thread_pool: create a new thread group per process

Fixes #576 (ThreadError: can't move to the enclosed thread group)

When we `Airbrake.notify` inside a `fork` block, `ThreadPool` tries to respawn
workers (which were killed by the `fork`) to the same `ThreadGroup`, which was
created in the parent thread. The problem is that that `ThreadGroup` is already
finalised in that parent thread and no new threads can be added anymore.  The
fix is to create a new `ThreadGroup` on `fork`. This way every process will have
its own `ThreadGroup`.

CHANGELOG.md

@@ -3,6 +3,10 @@ Airbrake Ruby Changelog
 
 ### master
 
+* Fixed `ThreadError: can't move to the enclosed thread group` when using
+  `notify` inside a `fork`
+  ([#577](https://github.com/airbrake/airbrake-ruby/pull/577))
+
 ### [v4.14.0][v4.14.0] (April 10, 2020)
 
 * Fixed a bug where some default filters are not appended

lib/airbrake-ruby/thread_pool.rb

@@ -83,6 +83,7 @@ def has_workers?
 
         if @pid != Process.pid && @workers.list.empty?
           @pid = Process.pid
+          @workers = ThreadGroup.new
           spawn_workers
         end
 

spec/thread_pool_spec.rb

@@ -85,12 +85,27 @@
       expect(subject).not_to have_workers
     end
 
-    it "respawns workers on fork()", skip: %w[jruby].include?(RUBY_ENGINE) do
+    it "respawns workers on fork()" do
+      skip('fork() is unsupported on JRuby') if %w[jruby].include?(RUBY_ENGINE)
+
       pid = fork { expect(subject).to have_workers }
       Process.wait(pid)
       subject.close
+
+      expect(Process.last_status).to be_success
       expect(subject).not_to have_workers
     end
+
+    it "ensures that a new thread group is created per process" do
+      skip('fork() is unsupported on JRuby') if %w[jruby].include?(RUBY_ENGINE)
+
+      subject << 1
+      pid = fork { subject.has_workers? }
+      Process.wait(pid)
+      subject.close
+
+      expect(Process.last_status).to be_success
+    end
   end
 
   describe "#close" do