RUBY-1934 clean up DNS server termination (#2903)

* RUBY-1934 clean up DNS server termination

The ticket originally called for pulling in another dependency to manage
the DNS server, but I don't think it's necessary. The async-container
code would be doing essentially what we already had, just with a cleaner
technique for sending the termination signal.

* need to require rubydns

* even cleaner

* RubyDNS is failing oddly with Ruby 2.7, so let's just skip it

spec/integration/reconnect_spec.rb

@@ -111,6 +111,8 @@
       # thread.kill should've similarly failed, but it doesn't.
       fails_on_jruby
 
+      minimum_mri_version '3.0.0'
+
       it 'recreates SRV monitor' do
         wait_for_discovery
 
@@ -181,8 +183,6 @@
       end
 
       around do |example|
-        require 'support/dns'
-
         rules = [
           ['_mongodb._tcp.test-fake.test.build.10gen.cc', :srv,
             [0, 0, 2799, 'localhost.test.build.10gen.cc'],

spec/integration/srv_monitoring_spec.rb

@@ -76,9 +76,7 @@
     # NotImplementedError: recvmsg_nonblock is not implemented
     fails_on_jruby
 
-    before(:all) do
-      require 'support/dns'
-    end
+    minimum_mri_version '3.0.0'
 
     around do |example|
       # Speed up the tests by listening on the fake ports we are using.

spec/integration/srv_spec.rb

@@ -12,10 +12,6 @@
     # NotImplementedError: recvmsg_nonblock is not implemented
     fails_on_jruby
 
-    before(:all) do
-      require 'support/dns'
-    end
-
     let(:uri) do
       "mongodb+srv://test-fake.test.build.10gen.cc/?tls=#{SpecConfig.instance.ssl?}&tlsInsecure=true"
     end

spec/support/common_shortcuts.rb

@@ -337,51 +337,33 @@ def stop_monitoring(*clients)
       [:tcp, "0.0.0.0", 5300],
     ]
 
-    def mock_dns(config)
-      semaphore = Mongo::Semaphore.new
-
-      thread = Thread.new do
-        RubyDNS::run_server(DNS_INTERFACES) do
-          config.each do |(query, type, *answers)|
-
-            resource_cls = Resolv::DNS::Resource::IN.const_get(type.to_s.upcase)
-            resources = answers.map do |answer|
-              resource_cls.new(*answer)
-            end
-            match(query, resource_cls) do |req|
-              req.add(resources)
-            end
+    # Starts the DNS server and returns it; should be run from within an
+    # Async block. Prefer #mock_dns instead, which does the setup for you.
+    def start_dns_server(config)
+      RubyDNS::run_server(DNS_INTERFACES) do
+        config.each do |(query, type, *answers)|
+          resource_cls = Resolv::DNS::Resource::IN.const_get(type.to_s.upcase)
+          resources = answers.map do |answer|
+            resource_cls.new(*answer)
           end
 
-          semaphore.signal
+          match(query, resource_cls) do |req|
+            req.add(resources)
+          end
         end
       end
+    end
 
-      semaphore.wait
+    # Starts and runs a DNS server, then yields to the attached block.
+    def mock_dns(config)
+      # only require rubydns when we need it; it's MRI-only.
+      require 'rubydns'
 
-      begin
+      Async do |task|
+        server = start_dns_server(config)
         yield
       ensure
-        10.times do
-          if $last_async_task
-            break
-          end
-          sleep 0.5
-        end
-
-        # Hack to stop the server - https://github.com/socketry/rubydns/issues/75
-        if $last_async_task.nil?
-          STDERR.puts "No async task - server never started?"
-        else
-          begin
-            $last_async_task.stop
-          rescue NoMethodError => e
-            STDERR.puts "Error stopping async task: #{e}"
-          end
-        end
-
-        thread.kill
-        thread.join
+        server.stop
       end
     end
 

spec/support/constraints.rb

@@ -17,6 +17,16 @@ def require_local_tls
     end
   end
 
+  def minimum_mri_version(version)
+    require_mri
+
+    before(:all) do
+      if RUBY_VERSION < version
+        skip "Ruby #{version} or greater is required"
+      end
+    end
+  end
+
   def forbid_x509_auth
     before(:all) do
       skip 'X.509 auth not allowed' if SpecConfig.instance.x509_auth?