Improve async string perf and fix reading chars with initial offset.

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

@@ -13088,7 +13088,7 @@ bool writeDataSizeToSnapshot
                 ||
                 (buff.Length >= offst + len)
                 ||
-                (buff.Length == (startOffsetByteCount >> 1) + 1),
+                (buff.Length >= (startOffsetByteCount >> 1) + 1),
                 "Invalid length sent to ReadPlpUnicodeChars()!"
             );
             charsLeft = len;
@@ -13152,7 +13152,10 @@ bool writeDataSizeToSnapshot
                         }
                         else
                         {
-                            newbuf = new char[offst + charsRead];
+                            // grow by an arbitrary number of packets to avoid needing to reallocate
+                            //  the newbuf on each loop iteration of long packet sequences which causes
+                            //  a performance problem as we spend large amounts of time copying and in gc
+                            newbuf = new char[offst + charsRead + (stateObj.GetPacketSize() * 8)];
                             rentedBuff = false;
                         }
 

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

@@ -1360,6 +1360,11 @@ internal bool SetPacketSize(int size)
             return false;
         }
 
+        internal int GetPacketSize()
+        {
+            return _inBuff.Length;
+        }
+
         ///////////////////////////////////////
         // Buffer read methods - data values //
         ///////////////////////////////////////