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Testing SMTP Server DNS Resolution Implementation in Postal

This RSpec test suite validates the DNS resolution and endpoint handling functionality of the SMTP Server class in the Postal project. It focuses on testing IPv4 and IPv6 address resolution for SMTP server configurations.

Test Coverage Overview

The test suite provides comprehensive coverage of DNS resolution scenarios for SMTP server endpoints.

Key areas tested include:

Dual-stack IPv4/IPv6 resolution
IPv4-only resolution
IPv6-only resolution
DNS resolver interaction verification

The tests ensure proper handling of different DNS record combinations and correct endpoint ordering.

Implementation Analysis

The testing approach utilizes RSpec’s behavior-driven development patterns with context-based test organization. It employs mock objects through RSpec’s allow/receive pattern to simulate DNS resolver responses, enabling controlled testing of various network address scenarios.

The implementation leverages RSpec’s nested describe and context blocks for clear test organization, with extensive use of test doubles for DNS resolution simulation.

Technical Details

Testing tools and configuration:

RSpec as the testing framework
Rails helper integration
DNS resolver mocking
Custom matchers for IP address validation
Subject/let blocks for shared test setup
Before blocks for mock configuration

Best Practices Demonstrated

The test suite exemplifies several testing best practices including isolated test cases, proper mock object usage, and clear test organization.

Notable practices include:

Descriptive context blocks for different scenarios
Isolated test setup using before blocks
Effective use of RSpec matchers
Clear separation of test cases
Comprehensive edge case coverage

postalserver/postal

spec/lib/smtp_client/server_spec.rb

            
# frozen_string_literal: true

require "rails_helper"

module SMTPClient

  RSpec.describe Server do
    let(:hostname) { "example.com" }
    let(:port) { 25 }
    let(:ssl_mode) { SSLModes::AUTO }

    subject(:server) { described_class.new(hostname, port: port, ssl_mode: ssl_mode) }

    describe "#endpoints" do
      context "when there are A and AAAA records" do
        before do
          allow(DNSResolver.local).to receive(:a).and_return(["1.2.3.4", "2.3.4.5"])
          allow(DNSResolver.local).to receive(:aaaa).and_return(["2a00::67a0:a::1234", "2a00::67a0:a::2345"])
        end

        it "asks the resolver for the A and AAAA records for the hostname" do
          server.endpoints
          expect(DNSResolver.local).to have_received(:a).with(hostname).once
          expect(DNSResolver.local).to have_received(:aaaa).with(hostname).once
        end

        it "returns endpoints for ipv6 addresses followed by ipv4" do
          expect(server.endpoints).to match [
            have_attributes(ip_address: "2a00::67a0:a::1234"),
            have_attributes(ip_address: "2a00::67a0:a::2345"),
            have_attributes(ip_address: "1.2.3.4"),
            have_attributes(ip_address: "2.3.4.5"),
          ]
        end
      end

      context "when there are just A records" do
        before do
          allow(DNSResolver.local).to receive(:a).and_return(["1.2.3.4", "2.3.4.5"])
          allow(DNSResolver.local).to receive(:aaaa).and_return([])
        end

        it "returns ipv4 endpoints" do
          expect(server.endpoints).to match [
            have_attributes(ip_address: "1.2.3.4"),
            have_attributes(ip_address: "2.3.4.5"),
          ]
        end
      end

      context "when there are just AAAA records" do
        before do
          allow(DNSResolver.local).to receive(:a).and_return([])
          allow(DNSResolver.local).to receive(:aaaa).and_return(["2a00::67a0:a::1234", "2a00::67a0:a::2345"])
        end

        it "returns ipv6 endpoints" do
          expect(server.endpoints).to match [
            have_attributes(ip_address: "2a00::67a0:a::1234"),
            have_attributes(ip_address: "2a00::67a0:a::2345"),
          ]
        end
      end
    end
  end

end