Make it possible to enable/disable PostgreSQL FDW for Geo

User are able to set `fdw: true` in their `config/database_geo.yml` to
use PostgreSQL FDW for Geo.

By default it is disabled, and if FDW isn't configured properly, it
isn't used anyway.

Closes gitlab-org/gitlab-ee#4558.

changelogs/unreleased-ee/tc-geo-fdw-configurable.yml

+---
+title: Make it possible to enable/disable PostgreSQL FDW for Geo
+merge_request: 4020
+author:
+type: added

config/database_geo.yml.postgresql

@@ -9,6 +9,7 @@ production:
   username: git
   password: "secure password"
   host: localhost
+  # fdw: false
 
 #
 # Development specific
@@ -21,6 +22,7 @@ development:
   username: postgres
   password: "secure password"
   host: localhost
+  # fdw: false
 
 #
 # Staging specific
@@ -33,6 +35,7 @@ staging:
   username: git
   password: "secure password"
   host: localhost
+  # fdw: false
 
 # Warning: The database defined as "test" will be erased and
 # re-generated from your development database when you run "rake".
@@ -45,3 +48,4 @@ test: &test
   username: postgres
   password:
   host: localhost
+  # fdw: false

ee/lib/gitlab/geo.rb

@@ -65,14 +65,22 @@ module Gitlab
       ::License.feature_available?(:geo)
     end
 
-    def self.fdw?
-      self.cache_value(:geo_fdw?) do
-        ::Geo::BaseRegistry.connection.execute(
+    def self.fdw_capable?
+      self.cache_value(:geo_fdw_capable?) do
+        ::Geo::TrackingBase.connection.execute(
           "SELECT COUNT(*) FROM information_schema.tables WHERE table_schema = '#{FDW_SCHEMA}' AND table_type = 'FOREIGN TABLE'"
         ).first.fetch('count').to_i.positive?
       end
     end
 
+    def self.fdw?
+      return false unless self.fdw_capable?
+
+      # FDW is disabled by default: https://gitlab.com/gitlab-org/gitlab-ee/issues/4558
+      # Enable it by setting `fdw: true` in config/database_geo.yml
+      Rails.configuration.geo_database.fetch('fdw', false)
+    end
+
     def self.fdw_table(table_name)
       FDW_SCHEMA + ".#{table_name}"
     end

spec/ee/spec/lib/gitlab/geo_spec.rb

@@ -18,24 +18,57 @@ describe Gitlab::Geo, :geo do
     end
   end
 
-  describe 'fdw?' do
+  describe 'fdw_capable?' do
     let(:fdw_check) { "SELECT COUNT(*) FROM information_schema.tables WHERE table_schema = 'gitlab_secondary' AND table_type = 'FOREIGN TABLE'" }
 
     before do
-      allow(::Geo::BaseRegistry.connection).to receive(:execute).with(anything).and_call_original
+      allow(::Geo::TrackingBase.connection).to receive(:execute).with(anything).and_call_original
     end
 
     it 'returns true when PostgreSQL FDW is enabled' do
-      expect(::Geo::BaseRegistry.connection).to receive(:execute).with(fdw_check).and_return([{ 'count' => 1 }])
+      expect(::Geo::TrackingBase.connection).to receive(:execute).with(fdw_check).and_return([{ 'count' => 1 }])
+
+      expect(described_class.fdw_capable?).to be_truthy
+    end
+
+    it 'returns false when PostgreSQL FDW is not enabled' do
+      expect(::Geo::TrackingBase.connection).to receive(:execute).with(fdw_check).and_return([{ 'count' => 0 }])
 
-      expect(described_class.fdw?).to be_truthy
+      expect(described_class.fdw_capable?).to be_falsey
     end
+  end
 
+  describe 'fdw?' do
     it 'returns false when PostgreSQL FDW is not enabled' do
-      expect(::Geo::BaseRegistry.connection).to receive(:execute).with(fdw_check).and_return([{ 'count' => 0 }])
+      allow(::Geo::TrackingBase.connection).to receive(:execute).and_return([{ 'count' => 0 }])
+      allow(Rails.configuration).to receive(:geo_database).and_return('fdw' => true)
 
       expect(described_class.fdw?).to be_falsey
     end
+
+    context 'fdw capable' do
+      before do
+        allow(described_class).to receive(:fdw_capable?).and_return(true)
+      end
+
+      it 'returns false by default' do
+        allow(Rails.configuration).to receive(:geo_database).and_return('fdw' => nil)
+
+        expect(described_class.fdw?).to be_falsey
+      end
+
+      it 'returns false if configured in `config/database_geo.yml`' do
+        allow(Rails.configuration).to receive(:geo_database).and_return('fdw' => false)
+
+        expect(described_class.fdw?).to be_falsey
+      end
+
+      it 'returns true if configured in `config/database_geo.yml`' do
+        allow(Rails.configuration).to receive(:geo_database).and_return('fdw' => true)
+
+        expect(described_class.fdw?).to be_truthy
+      end
+    end
   end
 
   describe 'primary?' do