`middleware_mutex` is racey.

#!/usr/bin/env ruby

# Taken from faraday internals
def middleware_mutex(&block)
	@middleware_mutex ||= begin
		require 'monitor'
		puts "I'm making a monitor!"
		Monitor.new
	end
	puts @middleware_mutex.inspect
	@middleware_mutex.synchronize(&block)
end


threads = 10.times.collect do
	Thread.new do
		middleware_mutex do
			puts "I got the power!"
		end
	end
end

threads.each(&:join)

Here is example output on 2.6:

koyoko% rvm use 2.6          
Using /home/samuel/.rvm/gems/ruby-2.6.3
^[[A%                                                                                                                                                         koyoko% ./middleware_mutex.rb
I'm making a monitor!
#<Monitor:0x000055fb2217ec60 @mon_mutex=#<Thread::Mutex:0x000055fb2217ec10>, @mon_mutex_owner_object_id=47268548572720, @mon_owner=nil, @mon_count=0>
I got the power!
#<Monitor:0x000055fb2217ec60 @mon_mutex=#<Thread::Mutex:0x000055fb2217ec10>, @mon_mutex_owner_object_id=47268548572720, @mon_owner=nil, @mon_count=0>
I got the power!
#<Monitor:0x000055fb2217ec60 @mon_mutex=#<Thread::Mutex:0x000055fb2217ec10>, @mon_mutex_owner_object_id=47268548572720, @mon_owner=nil, @mon_count=0>
I got the power!
#<Monitor:0x000055fb2217ec60 @mon_mutex=#<Thread::Mutex:0x000055fb2217ec10>, @mon_mutex_owner_object_id=47268548572720, @mon_owner=nil, @mon_count=0>
I got the power!
#<Monitor:0x000055fb2217ec60 @mon_mutex=#<Thread::Mutex:0x000055fb2217ec10>, @mon_mutex_owner_object_id=47268548572720, @mon_owner=nil, @mon_count=0>
I got the power!
#<Monitor:0x000055fb2217ec60 @mon_mutex=#<Thread::Mutex:0x000055fb2217ec10>, @mon_mutex_owner_object_id=47268548572720, @mon_owner=nil, @mon_count=0>
I got the power!
#<Monitor:0x000055fb2217ec60 @mon_mutex=#<Thread::Mutex:0x000055fb2217ec10>, @mon_mutex_owner_object_id=47268548572720, @mon_owner=nil, @mon_count=0>
I got the power!
#<Monitor:0x000055fb2217ec60 @mon_mutex=#<Thread::Mutex:0x000055fb2217ec10>, @mon_mutex_owner_object_id=47268548572720, @mon_owner=nil, @mon_count=0>
I got the power!
#<Monitor:0x000055fb2217ec60 @mon_mutex=#<Thread::Mutex:0x000055fb2217ec10>, @mon_mutex_owner_object_id=47268548572720, @mon_owner=nil, @mon_count=0>
I got the power!
#<Monitor:0x000055fb2217ec60 @mon_mutex=#<Thread::Mutex:0x000055fb2217ec10>, @mon_mutex_owner_object_id=47268548572720, @mon_owner=nil, @mon_count=0>
I got the power!
koyoko% ./middleware_mutex.rb
I'm making a monitor!
I'm making a monitor!
#<Monitor:0x000055f197e82388 @mon_mutex=#<Thread::Mutex:0x000055f197e82338>, @mon_mutex_owner_object_id=47248062026180, @mon_owner=nil, @mon_count=0>
I got the power!
I'm making a monitor!
#<Monitor:0x000055f197e81eb0 @mon_mutex=#<Thread::Mutex:0x000055f197e81e60>, @mon_mutex_owner_object_id=47248062025560, @mon_owner=nil, @mon_count=0>
I got the power!
I'm making a monitor!
#<Monitor:0x000055f197e81b90 @mon_mutex=#<Thread::Mutex:0x000055f197e81b40>, @mon_mutex_owner_object_id=47248062025160, @mon_owner=nil, @mon_count=0>
I got the power!
I'm making a monitor!
#<Monitor:0x000055f197e81938 @mon_mutex=#<Thread::Mutex:0x000055f197e818e8>, @mon_mutex_owner_object_id=47248062024860, @mon_owner=nil, @mon_count=0>
I got the power!
I'm making a monitor!
#<Monitor:0x000055f197e816e0 @mon_mutex=#<Thread::Mutex:0x000055f197e81690>, @mon_mutex_owner_object_id=47248062024560, @mon_owner=nil, @mon_count=0>
I got the power!
#<Monitor:0x000055f197e827c0 @mon_mutex=#<Thread::Mutex:0x000055f197e82590>, @mon_mutex_owner_object_id=47248062026720, @mon_owner=nil, @mon_count=0>
I got the power!
I'm making a monitor!
#<Monitor:0x000055f197e81438 @mon_mutex=#<Thread::Mutex:0x000055f197e813e8>, @mon_mutex_owner_object_id=47248062024220, @mon_owner=nil, @mon_count=0>
I got the power!
I'm making a monitor!
#<Monitor:0x000055f197e811e0 @mon_mutex=#<Thread::Mutex:0x000055f197e81190>, @mon_mutex_owner_object_id=47248062023920, @mon_owner=nil, @mon_count=0>
I got the power!
I'm making a monitor!
#<Monitor:0x000055f197e80f88 @mon_mutex=#<Thread::Mutex:0x000055f197e80f38>, @mon_mutex_owner_object_id=47248062023620, @mon_owner=nil, @mon_count=0>
I got the power!
I'm making a monitor!
#<Monitor:0x000055f197e80d30 @mon_mutex=#<Thread::Mutex:0x000055f197e80ce0>, @mon_mutex_owner_object_id=47248062023320, @mon_owner=nil, @mon_count=0>
I got the power!

