Add a reproduction test

test/transformation_spec.rb

@@ -145,6 +145,57 @@ def empty_ops
       end
     end
 
+    it "rebalances a tree containing a write-only shard" do
+      to = mk_template 'ReplicatingShard -> (SqlShard(host3), WriteOnlyShard -> SqlShard(host4))'
+
+      children = [
+        Gizzard::Shard.new(info("host1", "tbl_001_a", "SqlShard"), [], 1),
+        Gizzard::Shard.new(
+          info("localhost", "tbl_001_write_only", "WriteOnlyShard"),
+          [Gizzard::Shard.new(info("host2", "tbl_001_b", "SqlShard"), [], 1)],
+          1)
+      ]
+      trees = {
+        forwarding(0, 0, id("localhost", "tbl_001_rep")) =>
+          Gizzard::Shard.new(info("localhost", "tbl_001_rep", "ReplicatingShard", "", "", 0), children, 1)
+      }
+      dest_templates_and_weights = { to => 1 }
+      copy_wrapper = "BlockedShard"
+      batch_finish = false
+
+      rebalancer = Gizzard::Rebalancer.new(trees, dest_templates_and_weights, copy_wrapper, batch_finish)
+      rebalancer.transformations.size.should == 1
+      transformation = rebalancer.transformations.clone.shift[0]
+      transformation.operations.should == empty_ops.merge({
+        :prepare => [ create_shard('WriteOnlyShard'),
+                      create_shard('BlockedShard'),
+                      create_shard('SqlShard(host4)'),
+                      create_shard('BlockedShard'),
+                      create_shard('SqlShard(host3)'),
+                      add_link('WriteOnlyShard', 'BlockedShard'),
+                      add_link('BlockedShard', 'SqlShard(host3)'),
+                      add_link('ReplicatingShard', 'WriteOnlyShard'),
+                      add_link('BlockedShard', 'SqlShard(host4)'),
+                      add_link('ReplicatingShard', 'BlockedShard') ],
+        :copy =>    [ copy_shard('SqlShard(host1)', 'SqlShard(host3)'),
+                      copy_shard('SqlShard(host1)', 'SqlShard(host4)') ],
+        :cleanup => [ add_link('ReplicatingShard', 'SqlShard(host3)'),
+                      add_link('WriteOnlyShard', 'SqlShard(host4)'),
+                      remove_link('ReplicatingShard', 'BlockedShard'),
+                      remove_link('WriteOnlyShard', 'BlockedShard'),
+                      remove_link('BlockedShard', 'SqlShard(host4)'),
+                      remove_link('ReplicatingShard', 'SqlShard(host1)'),
+                      remove_link('WriteOnlyShard', 'SqlShard(host2)'),
+                      remove_link('ReplicatingShard', 'WriteOnlyShard'),
+                      delete_shard('BlockedShard'),
+                      delete_shard('BlockedShard'),
+                      delete_shard('SqlShard(host1)'),
+                      # FIXME: DATASERV-40: these deletes are currently occurring in :prepare
+                      delete_shard('WriteOnlyShard'),
+                      delete_shard('SqlShard(host2)') ]
+      })
+    end
+
     it "migrates the top level shard" do
       from = mk_template 'ReplicatingShard -> (SqlShard(host1), SqlShard(host2))'
       to   = mk_template 'FailingOverShard -> (SqlShard(host1), SqlShard(host2))'