Specify MemoryBIO contract better

Author: msimacek

graalpython/com.oracle.graal.python/src/com/oracle/graal/python/builtins/objects/ssl/PMemoryBIO.java

@@ -81,10 +81,13 @@ public ByteBuffer getBufferForReading() {
 
     /**
      * Update read position from a buffer previously obtained using {@link #getBufferForReading()}
+     * The caller is responsible for making sure the buffer was obtained for this BIO, that this BIO
+     * was not concurrently updated since the {@link #getBufferForReading()} call and that the
+     * position was not moved backwards.
      */
     public void applyRead(ByteBuffer buffer) {
         readPosition = buffer.position();
-        assert readPosition <= bytes.length;
+        assert buffer.array() == bytes;
         assert readPosition <= writePosition;
     }
 
@@ -100,11 +103,14 @@ public ByteBuffer getBufferForWriting() {
     }
 
     /**
-     * Update write position from a buffer previously obtained using {@link #getBufferForWriting()}
+     * Update write position from a buffer previously obtained using {@link #getBufferForWriting()}.
+     * The caller is responsible for making sure the buffer was obtained for this BIO, that this BIO
+     * was not concurrently updated since the {@link #getBufferForWriting()} call and that the
+     * position was not moved backwards.
      */
     public void applyWrite(ByteBuffer buffer) {
         writePosition = buffer.position();
-        assert writePosition <= bytes.length;
+        assert buffer.array() == bytes;
         assert readPosition <= writePosition;
     }