Handle fork_sibling failing repeatedly

Ref: #79

It can happen that the new mold was forked while at an unsafe
point, causing the middle process to crash.

When we detect this happens, we should abandon this mold.

Currently abandoning the mold cause a graceful shutdown,
in the future we could try creating a new mold to replace it.

Co-Authored-By: Étienne Barrié <[email protected]>

bin/dev-console

@@ -4,7 +4,7 @@
 # this script gives a quick dev environment for macOS
 # and Windows contributors.
 runner="docker"
-if command -v podman >/dev/null 2>&1; then
+if ! command -v docker >/dev/null 2>&1; then
   runner="podman"
 fi
 

lib/pitchfork.rb

@@ -33,9 +33,11 @@ module Pitchfork
   ClientShutdown = Class.new(EOFError)
 
   BootFailure = Class.new(StandardError)
+  ForkFailure = Class.new(StandardError)
 
   # :stopdoc:
 
+  FORK_TIMEOUT = 5
   FORK_LOCK = Monitor.new
   @socket_type = :SOCK_SEQPACKET
 
@@ -198,39 +200,54 @@ def clean_fork(setpgid: true, &block)
 
     def fork_sibling(role, &block)
       if REFORKING_AVAILABLE
+        r, w = IO.pipe
         # We double fork so that the new worker is re-attached back
         # to the master.
         # This requires either PR_SET_CHILD_SUBREAPER which is exclusive to Linux 3.4
         # or the master to be PID 1.
         if middle_pid = FORK_LOCK.synchronize { Process.fork } # parent
+          w.close
           # We need to wait(2) so that the middle process doesn't end up a zombie.
           # The process only call fork again an exit so it should be pretty fast.
           # However it might need to execute some `Process._fork` or `at_exit` callbacks,
           # so it case it takes more than 5 seconds to exit, we kill it with SIGBUS
           # to produce a crash report, as this is indicative of a nasty bug.
-          process_wait_with_timeout(middle_pid, 5, :BUS)
+          status = process_wait_with_timeout(middle_pid, FORK_TIMEOUT, :BUS)
+          pid_str = r.gets
+          r.close
+          if pid_str
+            Integer(pid_str)
+          else
+            raise ForkFailure, "fork_sibling didn't succeed in #{FORK_TIMEOUT} seconds"
+          end
         else # first child
+          r.close
           Process.setproctitle("<pitchfork fork_sibling(#{role})>")
-          clean_fork(&block) # detach into a grand child
+          pid = clean_fork do
+            # detach into a grand child
+            w.close
+            yield
+          end
+          w.puts(pid)
+          w.close
           exit
         end
       else
         clean_fork(&block)
       end
-
-      nil # it's tricky to return the PID
     end
 
     def process_wait_with_timeout(pid, timeout, timeout_signal = :KILL)
       (timeout * 200).times do
-        status = Process.wait(pid, Process::WNOHANG)
+        _, status = Process.waitpid2(pid, Process::WNOHANG)
         return status if status
         sleep 0.005 # 200 * 5ms => 1s
       end
 
       # The process didn't exit in the allotted time, so we kill it.
       Process.kill(timeout_signal, pid)
-      Process.wait(pid)
+      _, status = Process.waitpid2(pid)
+      status
     end
 
     def time_now(int = false)

lib/pitchfork/children.rb

@@ -66,13 +66,21 @@ def nr_alive?(nr)
       @workers.key?(nr)
     end
 
+    def abandon(worker)
+      @workers.delete(worker.nr)
+      @pending_workers.delete(worker.nr)
+    end
+
     def reap(pid)
       if child = @children.delete(pid)
         @pending_workers.delete(child.nr)
         @pending_molds.delete(child.pid)
         @molds.delete(child.pid)
         @workers.delete(child.nr)
         if @mold == child
+          @pending_workers.reject! do |nr, worker|
+            worker.generation == @mold.generation
+          end
           @mold = nil
         end
       end

lib/pitchfork/http_server.rb

@@ -315,7 +315,7 @@ def join
         end
       end
       stop # gracefully shutdown all workers on our way out
-      logger.info "master complete"
+      logger.info "master complete status=#{@exit_status}"
       @exit_status
     end
 
