Empty<byte>();
                 }
 
                 AssertValidState();
@@ -409,7 +409,19 @@ internal bool TryReadPlpBytes(ref byte[] buff, int offset, int len, out int tota
             // If total length is known up front, allocate the whole buffer in one shot instead of realloc'ing and copying over each time
             if (buff == null && _longlen != TdsEnums.SQL_PLP_UNKNOWNLEN)
             {
-                buff = new byte[(Math.Min((int)_longlen, len))];
+                if (_snapshot != null)
+                {
+                    // if there is a snapshot and it contains a stored plp buffer take it
+                    // and try to use it if it is the right length
+                    buff = _snapshot._plpBuffer;
+                    _snapshot._plpBuffer = null;
+                }
+
+                if ((ulong)(buff?.Length ?? 0) != _longlen)
+                {
+                    // if the buffer is null or the wrong length create one to use
+                    buff = new byte[(Math.Min((int)_longlen, len))];
+                }
             }
 
             if (_longlenleft == 0)
@@ -454,6 +466,12 @@ internal bool TryReadPlpBytes(ref byte[] buff, int offset, int len, out int tota
                 _longlenleft -= (ulong)bytesRead;
                 if (!result)
                 {
+                    if (_snapshot != null)
+                    {
+                        // a partial read has happened so store the target buffer in the snapshot
+                        // so it can be re-used when another packet arrives and we read again
+                        _snapshot._plpBuffer = buff;
+                    }
                     return false;
                 }
 
@@ -462,6 +480,12 @@ internal bool TryReadPlpBytes(ref byte[] buff, int offset, int len, out int tota
                     // Read the next chunk or cleanup state if hit the end
                     if (!TryReadPlpLength(false, out _))
                     {
+                        if (_snapshot != null)
+                        {
+                            // a partial read has happened so store the target buffer in the snapshot
+                            // so it can be re-used when another packet arrives and we read again
+                            _snapshot._plpBuffer = buff;
+                        }
                         return false;
                     }
                 }

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

@@ -1928,6 +1928,8 @@ internal void Restore(TdsParserStateObject stateObj)
             private TdsParserStateObject _stateObj;
             private StateObjectData _replayStateData;
 
+            internal byte[] _plpBuffer;
+
 #if DEBUG
             private int _rollingPend = 0;
             private int _rollingPendCount = 0;