Remove concurrent-ruby dependency, fixes #3830

Changes.md

@@ -2,6 +2,11 @@
 
 [Sidekiq Changes](https://github.com/mperham/sidekiq/blob/master/Changes.md) | [Sidekiq Pro Changes](https://github.com/mperham/sidekiq/blob/master/Pro-Changes.md) | [Sidekiq Enterprise Changes](https://github.com/mperham/sidekiq/blob/master/Ent-Changes.md)
 
+HEAD
+-----------
+
+- Remove concurrent-ruby gem dependency [#3830]
+
 5.1.3
 -----------
 

lib/sidekiq/processor.rb

@@ -4,7 +4,6 @@ require 'sidekiq/fetch'
 require 'sidekiq/job_logger'
 require 'sidekiq/job_retry'
 require 'thread'
-require 'concurrent/map'
 
 module Sidekiq
   ##
@@ -206,7 +205,9 @@ module Sidekiq
 
     PROCESSED = Counter.new
     FAILURE = Counter.new
-    WORKER_STATE = Concurrent::Map.new
+    # This is mutable global state but because each thread is storing
+    # its own unique key/value, there's no thread-safety issue AFAIK.
+    WORKER_STATE = {}
 
     def stats(job_hash, queue)
       tid = Sidekiq::Logging.tid

sidekiq.gemspec

@@ -19,6 +19,5 @@ Gem::Specification.new do |gem|
 
   gem.add_dependency 'redis', '>= 3.3.5', '< 5'
   gem.add_dependency 'connection_pool', '~> 2.2', '>= 2.2.0'
-  gem.add_dependency 'concurrent-ruby', '~> 1.0', '>= 1.0.5'
   gem.add_dependency 'rack-protection', '>= 1.5.0'
 end