Update strategies used for DB count approximation for read-only DB

With the Tablesample and Reltuple strategies, they both attempt
transactions to stick to the primary DB when load-balancing is used,
which won't work with the loadbalancer enabled by default on read-only
databases (such as Geo), as there is no writeable primary DB.

Changelog: fixed

lib/gitlab/database/count.rb

@@ -35,7 +35,17 @@ module Gitlab
       #
       # @param [Array]
       # @return [Hash] of Model -> count mapping
-      def self.approximate_counts(models, strategies: [TablesampleCountStrategy, ReltuplesCountStrategy, ExactCountStrategy])
+      def self.approximate_counts(models, strategies: [])
+        if strategies.empty?
+          # ExactCountStrategy is the only strategy working on read-only DBs, as others make
+          # use of tuple stats which use the primary DB to estimate tables size in a transaction.
+          strategies = if ::Gitlab::Database.read_write?
+                         [TablesampleCountStrategy, ReltuplesCountStrategy, ExactCountStrategy]
+                       else
+                         [ExactCountStrategy]
+                       end
+        end
+
         strategies.each_with_object({}) do |strategy, counts_by_model|
           models_with_missing_counts = models - counts_by_model.keys
 

spec/lib/gitlab/database/count_spec.rb

@@ -46,5 +46,49 @@ RSpec.describe Gitlab::Database::Count do
         subject
       end
     end
+
+    context 'default strategies' do
+      subject { described_class.approximate_counts(models) }
+
+      context 'with a read-only database' do
+        before do
+          allow(Gitlab::Database).to receive(:read_only?).and_return(true)
+        end
+
+        it 'only uses the ExactCountStrategy' do
+          allow_next_instance_of(Gitlab::Database::Count::TablesampleCountStrategy) do |instance|
+            expect(instance).not_to receive(:count)
+          end
+          allow_next_instance_of(Gitlab::Database::Count::ReltuplesCountStrategy) do |instance|
+            expect(instance).not_to receive(:count)
+          end
+          expect_next_instance_of(Gitlab::Database::Count::ExactCountStrategy) do |instance|
+            expect(instance).to receive(:count).and_return({})
+          end
+
+          subject
+        end
+      end
+
+      context 'with a read-write database' do
+        before do
+          allow(Gitlab::Database).to receive(:read_only?).and_return(false)
+        end
+
+        it 'uses the available strategies' do
+          [
+            Gitlab::Database::Count::TablesampleCountStrategy,
+            Gitlab::Database::Count::ReltuplesCountStrategy,
+            Gitlab::Database::Count::ExactCountStrategy
+          ].each do |strategy_klass|
+            expect_next_instance_of(strategy_klass) do |instance|
+              expect(instance).to receive(:count).and_return({})
+            end
+          end
+
+          subject
+        end
+      end
+    end
   end
 end