pool: Fix xrootd vector read limits

Fixes a bug that caused problems with ROOT 6: dCache provided
too large limits for the readv_ior_max and readv_iov_max
properties.

Target: trunk
Request: 2.10
Request: 2.9
Request: 2.8
Request: 2.7
Request: 2.6
Ticket: http://rt.dcache.org/Ticket/Display.html?id=8461
Require-notes: ues
Require-book: no
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: https://rb.dcache.org/r/7313/
(cherry picked from commit 92b3668)

modules/dcache-xrootd/src/main/java/org/dcache/xrootd/pool/XrootdPoolRequestHandler.java

@@ -96,6 +96,11 @@ public class XrootdPoolRequestHandler extends AbstractXrootdRequestHandler
     private static final int DEFAULT_FILESTATUS_FLAGS = 0;
     private static final int DEFAULT_FILESTATUS_MODTIME = 0;
 
+    private static final int MAX_JAVA_ARRAY = Integer.MAX_VALUE - 5;
+    private static final int READV_HEADER_LENGTH = 24;
+    private static final int READV_ELEMENT_LENGTH = 12;
+    private static final int READV_IOV_MAX = (MAX_JAVA_ARRAY - READV_HEADER_LENGTH) / READV_ELEMENT_LENGTH;
+
     /**
      * Store file descriptors of open files.
      */
@@ -557,10 +562,10 @@ protected Object doOnQuery(ChannelHandlerContext ctx, MessageEvent event, QueryR
                     s.append(0);
                     break;
                 case "readv_ior_max":
-                    s.append(_server.getMaxFrameSize());
+                    s.append(_server.getMaxFrameSize() - ReadResponse.READ_LIST_HEADER_SIZE);
                     break;
                 case "readv_iov_max":
-                    s.append(Integer.MAX_VALUE);
+                    s.append(READV_IOV_MAX);
                     break;
                 case "version":
                     s.append("dCache ").append(Version.of(XrootdPoolRequestHandler.class).getVersion());