Fixes #224. I believe this is a Postgres only issue

Gemfile.lock

@@ -171,6 +171,7 @@ GEM
 
 PLATFORMS
   arm64-darwin-22
+  arm64-darwin-23
   x86_64-darwin-21
   x86_64-darwin-23
   x86_64-linux

app/models/solid_queue/semaphore.rb

@@ -17,6 +17,17 @@ def signal(job)
       def signal_all(jobs)
         Proxy.signal_all(jobs)
       end
+
+      # Requires a unique index on key
+      def create_unique_by(attributes)
+        if connection.supports_insert_conflict_target?
+          insert({ **attributes }, unique_by: :key).any?
+        else
+          create!(**attributes)
+        end
+      rescue ActiveRecord::RecordNotUnique
+        false
+      end
     end
 
     class Proxy
@@ -41,18 +52,19 @@ def signal
       end
 
       private
+
         attr_accessor :job
 
         def attempt_creation
-          Semaphore.create!(key: key, value: limit - 1, expires_at: expires_at)
-          true
-        rescue ActiveRecord::RecordNotUnique
-          if limit == 1 then false
+          if Semaphore.create_unique_by(key: key, value: limit - 1, expires_at: expires_at)
+            true
           else
-            attempt_decrement
+            check_limit_or_decrement
           end
         end
 
+        def check_limit_or_decrement = limit == 1 ? false : attempt_decrement
+
         def attempt_decrement
           Semaphore.available.where(key: key).update_all([ "value = value - 1, expires_at = ?", expires_at ]) > 0
         end

test/integration/concurrency_controls_test.rb

@@ -183,7 +183,22 @@ class ConcurrencyControlsTest < ActiveSupport::TestCase
     assert job.reload.ready?
   end
 
+  test "verify transactions remain valid after Job creation conflicts via limits_concurrency" do
+    ActiveRecord::Base.transaction do
+      SequentialUpdateResultJob.perform_later(@result, name: "A", pause: 0.2.seconds)
+      SequentialUpdateResultJob.perform_later(@result, name: "B")
+
+      begin
+        assert_equal 2, SolidQueue::Job.count
+        assert true, "Transaction state valid"
+      rescue ActiveRecord::StatementInvalid
+        assert false, "Transaction state unexpectedly invalid"
+      end
+    end
+  end
+
   private
+
     def assert_stored_sequence(result, *sequences)
       expected = sequences.map { |sequence| "seq: " + sequence.map { |name| "s#{name}c#{name}" }.join }
       skip_active_record_query_cache do