1 files changed, 75 insertions, 0 deletions
diff --git a/lib/gitlab/database/partitioning/sliding_list_strategy.rb b/lib/gitlab/database/partitioning/sliding_list_strategy.rb
new file mode 100644
index 00000000000..21b86b43ae7
--- /dev/null
+++ b/lib/gitlab/database/partitioning/sliding_list_strategy.rb
@@ -0,0 +1,75 @@
+# frozen_string_literal: true
+
+module Gitlab
+  module Database
+    module Partitioning
+      class SlidingListStrategy
+        attr_reader :model, :partitioning_key, :next_partition_if, :detach_partition_if
+
+        delegate :table_name, to: :model
+
+        def initialize(model, partitioning_key, next_partition_if:, detach_partition_if:)
+          @model = model
+          @partitioning_key = partitioning_key
+          @next_partition_if = next_partition_if
+          @detach_partition_if = detach_partition_if
+
+          ensure_partitioning_column_ignored!
+        end
+
+        def current_partitions
+          Gitlab::Database::PostgresPartition.for_parent_table(table_name).map do |partition|
+            SingleNumericListPartition.from_sql(table_name, partition.name, partition.condition)
+          end.sort
+        end
+
+        def missing_partitions
+          if no_partitions_exist?
+            [initial_partition]
+          elsif next_partition_if.call(active_partition.value)
+            [next_partition]
+          else
+            []
+          end
+        end
+
+        def initial_partition
+          SingleNumericListPartition.new(table_name, 1)
+        end
+
+        def next_partition
+          SingleNumericListPartition.new(table_name, active_partition.value + 1)
+        end
+
+        def extra_partitions
+          possibly_extra = current_partitions[0...-1] # Never consider the most recent partition
+
+          possibly_extra.take_while { |p| detach_partition_if.call(p.value) }
+        end
+
+        def after_adding_partitions
+          active_value = active_partition.value
+          model.connection.change_column_default(model.table_name, partitioning_key, active_value)
+        end
+
+        def active_partition
+          # The current partitions list is sorted, so the last partition has the highest value
+          # This is the only partition that receives inserts.
+          current_partitions.last
+        end
+
+        def no_partitions_exist?
+          current_partitions.empty?
+        end
+
+        private
+
+        def ensure_partitioning_column_ignored!
+          unless model.ignored_columns.include?(partitioning_key.to_s)
+            raise "Add #{partitioning_key} to #{model.name}.ignored_columns to use it with SlidingListStrategy"
+          end
+        end
+      end
+    end
+  end
+end