Address FileThreadPool race condition

I saw a couple ThreadErrors around popping from an empty queue in
bugsnag: this got introduced when I moved away from catching all
ThreadErrors. There's a race condition between checking that the queue
is empty & attempting to pop from it.

This change locks the pop operation with a mutex to prevent the race.

Author: wfleming

lib/cc/engine/analyzers/file_thread_pool.rb

@@ -17,7 +17,7 @@ def run(&block)
 
           @workers = thread_count.times.map do
             Thread.new do
-              while !queue.empty? && (item = queue.pop(true))
+              while item = next_item(queue)
                 yield item
               end
             end
@@ -32,6 +32,11 @@ def join
 
         attr_reader :files, :concurrency, :workers
 
+        def next_item(queue)
+          @lock ||= Mutex.new
+          @lock.synchronize { queue.pop(true) unless queue.empty? }
+        end
+
         def build_queue
           Queue.new.tap do |queue|
             files.each do |file|