Here is example output on jruby:

koyoko% rvm use jruby        
Using /home/samuel/.rvm/gems/jruby-9.2.6.0
koyoko% ./middleware_mutex.rb
I'm making a monitor!
I'm making a monitor!
I'm making a monitor!
#<Monitor:0x7ea668b0 @mon_mutex=#<Thread::Mutex:0x3027cbd3>, @mon_owner=nil, @mon_count=0>
I got the power!
I'm making a monitor!I'm making a monitor!#<Monitor:0x53ca91a5 @mon_mutex=#<Thread::Mutex:0x57f50f04>, @mon_owner=nil, @mon_count=0>I'm making a monitor!
#<Monitor:0x1b5e79ce @mon_mutex=#<Thread::Mutex:0x159994d5>, @mon_owner=nil, @mon_count=0>
I got the power!
#<Monitor:0x67589eb3 @mon_mutex=#<Thread::Mutex:0x63671680>, @mon_owner=nil, @mon_count=1>#<Monitor:0x1b5e79ce @mon_mutex=#<Thread::Mutex:0x159994d5>, @mon_owner=nil, @mon_count=0>
I got the power!
#<Monitor:0x67589eb3 @mon_mutex=#<Thread::Mutex:0x63671680>, @mon_owner=nil, @mon_count=0>
I got the power!

#<Monitor:0x49c2c95a @mon_mutex=#<Thread::Mutex:0xcc04e06>, @mon_owner=nil, @mon_count=0>
I got the power!

I'm making a monitor!#<Monitor:0x1b5e79ce @mon_mutex=#<Thread::Mutex:0x159994d5>, @mon_owner=nil, @mon_count=0>
#<Monitor:0x6fa38df1 @mon_mutex=#<Thread::Mutex:0x756e0d98>, @mon_owner=nil, @mon_count=0>
I got the power!
I got the power!

#<Monitor:0x4a9f0f52 @mon_mutex=#<Thread::Mutex:0x600dfde7>, @mon_owner=nil, @mon_count=0>

I got the power!

I got the power!
I got the power!

Start with the standalone Ruby reproduction in the issue and trace Faraday's middleware_mutex implementation. Run it under MRI and JRuby to confirm the competing Monitor initialization, then inspect existing concurrency coverage if available. Done means concurrent callers consistently share one synchronization object without creating multiple monitors.