@@ -368,7 +368,7 @@ def monitor_loop(sleep = true)
       when Message::WorkerSpawned
         worker = @children.update(message)
         # TODO: should we send a message to the worker to acknowledge?
-        logger.info "worker=#{worker.nr} pid=#{worker.pid} registered"
+        logger.info "worker=#{worker.nr} pid=#{worker.pid} gen=#{worker.generation} registered"
       when Message::MoldSpawned
         new_mold = @children.update(message)
         logger.info("mold pid=#{new_mold.pid} gen=#{new_mold.generation} spawned")
@@ -409,6 +409,7 @@ def stop(graceful = true)
     end
 
     def worker_exit(worker)
+      logger.info "worker=#{worker.nr} pid=#{worker.pid} gen=#{worker.generation} exiting"
       proc_name status: "exiting"
 
       if @before_worker_exit
@@ -514,6 +515,12 @@ def murder_lazy_workers
     end
 
     def hard_timeout(worker)
+      if worker.pid.nil? # Not yet registered, likely never spawned
+        logger.error "worker=#{worker.nr} timed out during spawn, abandoning"
+        @children.abandon(worker)
+        return
+      end
+
       if @after_worker_hard_timeout
         begin
           @after_worker_hard_timeout.call(self, worker)
@@ -599,6 +606,8 @@ def spawn_missing_workers
         worker = Pitchfork::Worker.new(worker_nr)
 
         if REFORKING_AVAILABLE
+          worker.generation = @children.mold&.generation || 0
+
           unless @children.mold&.spawn_worker(worker)
             @logger.error("Failed to send a spawn_worker command")
           end
@@ -854,7 +863,7 @@ def worker_loop(worker)
             if @refork_condition.met?(worker, logger)
               proc_name status: "requests: #{worker.requests_count}, spawning mold"
               if spawn_mold(worker.generation)
-                logger.info("Refork condition met, promoting ourselves")
+                logger.info("worker=#{worker.nr} gen=#{worker.generation} Refork condition met, promoting ourselves")
               end
               @refork_condition.backoff!
             end
@@ -910,8 +919,18 @@ def mold_loop(mold)
             when false
               # no message, keep looping
             when Message::SpawnWorker
+              retries = 1
               begin
                 spawn_worker(Worker.new(message.nr, generation: mold.generation), detach: true)
+              rescue ForkFailure
+                if retries > 0
+                  @logger.fatal("mold pid=#{mold.pid} gen=#{mold.generation} Failed to spawn a worker. Retrying.")
+                  retries -= 1
+                  retry
+                else
+                  @logger.fatal("mold pid=#{mold.pid} gen=#{mold.generation} Failed to spawn a worker twice in a row. Corrupted mold process?")
+                  Process.exit(1)
+                end
               rescue => error
                 raise BootFailure, error.message
               end

test/integration/test_reforking.rb

@@ -64,6 +64,45 @@ def test_reforking_broken_after_mold_fork_hook
       assert_clean_shutdown(pid)
     end
 
