8238920: Better Buffer support

Reviewed-by: alanb, ahgross, rhalade, psandoz

src/java.base/share/classes/java/nio/Buffer.java

 /*
- * Copyright (c) 2000, 2016, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2000, 2020, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -345,8 +345,8 @@ public abstract class Buffer {
         if (newLimit > capacity | newLimit < 0)
             throw createLimitException(newLimit);
         limit = newLimit;
-        if (position > limit) position = limit;
-        if (mark > limit) mark = -1;
+        if (position > newLimit) position = newLimit;
+        if (mark > newLimit) mark = -1;
         return this;
     }
 
@@ -637,16 +637,18 @@ public abstract class Buffer {
      * @return  The current position value, before it is incremented
      */
     final int nextGetIndex() {                          // package-private
-        if (position >= limit)
+        int p = position;
+        if (p >= limit)
             throw new BufferUnderflowException();
-        return position++;
+        position = p + 1;
+        return p;
     }
 
     final int nextGetIndex(int nb) {                    // package-private
-        if (limit - position < nb)
-            throw new BufferUnderflowException();
         int p = position;
-        position += nb;
+        if (limit - p < nb)
+            throw new BufferUnderflowException();
+        position = p + nb;
         return p;
     }
 
@@ -658,16 +660,18 @@ public abstract class Buffer {
      * @return  The current position value, before it is incremented
      */
     final int nextPutIndex() {                          // package-private
-        if (position >= limit)
+        int p = position;
+        if (p >= limit)
             throw new BufferOverflowException();
-        return position++;
+        position = p + 1;
+        return p;
     }
 
     final int nextPutIndex(int nb) {                    // package-private
-        if (limit - position < nb)
-            throw new BufferOverflowException();
         int p = position;
-        position += nb;
+        if (limit - p < nb)
+            throw new BufferOverflowException();
+        position = p + nb;
         return p;
     }