notifier: respawn workers if they're all dead

Potentially fixes airbrake/airbrake#463
(falling back to sync delivery because there are no running async
workers)

I noticed I can reliably reproduce this message when I invoke the Rails
console and manually send a notice via `Airbrake.notify`. It happens
because by default Rails launches [Spring][1], which preloads the app
by using the `fork()` system call. And when that happens, as we know,
our worker threads die.

The fix is simply to respawn workers. It works reliably with the Rails
console and I assume it will fix the above mentioned issue.

[1]: https://github.com/rails/spring/

CHANGELOG.md

@@ -7,6 +7,8 @@ Airbrake Ruby Changelog
   ([#25](https://github.com/airbrake/airbrake-ruby/pull/25))
 * Made sure that generated notices always have a backtrace
   ([#21](https://github.com/airbrake/airbrake-ruby/pull/21))
+* Made the asynchronous delivery mechanism more robust
+  ([#26](https://github.com/airbrake/airbrake-ruby/pull/26))
 
 ### [v1.0.2][v1.0.2] (January 3, 2016)
 

lib/airbrake-ruby/async_sender.rb

@@ -13,10 +13,9 @@ def initialize(config)
       @unsent = SizedQueue.new(config.queue_size)
       @sender = SyncSender.new(config)
       @closed = false
-      @workers = ThreadGroup.new
+      @workers = nil
 
-      config.workers.times { @workers.add(spawn_worker) }
-      @workers.enclose
+      spawn_workers
     end
 
     ##
@@ -77,6 +76,18 @@ def has_workers?
       !@closed && @workers.list.any?
     end
 
+    ##
+    # Spawns N workers (according to the config option) and creates a new
+    # enclosed ThreadGroup.
+    #
+    # @since v1.0.3
+    # @return [void]
+    def spawn_workers
+      @workers = ThreadGroup.new
+      @config.workers.times { @workers.add(spawn_worker) }
+      @workers.enclose
+    end
+
     private
 
     def spawn_worker

lib/airbrake-ruby/notifier.rb

@@ -118,15 +118,17 @@ def convert_to_exception(ex)
       e
     end
 
-    def send_notice(exception, params, sender = default_sender)
+    def send_notice(exception, params, sender = select_sender)
       notice = build_notice(exception, params)
       @filter_chain.refine(notice)
       return if notice.ignored?
 
       sender.send(notice)
     end
 
-    def default_sender
+    def select_sender
+      @async_sender.spawn_workers unless @async_sender.has_workers?
+
       if @async_sender.has_workers?
         @async_sender
       else

spec/notifier_spec.rb

@@ -377,17 +377,38 @@ def to_json(*)
       out = StringIO.new
 
       airbrake = described_class.new(airbrake_params.merge(logger: Logger.new(out)))
-      airbrake.
-        instance_variable_get(:@async_sender).
-        instance_variable_get(:@workers).
-        list.
-        each(&:kill)
+      async_sender = airbrake.instance_variable_get(:@async_sender)
+      # Make sure we don't revive workers.
+      allow(async_sender).to receive(:spawn_workers).and_return(nil)
+      async_sender.instance_variable_get(:@workers).list.each(&:kill)
 
       sleep 1
 
+      expect(async_sender.has_workers?).to be_falsey
       expect(airbrake.notify('bingo')).to be_nil
+      expect(async_sender.has_workers?).to be_falsey
+
       expect(out.string).to match(/falling back to sync delivery/)
     end
+
+    it "respawns workers if the workers list is empty" do
+      out = StringIO.new
+
+      airbrake = described_class.new(airbrake_params.merge(logger: Logger.new(out)))
+      async_sender = airbrake.instance_variable_get(:@async_sender)
+      async_sender.instance_variable_get(:@workers).list.each(&:kill)
+
+      sleep 1
+
+      expect(async_sender.has_workers?).to be_falsey
+      expect(airbrake.notify('bingo')).to be_nil
+
+      sleep 1
+
+      expect(async_sender.has_workers?).to be_truthy
+
+      expect(out.string).not_to match(/falling back to sync delivery/)
+    end
   end
 
   describe "#add_filter" do