Java/C#/Python prefixed size support (#4445)

* initial changes to support size prefixed buffers in Java

* add slice equivalent to CSharp ByteBuffer

* resolve TODO for slicing in CSharp code generation

* add newly generated Java and CSharp test sources

* fix typo in comment

* add FinishSizePrefixed methods to CSharp FlatBufferBuilder as well

* add option to allow writing the prefix as well

* generate size-prefixed monster binary as well

* extend JavaTest to test the size prefixed binary as well

* use constants for size prefix length

* fuse common code for getRootAs and getSizePrefixedRootAs

* pulled file identifier out of if

* add FinishSizePrefixed, GetSizePrefixedRootAs support for Python

* Revert "extend JavaTest to test the size prefixed binary as well"

This reverts commit 68be4420dda47e8d0600bb19691f03be71503a68.

* Revert "generate size-prefixed monster binary as well"

This reverts commit 2939516fdf78df4f061c627221e232b312301417.

* fix ByteBuffer.cs Slice() method; add proper CSharp and Java tests

* fix unused parameter

* increment version number

* pulled out generated methods into separate utility class

* pulled out generated methods into separate utility class for Python

* fix indentation

* remove unnecessary comment

* fix newline and copyright

* add ByteBufferUtil to csproj compilation

* hide ByteBuffer's internal data; track offset into parent's array

* test unsafe versions as well; compile and run in debug mode

* clarify help text for size prefix

* move ByteBuffer slicing behavior to subclass

* fix protection levels

* add size prefix support for text generation

* add ByteBufferSlice to csproj compilation

* revert size prefix handling for nested buffers

* use duplicate instead of slice for removing size prefix

* remove slice subclass and use duplicate for removing size prefix

* remove slice specific tests

* remove superfluous command line option

3 files changed, 110 insertions, 5 deletions

diff --git a/java/com/google/flatbuffers/ByteBufferUtil.java b/java/com/google/flatbuffers/ByteBufferUtil.java
new file mode 100644
index 00000000..624dc4e2
--- /dev/null
+++ b/java/com/google/flatbuffers/ByteBufferUtil.java
@@ -0,0 +1,58 @@
+/*
+ * Copyright 2017 Google Inc. All rights reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.google.flatbuffers;
+
+import static com.google.flatbuffers.Constants.*;
+
+import java.nio.ByteBuffer;
+
+/// @file
+/// @addtogroup flatbuffers_java_api
+/// @{
+
+/**
+ * Class that collects utility functions around `ByteBuffer`.
+ */
+public class ByteBufferUtil {
+
+	/**
+     * Extract the size prefix from a `ByteBuffer`.
+     * 
+     * @param bb a size-prefixed buffer
+     * @return the size prefix
+     */
+    public static int getSizePrefix(ByteBuffer bb) {
+        return bb.getInt(bb.position());
+    }
+
+	/**
+     * Create a duplicate of a size-prefixed `ByteBuffer` that has its position
+     * advanced just past the size prefix.
+     * 
+     * @param bb a size-prefixed buffer
+     * @return a new buffer on the same underlying data that has skipped the
+     *         size prefix
+     */
+    public static ByteBuffer removeSizePrefix(ByteBuffer bb) {
+        ByteBuffer s = bb.duplicate();
+        s.position(s.position() + SIZE_PREFIX_LENGTH);
+        return s;
+    }
+
+}
+
+/// @}
diff --git a/java/com/google/flatbuffers/Constants.java b/java/com/google/flatbuffers/Constants.java
index f5906314..751f4a65 100644
--- a/java/com/google/flatbuffers/Constants.java
+++ b/java/com/google/flatbuffers/Constants.java
@@ -37,6 +37,8 @@ public class Constants {
     static final int SIZEOF_DOUBLE = 8;
     /** The number of bytes in a file identifier. */
     static final int FILE_IDENTIFIER_LENGTH = 4;
+    /** The number of bytes in a size prefix. */
+    public static final int SIZE_PREFIX_LENGTH = 4;
 }
 
 /// @endcond
diff --git a/java/com/google/flatbuffers/FlatBufferBuilder.java b/java/com/google/flatbuffers/FlatBufferBuilder.java
index f98fbedc..44f107ae 100644
--- a/java/com/google/flatbuffers/FlatBufferBuilder.java
+++ b/java/com/google/flatbuffers/FlatBufferBuilder.java
@@ -816,10 +816,14 @@ public class FlatBufferBuilder {
      * Finalize a buffer, pointing to the given `root_table`.
      *
      * @param root_table An offset to be added to the buffer.
+     * @param size_prefix Whether to prefix the size to the buffer.
      */
-    public void finish(int root_table) {
-        prep(minalign, SIZEOF_INT);
+    protected void finish(int root_table, boolean size_prefix) {
+        prep(minalign, SIZEOF_INT + (size_prefix ? SIZEOF_INT : 0));
         addOffset(root_table);
+        if (size_prefix) {
+            addInt(bb.capacity() - space);
+        }
         bb.position(space);
         finished = true;
     }
@@ -828,18 +832,59 @@ public class FlatBufferBuilder {
      * Finalize a buffer, pointing to the given `root_table`.
      *
      * @param root_table An offset to be added to the buffer.
+     */
+    public void finish(int root_table) {
+        finish(root_table, false);
+    }
+
+    /**
+     * Finalize a buffer, pointing to the given `root_table`, with the size prefixed.
+     *
+     * @param root_table An offset to be added to the buffer.
+     */
+    public void finishSizePrefixed(int root_table) {
+        finish(root_table, true);
+    }
+
+    /**
+     * Finalize a buffer, pointing to the given `root_table`.
+     *
+     * @param root_table An offset to be added to the buffer.
      * @param file_identifier A FlatBuffer file identifier to be added to the buffer before
      * `root_table`.
+     * @param size_prefix Whether to prefix the size to the buffer.
      */
-    public void finish(int root_table, String file_identifier) {
-        prep(minalign, SIZEOF_INT + FILE_IDENTIFIER_LENGTH);
+    protected void finish(int root_table, String file_identifier, boolean size_prefix) {
+        prep(minalign, SIZEOF_INT + FILE_IDENTIFIER_LENGTH + (size_prefix ? SIZEOF_INT : 0));
         if (file_identifier.length() != FILE_IDENTIFIER_LENGTH)
             throw new AssertionError("FlatBuffers: file identifier must be length " +
                                      FILE_IDENTIFIER_LENGTH);
         for (int i = FILE_IDENTIFIER_LENGTH - 1; i >= 0; i--) {
             addByte((byte)file_identifier.charAt(i));
         }
-        finish(root_table);
+        finish(root_table, size_prefix);
+    }
+
+    /**
+     * Finalize a buffer, pointing to the given `root_table`.
+     *
+     * @param root_table An offset to be added to the buffer.
+     * @param file_identifier A FlatBuffer file identifier to be added to the buffer before
+     * `root_table`.
+     */
+    public void finish(int root_table, String file_identifier) {
+        finish(root_table, file_identifier, false);
+    }
+
+    /**
+     * Finalize a buffer, pointing to the given `root_table`, with the size prefixed.
+     *
+     * @param root_table An offset to be added to the buffer.
+     * @param file_identifier A FlatBuffer file identifier to be added to the buffer before
+     * `root_table`.
+     */
+    public void finishSizePrefixed(int root_table, String file_identifier) {
+        finish(root_table, file_identifier, true);
     }
 
     /**