Stop profiling in forked child process

Otherwise the child will hang forever on exit when trying to
dump the profiling data. Which is problematic when trying to
profile some programs.

There might be more elegant ways to handle it, this is mostly
a proof of concept.

Also it's not quite fully working, the test passes alone, but
but if ran after some other specific tests, it end up in a
SIGTRAP, which I'm not sure why:

```
bundle exec rake SEED=26641 TESTOPTS="-v"
...

 1) Failure:
TestVernier#test_that_forked_children_do_not_hang [test/test_vernier.rb:15]:
Expected #<Process::Status: pid 63146 SIGTRAP (signal 5)>
 to be success?.

```

Author: byroot

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 +57,13 @@ def self.stop_profile
     result
   end
 
+  def self.cancel_profile
+    if @collector
+      @collector.stop
+      @collector = nil
+    end
+  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