thread_pool: create a new thread group per process

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

@@ -85,11 +85,31 @@
       expect(subject).not_to have_workers
     end
 
-    it "respawns workers on fork()", skip: %w[jruby].include?(RUBY_ENGINE) do
-      pid = fork { expect(subject).to have_workers }
-      Process.wait(pid)
-      subject.close
-      expect(subject).not_to have_workers
+    describe "forking behavior" do
+      before do
+        skip('fork() is unsupported on JRuby') if %w[jruby].include?(RUBY_ENGINE)
+        unless Process.respond_to?(:last_status)
+          skip('Process.last_status is unsupported on this Ruby')
+        end
+      end
+
+      it "respawns workers on fork()" do
+        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
+        subject << 1
+        pid = fork { subject.has_workers? }
+        Process.wait(pid)
+        subject.close
+
+        expect(Process.last_status).to be_success
+      end
     end
   end