Allow to mark workers as fork unsafe

When some part of an application is hard or impossible to make
fork-safe, but rarely called, it's possible to simply mark the worker
as fork unsafe, and rely on its siblings to fork the next generation.

In the eventually where all the workers end up marked fork unsafe,
no new generation will be spawned, unless of course one of the unsafe
workers die and is replaced by a fresh one.

So this is only meant as a workaround for infrequently called things.

lib/pitchfork/http_server.rb

@@ -811,7 +811,7 @@ def worker_loop(worker)
           # timeout so we can update .deadline and keep parent from SIGKILL-ing us
           worker.update_deadline(@timeout)
 
-          if @refork_condition && !worker.outdated?
+          if @refork_condition && Info.fork_safe? && !worker.outdated?
             if @refork_condition.met?(worker, logger)
               if spawn_mold(worker.generation)
                 logger.info("Refork condition met, promoting ourselves")

lib/pitchfork/info.rb

@@ -5,10 +5,19 @@
 module Pitchfork
   module Info
     @workers_count = 0
+    @fork_safe = true
 
     class << self
       attr_accessor :workers_count
 
+      def fork_safe?
+        @fork_safe
+      end
+
+      def no_longer_fork_safe!
+        @fork_safe = false
+      end
+
       def live_workers_count
         now = Pitchfork.time_now(true)
         (0...workers_count).count do |nr|

test/integration/fork_unsafe.ru

@@ -0,0 +1,6 @@
+use Rack::ContentLength
+use Rack::ContentType, "text/plain"
+run lambda { |env|
+  Pitchfork::Info.no_longer_fork_safe!
+  [ 200, {}, [ env.inspect << "\n" ] ]
+}

test/integration/test_reforking.rb

@@ -64,6 +64,28 @@ def test_reforking_broken_after_mold_fork_hook
       assert_clean_shutdown(pid)
     end
 
+    def test_fork_unsafe
+      addr, port = unused_port
+
+      pid = spawn_server(app: File.join(ROOT, "test/integration/fork_unsafe.ru"), config: <<~CONFIG)
+        listen "#{addr}:#{port}"
+        worker_processes 2
+        refork_after [5, 5]
+      CONFIG
+
+      assert_healthy("http://#{addr}:#{port}")
+      assert_stderr "worker=0 gen=0 ready"
+      assert_stderr "worker=1 gen=0 ready"
+
+      20.times do
+        assert_equal true, healthy?("http://#{addr}:#{port}")
+      end
+
+      refute_match("Refork condition met, promoting ourselves", read_stderr)
+
+      assert_clean_shutdown(pid)
+    end
+
     def test_reforking_on_USR2
       addr, port = unused_port