RUBY-899 Don't select servers in ServerSelector if cluster returns a nil list

Author: estolfo

lib/mongo/server_selector/selectable.rb

@@ -89,9 +89,9 @@ def select_server(cluster)
           if cluster.single?
             servers = cluster.servers
           elsif cluster.sharded?
-            servers = near_servers(cluster.servers)
+            servers = near_servers(cluster.servers) if cluster.servers
           else
-            servers = select(cluster.servers)
+            servers = select(cluster.servers) if cluster.servers
           end
           return servers.first if servers && !servers.compact.empty?
           cluster.scan!

spec/mongo/server_selector_spec.rb

@@ -98,4 +98,55 @@
       end
     end
   end
+
+  shared_context 'a ServerSelector' do
+
+    context 'when cluster#servers is nil' do
+
+      let(:servers) { nil }
+
+      let(:cluster) do
+        double('cluster').tap do |c|
+          allow(c).to receive(:servers).and_return(servers)
+          allow(c).to receive(:single?).and_return(single)
+          allow(c).to receive(:sharded?).and_return(sharded)
+          allow(c).to receive(:scan!).and_return(true)
+        end
+      end
+
+      let(:read_pref) do
+        described_class.get({ mode: :primary }, server_selection_timeout: 1)
+      end
+
+      it 'raises a NoServerAvailable error' do
+        expect do
+          read_pref.select_server(cluster)
+        end.to raise_exception(Mongo::ServerSelector::NoServerAvailable)
+      end
+    end
+  end
+
+  context 'when the cluster has a Single topology' do
+
+    let(:single) { true }
+    let(:sharded) { false }
+
+    it_behaves_like 'a ServerSelector'
+  end
+
+  context 'when the cluster has a ReplicaSet topology' do
+
+    let(:single) { false }
+    let(:sharded) { false }
+
+    it_behaves_like 'a ServerSelector'
+  end
+
+  context 'when the cluster has a Sharded topology' do
+
+    let(:single) { false }
+    let(:sharded) { true }
+
+    it_behaves_like 'a ServerSelector'
+  end
 end