Merge branch 'hash-concurrent-foreign-key-names' into 'master'

Hash concurrent foreign key names similar to Rails

See merge request !9415
Conflicts:
	lib/gitlab/database/migration_helpers.rb
	spec/lib/gitlab/database/migration_helpers_spec.rb

lib/gitlab/database/migration_helpers.rb

@@ -26,6 +26,63 @@ module Gitlab
         add_index(table_name, column_name, options)
       end
 
+      # Adds a foreign key with only minimal locking on the tables involved.
+      #
+      # This method only requires minimal locking when using PostgreSQL. When
+      # using MySQL this method will use Rails' default `add_foreign_key`.
+      #
+      # source - The source table containing the foreign key.
+      # target - The target table the key points to.
+      # column - The name of the column to create the foreign key on.
+      # on_delete - The action to perform when associated data is removed,
+      #             defaults to "CASCADE".
+      def add_concurrent_foreign_key(source, target, column:, on_delete: :cascade)
+        # Transactions would result in ALTER TABLE locks being held for the
+        # duration of the transaction, defeating the purpose of this method.
+        if transaction_open?
+          raise 'add_concurrent_foreign_key can not be run inside a transaction'
+        end
+
+        # While MySQL does allow disabling of foreign keys it has no equivalent
+        # of PostgreSQL's "VALIDATE CONSTRAINT". As a result we'll just fall
+        # back to the normal foreign key procedure.
+        if Database.mysql?
+          return add_foreign_key(source, target,
+                                 column: column,
+                                 on_delete: on_delete)
+        end
+
+        disable_statement_timeout
+
+        key_name = concurrent_foreign_key_name(source, column)
+
+        # Using NOT VALID allows us to create a key without immediately
+        # validating it. This means we keep the ALTER TABLE lock only for a
+        # short period of time. The key _is_ enforced for any newly created
+        # data.
+        execute <<-EOF.strip_heredoc
+        ALTER TABLE #{source}
+        ADD CONSTRAINT #{key_name}
+        FOREIGN KEY (#{column})
+        REFERENCES #{target} (id)
+        ON DELETE #{on_delete} NOT VALID;
+        EOF
+
+        # Validate the existing constraint. This can potentially take a very
+        # long time to complete, but fortunately does not lock the source table
+        # while running.
+        execute("ALTER TABLE #{source} VALIDATE CONSTRAINT #{key_name};")
+      end
+
+      # Returns the name for a concurrent foreign key.
+      #
+      # PostgreSQL constraint names have a limit of 63 bytes. The logic used
+      # here is based on Rails' foreign_key_name() method, which unfortunately
+      # is private so we can't rely on it directly.
+      def concurrent_foreign_key_name(table, column)
+        "fk_#{Digest::SHA256.hexdigest("#{table}_#{column}_fk").first(10)}"
+      end
+
       # Long-running migrations may take more than the timeout allowed by
       # the database. Disable the session's statement timeout to ensure
       # migrations don't get killed prematurely. (PostgreSQL only)

spec/lib/gitlab/database/migration_helpers_spec.rb

@@ -59,6 +59,81 @@ describe Gitlab::Database::MigrationHelpers, lib: true do
     end
   end
 
+  describe '#add_concurrent_foreign_key' do
+    context 'inside a transaction' do
+      it 'raises an error' do
+        expect(model).to receive(:transaction_open?).and_return(true)
+
+        expect do
+          model.add_concurrent_foreign_key(:projects, :users, column: :user_id)
+        end.to raise_error(RuntimeError)
+      end
+    end
+
+    context 'outside a transaction' do
+      before do
+        allow(model).to receive(:transaction_open?).and_return(false)
+      end
+
+      context 'using MySQL' do
+        it 'creates a regular foreign key' do
+          allow(Gitlab::Database).to receive(:mysql?).and_return(true)
+
+          expect(model).to receive(:add_foreign_key).
+            with(:projects, :users, column: :user_id, on_delete: :cascade)
+
+          model.add_concurrent_foreign_key(:projects, :users, column: :user_id)
+        end
+      end
+
+      context 'using PostgreSQL' do
+        before do
+          allow(Gitlab::Database).to receive(:mysql?).and_return(false)
+        end
+
+        it 'creates a concurrent foreign key' do
+          expect(model).to receive(:disable_statement_timeout)
+          expect(model).to receive(:execute).ordered.with(/NOT VALID/)
+          expect(model).to receive(:execute).ordered.with(/VALIDATE CONSTRAINT/)
+
+          model.add_concurrent_foreign_key(:projects, :users, column: :user_id)
+        end
+      end
+    end
+  end
+
+  describe '#concurrent_foreign_key_name' do
+    it 'returns the name for a foreign key' do
+      name = model.concurrent_foreign_key_name(:this_is_a_very_long_table_name,
+                                               :with_a_very_long_column_name)
+
+      expect(name).to be_an_instance_of(String)
+      expect(name.length).to eq(13)
+    end
+  end
+
+  describe '#disable_statement_timeout' do
+    context 'using PostgreSQL' do
+      it 'disables statement timeouts' do
+        expect(Gitlab::Database).to receive(:postgresql?).and_return(true)
+
+        expect(model).to receive(:execute).with('SET statement_timeout TO 0')
+
+        model.disable_statement_timeout
+      end
+    end
+
+    context 'using MySQL' do
+      it 'does nothing' do
+        expect(Gitlab::Database).to receive(:postgresql?).and_return(false)
+
+        expect(model).not_to receive(:execute)
+
+        model.disable_statement_timeout
+      end
+    end
+  end
+
   describe '#update_column_in_batches' do
     before do
       create_list(:empty_project, 5)