Feature/search 1785 exception in explicit sharding policy (#115)

* SEARCH-1785 Add test cases and fix helper method so these fail.

* SEARCH-1785 Throw exception when explicit sharding policy fails to create enough replicas.

* SEARCH-1785 Update exception message to provide instructions to user.

src/main/java/org/alfresco/util/shard/ExplicitShardingPolicy.java

@@ -21,6 +21,8 @@ package org.alfresco.util.shard;
 import java.util.LinkedList;
 import java.util.List;
 
+import org.alfresco.error.AlfrescoRuntimeException;
+
 /**
  * Common ACL based index sharding behaviour for SOLR and the repository
  * 
@@ -67,6 +69,13 @@ public class ExplicitShardingPolicy
             {
                 if (test % numNodes == nodeInstance - 1)
                 {
+                    // This algorithm fails for some sets of parameters. (See SEARCH-1785)
+                    if (shardIds.contains(shard % numShards))
+                    {
+                        throw new AlfrescoRuntimeException("Sharding configuration not supported - unable to create shard list for node " + nodeInstance
+                                + " (shards:" + numShards + ", replication:" + replicationFactor + ", nodes:" + numNodes + ")."
+                                + " Please set up the shards manually or use a different sharding configuration.");
+                    }
                     shardIds.add(shard % numShards);
                 }
                 test++;

src/test/java/org/alfresco/util/shard/ExplicitShardingPolicyTest.java

@@ -20,8 +20,11 @@ package org.alfresco.util.shard;
 
 import static org.junit.Assert.*;
 
+import java.util.HashSet;
 import java.util.List;
+import java.util.Set;
 
+import org.alfresco.error.AlfrescoRuntimeException;
 import org.junit.Test;
 
 /**
@@ -117,6 +120,20 @@ public class ExplicitShardingPolicyTest
         buildAndTest(10, 2, 4);
     }
 
+    /** ExplicitShardingPolicy algorithm fails for 2 shards, 3 replicas, 3 nodes. (See SEARCH-1785) */
+    @Test(expected = AlfrescoRuntimeException.class)
+    public void search1785_233()
+    {
+        buildAndTest(2, 3, 3);
+    }
+
+    /** ExplicitShardingPolicy algorithm fails for 4 shards, 3 replicas, 6 nodes. (See SEARCH-1785) */
+    @Test (expected = AlfrescoRuntimeException.class)
+    public void search1785_436()
+    {
+        buildAndTest(4, 3, 6);
+    }
+    
     @Test
     public void check_10_2()
     {
@@ -163,7 +180,8 @@ public class ExplicitShardingPolicyTest
         int[] found = new int[numShards];
         for (int i = 0; i < numNodes; i++)
         {
-            List<Integer> shardIds = policy.getShardIdsForNode(i + 1);
+            // Convert to a set to remove any duplicates.
+            Set<Integer> shardIds = new HashSet<>(policy.getShardIdsForNode(i + 1));
             assertEquals(numShards * replicationFactor / numNodes, shardIds.size());
             for (Integer shardId : shardIds)
             {