Stub DNS in the spec suite

This implements stubbing throughout the spec suite for DNS queries,
of which there are many. It includes a few tweaks to continue to allow
resolution for local addresses. This solves an issue where non-standard
DNS setups prevent most of the spec suite from working locally, and
should also provide speed improvements to the CI pipeline on average
by removing a source of external requests.

spec/lib/gitlab/tcp_checker_spec.rb

@@ -2,7 +2,7 @@
 
 require 'spec_helper'
 
-describe Gitlab::TcpChecker do
+describe Gitlab::TcpChecker, :permit_dns do
   before do
     @server = TCPServer.new('localhost', 0)
     _, @port, _, @ip = @server.addr

spec/lib/gitlab/url_blocker_spec.rb

@@ -2,7 +2,7 @@
 
 require 'spec_helper'
 
-describe Gitlab::UrlBlocker do
+describe Gitlab::UrlBlocker, :stub_invalid_dns_only do
   include StubRequests
 
   describe '#validate!' do

spec/support/dns.rb

+# frozen_string_literal: true
+
+require Rails.root.join("spec/support/helpers/dns_helpers")
+
+RSpec.configure do |config|
+  config.include DnsHelpers
+
+  config.before do
+    block_dns!
+  end
+
+  config.before(:each, :permit_dns) do
+    permit_dns!
+  end
+
+  config.before(:each, :stub_invalid_dns_only) do
+    permit_dns!
+    stub_invalid_dns!
+  end
+end

spec/support/helpers/dns_helpers.rb

+# frozen_string_literal: true
+
+module DnsHelpers
+  def block_dns!
+    stub_all_dns!
+    stub_invalid_dns!
+    permit_local_dns!
+  end
+
+  def permit_dns!
+    allow(Addrinfo).to receive(:getaddrinfo).and_call_original
+  end
+
+  def stub_all_dns!
+    allow(Addrinfo).to receive(:getaddrinfo).with(anything, anything, nil, :STREAM).and_return([])
+    allow(Addrinfo).to receive(:getaddrinfo).with(anything, anything, nil, :STREAM, anything, anything).and_return([])
+  end
+
+  def stub_invalid_dns!
+    allow(Addrinfo).to receive(:getaddrinfo).with(/foobar\.\w|(\d{1,3}\.){4,}\d{1,3}/i, anything, nil, :STREAM) do
+      raise SocketError.new("getaddrinfo: Name or service not known")
+    end
+  end
+
+  def permit_local_dns!
+    local_addresses = /(127|10)\.0\.0\.\d{1,3}|(192\.168|172\.16)\.\d{1,3}\.\d{1,3}|0\.0\.0\.0|localhost/i
+    allow(Addrinfo).to receive(:getaddrinfo).with(local_addresses, anything, nil, :STREAM).and_call_original
+    allow(Addrinfo).to receive(:getaddrinfo).with(local_addresses, anything, nil, :STREAM, anything, anything).and_call_original
+  end
+end