Use COPY to populate schema_migrations table

This is intended to reduce conflict potential in the schema file.

See https://gitlab.com/gitlab-org/gitlab/-/issues/212051.

config/initializers/active_record_schema_versions.rb

+# frozen_string_literal: true
+
+# Patch to use COPY in db/structure.sql when populating schema_migrations table
+# This is intended to reduce potential for merge conflicts in db/structure.sql
+ActiveRecord::ConnectionAdapters::PostgreSQLAdapter.prepend(Gitlab::Database::PostgresqlAdapter::SchemaVersionsCopyMixin)

lib/gitlab/database/postgresql_adapter/schema_versions_copy_mixin.rb

+# frozen_string_literal: true
+
+module Gitlab
+  module Database
+    module PostgresqlAdapter
+      module SchemaVersionsCopyMixin
+        extend ActiveSupport::Concern
+
+        def dump_schema_information # :nodoc:
+          versions = schema_migration.all_versions
+          copy_versions_sql(versions) if versions.any?
+        end
+
+        private
+
+        def copy_versions_sql(versions)
+          sm_table = quote_table_name(schema_migration.table_name)
+
+          sql = +"COPY #{sm_table} (version) FROM STDIN;\n"
+          sql << versions.map { |v| Integer(v) }.sort.join("\n")
+          sql << "\n\\.\n"
+
+          sql
+        end
+      end
+    end
+  end
+end

spec/lib/gitlab/database/postgresql_adapter/schema_versions_copy_mixin_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe Gitlab::Database::PostgresqlAdapter::SchemaVersionsCopyMixin do
+  let(:schema_migration) { double('schem_migration', table_name: table_name, all_versions: versions) }
+  let(:versions) { %w(5 2 1000 200 4 93 2) }
+  let(:table_name) { "schema_migrations" }
+
+  let(:instance) do
+    Object.new.extend(described_class)
+  end
+
+  before do
+    allow(instance).to receive(:schema_migration).and_return(schema_migration)
+    allow(instance).to receive(:quote_table_name).with(table_name).and_return("\"#{table_name}\"")
+  end
+
+  subject { instance.dump_schema_information }
+
+  it 'uses COPY FROM STDIN' do
+    expect(subject.split("\n").first).to match(/COPY "schema_migrations" \(version\) FROM STDIN;/)
+  end
+
+  it 'contains a sorted list of versions by their numeric value' do
+    version_lines = subject.split("\n")[1..-2].map(&:to_i)
+
+    expect(version_lines).to eq(versions.map(&:to_i).sort)
+  end
+
+  it 'contains a end-of-data marker' do
+    expect(subject).to end_with("\\.\n")
+  end
+
+  context 'with non-Integer versions' do
+    let(:versions) { %w(5 2 4 abc) }
+
+    it 'raises an error' do
+      expect { subject }.to raise_error(/invalid value for Integer/)
+    end
+  end
+end