+    def test_broken_mold
+      addr, port = unused_port
+
+      pid = spawn_server(app: File.join(ROOT, "test/integration/env.ru"), config: <<~CONFIG)
+        listen "#{addr}:#{port}"
+        worker_processes 2
+        spawn_timeout 2
+        refork_after [5, 5]
+        after_mold_fork do |_server, mold|
+          if mold.generation > 0 && !File.exist?("broke-once.txt")
+            File.write("broke-once.txt", "$$")
+            def Process.fork
+              # simulate some issue causing children to fail.
+              # Typically some native background thread holding a lock
+              # right when we fork.
+              Process.spawn("false")
+            end
+          end
+        end
+      CONFIG
+
+      assert_healthy("http://#{addr}:#{port}")
+      assert_stderr "worker=0 gen=0 ready"
+      assert_stderr "worker=1 gen=0 ready", timeout: 5
+
+      9.times do
+        assert_equal true, healthy?("http://#{addr}:#{port}")
+      end
+
+      assert_stderr "Refork condition met, promoting ourselves", timeout: 3
+
+      assert_stderr "Failed to spawn a worker. Retrying."
+      assert_stderr "Failed to spawn a worker twice in a row. Corrupted mold process?"
+      assert_stderr "No mold alive, shutting down"
+      assert_stderr "timed out during spawn, abandoning", timeout: 5
+
+      assert_exited(pid, 1, timeout: 5)
+    end
+
     def test_fork_unsafe
       addr, port = unused_port
 

test/integration_test_helper.rb

@@ -97,8 +97,10 @@ def unused_port
     private
 
     def assert_stderr(pattern, timeout: 1)
-      wait_stderr?(pattern, timeout)
-      assert_match(pattern, read_stderr)
+      print_stderr_on_error do
+        wait_stderr?(pattern, timeout)
+        assert_match(pattern, read_stderr)
+      end
     end
 
     def read_stderr
@@ -115,26 +117,30 @@ def wait_stderr?(pattern, timeout)
       false
     end
 
-    def assert_clean_shutdown(pid, timeout = 4)
-      Process.kill("QUIT", pid)
-      status = nil
-      (timeout * 2).times do
-        Process.kill(0, pid)
-        break if status = Process.wait2(pid, Process::WNOHANG)
-        sleep 0.5
+    def assert_clean_shutdown(pid, timeout: 4)
+      print_stderr_on_error do
+        Process.kill("QUIT", pid)
+        status = nil
+        (timeout * 2).times do
+          Process.kill(0, pid)
+          break if status = Process.wait2(pid, Process::WNOHANG)
+          sleep 0.5
+        end
+        assert status, "process pid=#{pid} didn't exit in #{timeout} seconds"
+        assert_predicate status[1], :success?
       end
-      assert status, "process pid=#{pid} didn't exit in #{timeout} seconds"
-      assert_predicate status[1], :success?
     end
 
-    def assert_exited(pid, exitstatus, timeout = 4)
-      status = nil
-      (timeout * 2).times do
-        break if status = Process.wait2(pid, Process::WNOHANG)
-        sleep 0.5
+    def assert_exited(pid, exitstatus, timeout: 4)
+      print_stderr_on_error do
+        status = nil
+        (timeout * 2).times do
+          break if status = Process.wait2(pid, Process::WNOHANG)
+          sleep 0.5
+        end
+        assert status, "process pid=#{pid} didn't exit in #{timeout} seconds"
+        assert_equal exitstatus, status[1].exitstatus
       end
-      assert status, "process pid=#{pid} didn't exit in #{timeout} seconds"
-      assert_equal exitstatus, status[1].exitstatus
     end
 
     def assert_healthy(host, timeout = 2)
@@ -179,5 +185,14 @@ def spawn(*args)
       @_pids << pid
       pid
     end
+
+    def print_stderr_on_error
+      yield
+    rescue Minitest::Assertion
+      puts '=' * 40
+      puts read_stderr
+      puts '=' * 40
+      raise
+    end
   end
 end

test/unit/test_children.rb

@@ -22,9 +22,11 @@ def test_message_worker_spawned
       worker = Worker.new(0)
       @children.register(worker)
       assert_predicate @children, :pending_workers?
+      assert @children.nr_alive?(0)
 
       @children.update(Message::WorkerSpawned.new(0, 42, 0, pipe))
       refute_predicate @children, :pending_workers?
+      assert @children.nr_alive?(0), @children.inspect
       assert_equal 42, worker.pid
       assert_equal [worker], @children.workers
     end