Re-use issue/MR counts for the pagination system

This changes the issue and MR index pages so the pagination system
re-uses the output of the COUNT(*) query used to calculate the number of
rows per state (opened, closed, etc). This removes the need for an
additional COUNT(*) on both pages.

1 files changed, 50 insertions, 0 deletions

diff --git a/lib/gitlab/issuables_count_for_state.rb b/lib/gitlab/issuables_count_for_state.rb
new file mode 100644
index 00000000000..505810964bc
--- /dev/null
+++ b/lib/gitlab/issuables_count_for_state.rb
@@ -0,0 +1,50 @@
+module Gitlab
+  # Class for counting and caching the number of issuables per state.
+  class IssuablesCountForState
+    # The name of the RequestStore cache key.
+    CACHE_KEY = :issuables_count_for_state
+
+    # The state values that can be safely casted to a Symbol.
+    STATES = %w[opened closed merged all].freeze
+
+    # finder - The finder class to use for retrieving the issuables.
+    def initialize(finder)
+      @finder = finder
+      @cache =
+        if RequestStore.active?
+          RequestStore[CACHE_KEY] ||= initialize_cache
+        else
+          initialize_cache
+        end
+    end
+
+    def for_state_or_opened(state = nil)
+      self[state || :opened]
+    end
+
+    # Returns the count for the given state.
+    #
+    # state - The name of the state as either a String or a Symbol.
+    #
+    # Returns an Integer.
+    def [](state)
+      state = state.to_sym if cast_state_to_symbol?(state)
+
+      cache_for_finder[state] || 0
+    end
+
+    private
+
+    def cache_for_finder
+      @cache[@finder]
+    end
+
+    def cast_state_to_symbol?(state)
+      state.is_a?(String) && STATES.include?(state)
+    end
+
+    def initialize_cache
+      Hash.new { |hash, finder| hash[finder] = finder.count_by_state }
+    end
+  end
+end