Fix more test failures for UnsafeBuffer

src/main/java/io/netty/buffer/api/ByteCursor.java

@@ -45,6 +45,11 @@ public interface ByteCursor {
     /**
      * Return the last 8 bytes read by {@link #readLong()}.
      * If {@link #readLong()} has not been called on this cursor before, then {@code -1} is returned.
+     * <p>
+     * The long value is in the big-endian format, such that the highest-order by of the long value, is the byte that
+     * would otherwise have been produced by a {@link #readByte()} / {@link #getByte()} pair.
+     * This means that cursors that iterate in reverse, e.g. from {@link Buffer#openReverseCursor()}, return longs in a
+     * "mirrored" or "reversed" big-endian format.
      *
      * @return The 8 bytes, in big-endian format, that was read by the most recent successful call to
      * {@link #readLong()}.

src/main/java/io/netty/buffer/api/unsafe/UnsafeBuffer.java

@@ -87,7 +87,7 @@ public class UnsafeBuffer extends RcSupport<Buffer, UnsafeBuffer> implements Buf
 
     @Override
     public int capacity() {
-        return rsize;
+        return Math.max(0, rsize); // Use Math.max to make capacity of closed buffer equal to zero.
     }
 
     @Override
@@ -147,6 +147,9 @@ public class UnsafeBuffer extends RcSupport<Buffer, UnsafeBuffer> implements Buf
 
     @Override
     public Buffer slice(int offset, int length) {
+        if (length < 0) {
+            throw new IllegalArgumentException("Length cannot be negative: " + length + '.');
+        }
         checkGet(offset, length);
         ArcDrop<UnsafeBuffer> drop = (ArcDrop<UnsafeBuffer>) unsafeGetDrop();
         drop.increment();
@@ -1240,6 +1243,8 @@ public class UnsafeBuffer extends RcSupport<Buffer, UnsafeBuffer> implements Buf
     }
 
     void makeInaccessible() {
+        roff = 0;
+        woff = 0;
         rsize = CLOSED_SIZE;
         wsize = CLOSED_SIZE;
         readOnly = false;
@@ -1279,19 +1284,6 @@ public class UnsafeBuffer extends RcSupport<Buffer, UnsafeBuffer> implements Buf
         }
     }
 
-    private RuntimeException checkWriteState(IndexOutOfBoundsException ioobe, int offset) {
-        if (rsize == CLOSED_SIZE) {
-            return bufferIsClosed();
-        }
-        if (wsize != rsize) {
-            return bufferIsReadOnly();
-        }
-
-        IndexOutOfBoundsException exception = outOfBounds(offset);
-        exception.addSuppressed(ioobe);
-        return exception;
-    }
-
     private RuntimeException readAccessCheckException(int index) {
         if (rsize == CLOSED_SIZE) {
             throw bufferIsClosed();