clarify compatability paths through TryReadPlpBytes

use as instead of casts for safety

Author: Wraith2

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

@@ -6493,7 +6493,7 @@ private TdsOperationStatus TryReadByteArrayWithContinue(TdsParserStateObject sta
             {
                 if (isContinuing || isStarting)
                 {
-                    temp = (byte[])stateObj.TryTakeSnapshotStorage();
+                    temp = stateObj.TryTakeSnapshotStorage() as byte[];
                     Debug.Assert(bytes == null || bytes.Length == length, "stored buffer length must be null or must have been created with the correct length");
                 }
                 if (temp != null)
@@ -12915,7 +12915,7 @@ internal TdsOperationStatus TryReadPlpUnicodeCharsWithContinue(TdsParserStateObj
             {
                 if (isContinuing || isStarting)
                 {
-                    temp = (char[])stateObj.TryTakeSnapshotStorage();
+                    temp = stateObj.TryTakeSnapshotStorage() as char[];
                     Debug.Assert(temp == null || length == int.MaxValue || temp.Length == length, "stored buffer length must be null or must have been created with the correct length");
                 }
                 if (temp != null)

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

@@ -6970,7 +6970,7 @@ private TdsOperationStatus TryReadByteArrayWithContinue(TdsParserStateObject sta
             {
                 if (isContinuing || isStarting)
                 {
-                    temp = (byte[])stateObj.TryTakeSnapshotStorage();
+                    temp = stateObj.TryTakeSnapshotStorage() as byte[];
                     Debug.Assert(bytes == null || bytes.Length == length, "stored buffer length must be null or must have been created with the correct length");
                 }
                 if (temp != null)
@@ -13458,7 +13458,7 @@ internal TdsOperationStatus TryReadPlpUnicodeCharsWithContinue(TdsParserStateObj
             {
                 if (isContinuing || isStarting)
                 {
-                    temp = (char[])stateObj.TryTakeSnapshotStorage();
+                    temp = stateObj.TryTakeSnapshotStorage() as char[];
                     Debug.Assert(temp == null || length == int.MaxValue || temp.Length == length, "stored buffer length must be null or must have been created with the correct length");
                 }
                 if (temp != null)

src/Microsoft.Data.SqlClient/src/Microsoft/Data/SqlClient/SqlCachedBuffer.cs

@@ -80,7 +80,7 @@ internal static TdsOperationStatus TryCreate(SqlMetaDataPriv metadata, TdsParser
                     byte[] byteArr = new byte[cb];
                     // pass false for the writeDataSizeToSnapshot parameter because we want to only take data
                     // from the current packet and not try to do a continue-capable multi packet read
-                    result = stateObj.TryReadPlpBytes(ref byteArr, 0, cb, out cb, writeDataSizeToSnapshot: false);
+                    result = stateObj.TryReadPlpBytes(ref byteArr, 0, cb, out cb, writeDataSizeToSnapshot: false, compatibilityMode: false);
                     if (result != TdsOperationStatus.Done)
                     {
                         if (result == TdsOperationStatus.NeedMoreData && isAvailable && cb == byteArr.Length)

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

@@ -1706,7 +1706,7 @@ internal TdsOperationStatus TryReadStringWithEncoding(int length, System.Text.En
                     {
                         if (isContinuing || isStarting)
                         {
-                            buf = (byte[])TryTakeSnapshotStorage();
+                            buf = TryTakeSnapshotStorage() as byte[];
                             Debug.Assert(buf == null || buf.Length == length, "stored buffer length must be null or must have been created with the correct length");
                         }
                         if (buf != null)
@@ -1850,19 +1850,25 @@ internal int ReadPlpBytesChunk(byte[] buff, int offset, int len)
 
         internal TdsOperationStatus TryReadPlpBytes(ref byte[] buff, int offset, int len, out int totalBytesRead)
         {
-            ( _, bool isStarting, bool isContinuing) = GetSnapshotStatuses();
-            return TryReadPlpBytes(ref buff, offset, len, out totalBytesRead, isStarting || isContinuing);
+            bool isStarting = false;
+            bool isContinuing = false;
+            bool compatibilityMode = LocalAppContextSwitches.UseCompatibilityAsyncBehaviour;
+            if (!compatibilityMode)
+            {
+                (_, isStarting, isContinuing) = GetSnapshotStatuses();
+            }
+            return TryReadPlpBytes(ref buff, offset, len, out totalBytesRead, isStarting || isContinuing, compatibilityMode);
         }
         // Reads the requested number of bytes from a plp data stream, or the entire data if
         // requested length is -1 or larger than the actual length of data. First call to this method
         //  should be preceeded by a call to ReadPlpLength or ReadDataLength.
         // Returns the actual bytes read.
         // NOTE: This method must be retriable WITHOUT replaying a snapshot
         // Every time you call this method increment the offset and decrease len by the value of totalBytesRead
-        internal TdsOperationStatus TryReadPlpBytes(ref byte[] buff, int offset, int len, out int totalBytesRead, bool writeDataSizeToSnapshot)
+        internal TdsOperationStatus TryReadPlpBytes(ref byte[] buff, int offset, int len, out int totalBytesRead, bool writeDataSizeToSnapshot, bool compatibilityMode)
         {
             totalBytesRead = 0;
-            
+
             if (_longlen == 0)
             {
                 Debug.Assert(_longlenleft == 0);
@@ -1888,17 +1894,19 @@ internal TdsOperationStatus TryReadPlpBytes(ref byte[] buff, int offset, int len
                 {
                     // 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 = (byte[])TryTakeSnapshotStorage();
+                    buff = TryTakeSnapshotStorage() as byte[];
                     if (buff != null)
                     {
                         offset = _snapshot.GetPacketDataOffset();
                         totalBytesRead = offset;
                     }
                 }
-                else if (_snapshot != null)
+                else if (compatibilityMode && _snapshot != null && _snapshotStatus != SnapshotStatus.NotActive)
                 {
                     // legacy replay path perf optimization
-                    buff = (byte[])TryTakeSnapshotStorage();
+                    // 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 = TryTakeSnapshotStorage() as byte[];
                 }
 
                 if ((ulong)(buff?.Length ?? 0) != _longlen)
@@ -1957,11 +1965,13 @@ internal TdsOperationStatus TryReadPlpBytes(ref byte[] buff, int offset, int len
                         // so it can be re-used when another packet arrives and we read again
                         SetSnapshotStorage(buff);
                         SetSnapshotDataSize(bytesRead);
-                        
+
                     }
-                    else if (_snapshot != null)
+                    else if (compatibilityMode && _snapshot != null)
                     {
                         // legacy replay path perf optimization
+                        // 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
                         SetSnapshotStorage(buff);
                     }
                     return result;
@@ -1973,18 +1983,19 @@ internal TdsOperationStatus TryReadPlpBytes(ref byte[] buff, int offset, int len
                     result = TryReadPlpLength(false, out _);
                     if (result != TdsOperationStatus.Done)
                     {
-                        if (result == TdsOperationStatus.NeedMoreData)
+                        if (writeDataSizeToSnapshot)
                         {
-                            if (writeDataSizeToSnapshot)
+                            if (result == TdsOperationStatus.NeedMoreData)
                             {
                                 SetSnapshotStorage(buff);
                                 SetSnapshotDataSize(bytesRead);
                             }
-                            else if (_snapshot != null)
-                            {
-                                // legacy replay path perf optimization
-                                SetSnapshotStorage(buff);
-                            }
+                        } 
+                        else if (compatibilityMode && _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
+                            SetSnapshotStorage(buff);
                         }
                         return result;
                     }