nfs4: FileTracker should return a copy of stateid

Motivation:
As parallel threads modifies open-stateid sequence number the
FileTracker should always return a copy to avoid that sequence
is modified before reply is sent.

Modification:
Update FileTracker#addOpen and FileTracker#downgradeOpen to return a
copy of stateid.

Result:
Spec compliant behaviour in concurrent environment.

Fixes: #96
Acked-by: Albert Rossi
Target: master, 0.24, 0.23
(cherry picked from commit 73d73af)
Signed-off-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>

core/src/main/java/org/dcache/nfs/v4/FileTracker.java

@@ -100,7 +100,7 @@ public NFS4Client getClient() {
      * @param inode of opened file.
      * @param shareAccess type of access required.
      * @param shareDeny type of access to deny others.
-     * @return stateid associated with open.
+     * @return a snapshot of the stateid associated with open.
      * @throws ShareDeniedException if share reservation conflicts with an existing open.
      * @throws ChimeraNFSException
      */
@@ -134,7 +134,8 @@ public stateid4 addOpen(NFS4Client client, StateOwner owner, Inode inode, int sh
                         os.shareAccess |= shareAccess;
                         os.shareDeny |= shareDeny;
                         os.stateid.seqid++;
-                        return os.stateid;
+                        //we need to return copy to avoid modification by concurrent opens
+                        return new stateid4(os.stateid.other, os.stateid.seqid);
                 }
             }
 
@@ -144,7 +145,8 @@ public stateid4 addOpen(NFS4Client client, StateOwner owner, Inode inode, int sh
             opens.add(openState);
             state.addDisposeListener(s -> removeOpen(inode, stateid));
             stateid.seqid++;
-            return stateid;
+            //we need to return copy to avoid modification by concurrent opens
+            return new stateid4(stateid.other, stateid.seqid);
         } finally {
             lock.unlock();
         }
@@ -158,7 +160,7 @@ public stateid4 addOpen(NFS4Client client, StateOwner owner, Inode inode, int sh
      * @param inode of opened file.
      * @param shareAccess type of access required.
      * @param shareDeny type of access to deny others.
-     * @return stateid associated with open.
+     * @return a snapshot of the stateid associated with open.
      * @throws ChimeraNFSException
      */
     public stateid4 downgradeOpen(NFS4Client client, stateid4 stateid, Inode inode, int shareAccess, int shareDeny) throws ChimeraNFSException {
@@ -187,7 +189,8 @@ public stateid4 downgradeOpen(NFS4Client client, stateid4 stateid, Inode inode,
             os.shareDeny = shareDeny;
 
             os.stateid.seqid++;
-            return os.stateid;
+            //we need to return copy to avoid modification by concurrent opens
+            return new stateid4(os.stateid.other, os.stateid.seqid);
         } finally {
             lock.unlock();
         }