don't get stuck in a loop

set a max number of attempts to create the random sample of subjects

lib/postgresql_selection.rb

@@ -1,6 +1,8 @@
 class PostgresqlSelection
   attr_reader :workflow, :user, :opts
 
+  MAX_RANDOM_SAMPLE_ATTEMPTS = 100
+
   def initialize(workflow, user=nil)
     @workflow, @user = workflow, user
   end
@@ -65,8 +67,11 @@ def select_results_in_order
 
   def construct_random_sample_to_limit
     results = []
+    attempt = 0
     until results.length >= limit do
       results = results | sample.limit(limit).pluck(:id)
+      attempt += 1
+      break if attempt >= MAX_RANDOM_SAMPLE_ATTEMPTS
     end
     results
   end

spec/lib/postgresql_selection_spec.rb

@@ -27,7 +27,6 @@ def update_sms_priorities
       sms_scope.count - _seen_count
     end
 
-
     context "when a user has only seen a few subjects" do
       let(:seen_count) { 5 }
       let!(:uss) do
@@ -87,6 +86,14 @@ def update_sms_priorities
       subject { PostgresqlSelection.new(workflow, user) }
 
       it_behaves_like "select for incomplete_project"
+
+      it "should give up trying to construct a random list after set number of attempts" do
+        unreachable_limit = SetMemberSubject.count + 1
+        allow_any_instance_of(PostgresqlSelection).to receive(:available_count).and_return(unreachable_limit + 1)
+        allow_any_instance_of(PostgresqlSelection).to receive(:limit).and_return(unreachable_limit)
+        results = subject.select
+        expect(results).to eq(results)
+      end
     end
 
     context "grouped selection" do