Stop profiling in forked child process

lib/vernier.rb

@@ -5,6 +5,7 @@
 require_relative "vernier/stack_table"
 require_relative "vernier/result"
 require_relative "vernier/hooks"
+require_relative "vernier/fork"
 require_relative "vernier/vernier"
 require_relative "vernier/output/firefox"
 require_relative "vernier/output/top"
@@ -56,6 +57,11 @@ def self.stop_profile
     result
   end
 
+  def self.cancel_profile
+    @collector&.cancel
+    @collector = nil
+  end
+
   def self.trace_retained(**profile_options, &block)
     profile(**profile_options.merge(mode: :retained), &block)
   end

lib/vernier/collector.rb

@@ -71,6 +71,15 @@ def record_interval(category, name = category)
       )
     end
 
+    def cancel
+      finish
+      @thread_names.cancel
+      @hooks.each do |hook|
+        hook.disable
+      end
+      nil
+    end
+
     def stop
       result = finish
 

lib/vernier/fork.rb

@@ -0,0 +1,17 @@
+# frozen_string_literal: true
+
+module Vernier
+  if ::Process.respond_to?(:_fork)
+    module ForkHooks
+      def _fork
+        pid = super
+        if pid == 0 # We're in the child
+          Vernier.cancel_profile
+        end
+        pid
+      end
+    end
+
+    ::Process.singleton_class.prepend(ForkHooks)
+  end
+end

lib/vernier/thread_names.rb

@@ -13,6 +13,10 @@ def [](object_id)
       @names[object_id] || "thread obj_id:#{object_id}"
     end
 
+    def cancel
+      @tp.disable
+    end
+
     def finish
       collect_running
       @tp.disable

test/test_vernier.rb

@@ -6,4 +6,12 @@ class TestVernier < Minitest::Test
   def test_that_it_has_a_version_number
     refute_nil ::Vernier::VERSION
   end
+
+  def test_that_forked_children_do_not_hang
+    pid = Process.fork do
+      # noop
+    end
+    _, status = Process.waitpid2(pid)
+    assert_predicate status, :success?
+  end
 end