Terminate Riemann::Tools::HttpCheck threads between tests

Under normal operation, we have a single instance of the
`Riemann::Tools::HttpCheck` class for the lifetime of the monitoring
process.

But when testing, we create a new instance for each test, each with its
resolvers and worker thread pools.  The test process will have more and
more threads, until it eventually hit the OS limit of the maximum number
of threads for a process and cause an exception:

> ThreadError: can't create Thread: Resource temporarily unavailable

Make sure that the threads are terminated at the end of each test to
avoid running out of resources if they are limited.

lib/riemann/tools/http_check.rb

@@ -37,10 +37,15 @@ def initialize
         @resolve_queue = Queue.new
         @work_queue = Queue.new
 
+        @resolvers = []
+        @workers = []
+
         opts[:resolvers].times do
-          Thread.new do
+          @resolvers << Thread.new do
             loop do
               uri = @resolve_queue.pop
+              Thread.exit unless uri
+
               host = uri.host
 
               addresses = Resolv::DNS.new.getaddresses(host)
@@ -59,9 +64,11 @@ def initialize
         end
 
         opts[:workers].times do
-          Thread.new do
+          @workers << Thread.new do
             loop do
               uri, addresses = @work_queue.pop
+              Thread.exit unless uri
+
               test_uri_addresses(uri, addresses)
             end
           end
@@ -70,6 +77,23 @@ def initialize
         super
       end
 
+      # Under normal operation, we have a single instance of this class for the
+      # lifetime of the process.  But when testing, we create a new instance
+      # for each test, each with its resolvers and worker threads.  The test
+      # process may end-up with a lot of running threads, hitting the OS limit
+      # of max threads by process and being unable to create more thread:
+      #
+      # ThreadError: can't create Thread: Resource temporarily unavailable
+      #
+      # To avoid this situation, we provide this method.
+      def shutdown
+        @resolve_queue.close
+        @resolvers.map(&:join)
+
+        @work_queue.close
+        @workers.map(&:join)
+      end
+
       def tick
         report(
           service: 'riemann http-check resolvers utilization',

spec/riemann/tools/http_check_spec.rb

@@ -116,6 +116,10 @@ def protected!
         @server&.shutdown
       end
 
+      after(:each) do
+        subject.shutdown
+      end
+
       let(:test_webserver_port) { @server.config[:Port] }
 
       let(:reported_uri) { uri }