pool: Fix pool selection bugs in migration module

There were two bugs in migration module:

- Random pool selection would select pools even when they are full.

- Proportional pool selection would trigger a NPE as it would return
  null when all pools are full.

Target: trunk
Request: 2.10
Request: 2.9
Request: 2.8
Request: 2.7
Request: 2.6
Require-notes: yes
Require-book: no
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: https://rb.dcache.org/r/7362/
(cherry picked from commit 55de0de)

Conflicts:
	modules/dcache/src/main/java/org/dcache/pool/migration/Task.java

(cherry picked from commit d85629d)

modules/dcache/src/main/java/org/dcache/pool/migration/PoolSelectionStrategy.java

@@ -1,5 +1,7 @@
 package org.dcache.pool.migration;
 
+import javax.annotation.Nullable;
+
 import java.util.List;
 
 import diskCacheV111.vehicles.PoolManagerPoolInformation;
@@ -9,5 +11,6 @@
  */
 public interface PoolSelectionStrategy
 {
+    @Nullable
     PoolManagerPoolInformation select(List<PoolManagerPoolInformation> pools);
 }

modules/dcache/src/main/java/org/dcache/pool/migration/RandomPoolSelectionStrategy.java

@@ -1,10 +1,15 @@
 package org.dcache.pool.migration;
 
+import com.google.common.base.Predicate;
+
 import java.util.List;
 import java.util.Random;
 
+import diskCacheV111.pools.PoolCostInfo;
 import diskCacheV111.vehicles.PoolManagerPoolInformation;
 
+import static com.google.common.collect.Iterables.*;
+
 /**
  * Implements a pool selection strategy in which the pool is selected
  * randomly from the list.
@@ -18,6 +23,19 @@ public class RandomPoolSelectionStrategy
     public PoolManagerPoolInformation
         select(List<PoolManagerPoolInformation> pools)
     {
-        return pools.get(_random.nextInt(pools.size()));
+        Iterable<PoolManagerPoolInformation> nonFullPools =
+                filter(pools, new Predicate<PoolManagerPoolInformation>()
+                {
+                    @Override
+                    public boolean apply(PoolManagerPoolInformation pool)
+                    {
+                        PoolCostInfo.PoolSpaceInfo info = pool.getPoolCostInfo().getSpaceInfo();
+                        return info.getFreeSpace() >= info.getGap();
+                    }
+                });
+        if (isEmpty(nonFullPools)) {
+            return null;
+        }
+        return get(nonFullPools, _random.nextInt(size(nonFullPools)));
     }
 }

modules/dcache/src/main/java/org/dcache/pool/migration/Task.java

@@ -207,10 +207,14 @@ private CellPath selectPool()
     {
         List<PoolManagerPoolInformation> pools =
             _definition.poolList.getPools();
-        if (pools.isEmpty()) {
-            throw new NoSuchElementException("No pools available");
+        PoolManagerPoolInformation pool = _definition.selectionStrategy.select(pools);
+        if (pool == null) {
+            if (pools.isEmpty()) {
+                throw new NoSuchElementException("No pools available.");
+            }
+            throw new NoSuchElementException("All target pools are full.");
         }
-        return new CellPath(_definition.selectionStrategy.select(pools).getName());
+        return new CellPath(pool.getName());
     }