RUBY-899 Return empty list instead of nil from cluster#servers and test server selector

Author: estolfo

lib/mongo/cluster.rb

@@ -181,7 +181,7 @@ def scan!
     #
     # @since 2.0.0
     def servers
-      topology.servers(@servers)
+      topology.servers(@servers.compact).compact
     end
 
     # Create a cluster for the provided client, for use when we don't want the

lib/mongo/cluster/topology/unknown.rb

@@ -106,6 +106,7 @@ def replica_set_name; nil; end
         #
         # @since 2.0.0
         def servers(servers)
+          []
         end
 
         # An unknown topology is not sharded.

spec/mongo/cluster_spec.rb

@@ -149,5 +149,65 @@
         end
       end
     end
+
+    context 'when the cluster has no servers' do
+
+      let(:cluster) do
+        described_class.new(ADDRESSES)
+      end
+
+      let(:servers) do
+        [nil,nil]
+      end
+
+      before do
+        cluster.instance_variable_set(:@servers, servers)
+        cluster.instance_variable_set(:@topology, topology)
+      end
+
+      context 'when topology is Single' do
+
+        let(:topology) do
+          Mongo::Cluster::Topology::Single.new({})
+        end
+
+        it 'returns an empty array' do
+          expect(cluster.servers).to eq([])
+        end
+      end
+
+      context 'when topology is ReplicaSet' do
+
+        let(:topology) do
+          Mongo::Cluster::Topology::ReplicaSet.new({})
+        end
+
+        it 'returns an empty array' do
+          expect(cluster.servers).to eq([])
+        end
+      end
+
+      context 'when topology is Sharded' do
+
+        let(:topology) do
+          Mongo::Cluster::Topology::Sharded.new({})
+        end
+
+        it 'returns an empty array' do
+          expect(cluster.servers).to eq([])
+        end
+      end
+
+      context 'when topology is Unknown' do
+
+        let(:topology) do
+          Mongo::Cluster::Topology::Unknown.new({})
+        end
+
+        it 'returns an empty array' do
+          expect(cluster.servers).to eq([])
+        end
+      end
+    end
   end
 end

spec/mongo/server_selector_spec.rb

@@ -98,4 +98,55 @@
       end
     end
   end
+
+  shared_context 'a ServerSelector' do
+
+    context 'when cluster#servers is empty' do
+
+      let(:servers) { [] }
+
+      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