Faraday: `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!
ioquatix
All 9 comments
If you run this on JRuby, @middleware_mutex ||= begin is not atomic and you not only end up executing the block multiple times, @middleware_mutex actually gets set several times to different monitors. Looks like this can also happen on 2.6 since puts can introduce blocking/thread context switch.
ioquatix
on 23 Oct 2019
_Thanks for analysis!_
olleolleolle
on 23 Oct 2019
It's quite interesting to see ||= doesn't guarantee single assignment, even on MRI.
eregon
on 26 Oct 2019
@eregon Yeah, I was surprised by that too. Can we dump the instructions to see what is actually going on?
ioquatix
on 26 Oct 2019
@ioquatix
$ xsel -b | ruby --dump=insns
...
== disasm: #<ISeq:middleware_mutex@-:1 (1,0)-(9,3)> (catch: FALSE)
local table (size: 1, argc: 0 [opts: 0, rest: -1, post: 0, block: 0, kw: -1@-1, kwrest: -1])
[ 1] block@0<Block>
0000 putnil ( 2)[LiCa]
0001 defined instance-variable, :@middleware_mutex, false
0005 branchunless 14
0007 getinstancevariable :@middleware_mutex, <is:0>
0010 dup
0011 branchif 42
0013 pop
0014 putself ( 3)[Li]
0015 putstring "monitor"
0017 opt_send_without_block <callinfo!mid:require, argc:1, FCALL|ARGS_SIMPLE>, <callcache>
0020 pop
0021 putself ( 4)[Li]
0022 putstring "I'm making a monitor!"
0024 opt_send_without_block <callinfo!mid:puts, argc:1, FCALL|ARGS_SIMPLE>, <callcache>
0027 pop
0028 opt_getinlinecache 35, <is:1> ( 5)[Li]
0031 getconstant :Monitor
0033 opt_setinlinecache <is:1>
0035 opt_send_without_block <callinfo!mid:new, argc:0, ARGS_SIMPLE>, <callcache>
0038 dup ( 2)
0039 setinstancevariable :@middleware_mutex, <is:0>
0042 pop
0043 putself ( 7)[Li]
0044 getinstancevariable :@middleware_mutex, <is:0>
0047 opt_send_without_block <callinfo!mid:inspect, argc:0, ARGS_SIMPLE>, <callcache>
0050 opt_send_without_block <callinfo!mid:puts, argc:1, FCALL|ARGS_SIMPLE>, <callcache>
0053 pop
0054 getinstancevariable :@middleware_mutex, <is:0> ( 8)[Li]
0057 getblockparamproxy block@0, 0
0060 send <callinfo!mid:synchronize, argc:0, ARGS_BLOCKARG>, <callcache>, nil
0064 leave ( 9)[Re]
So ||= is like:
if defined?(@middleware_mutex)
@middleware_mutex
else
@middleware_mutex = expression
end
To make it not override in MRI we'd need:
if defined?(@middleware_mutex)
@middleware_mutex
else
result = expression
# special bytecode to do `@middleware_mutex = result unless defined?(@middleware_mutex)` atomically
end
Of course that has a cost as it's an extra check before assigning.
And the block would still execute multiple times, trying to fix that automatically would likely lead to deadlock and high overhead, so that part is best left to user code.
eregon
on 26 Oct 2019
Oh, I totally agree this isn't something we can fix automatically, the code is horribly broken from the POV of thread safety and making ||= atomic won't fix that.
ioquatix
on 26 Oct 2019
Wow thanks for the analysis @ioquatix and once again your expertise around this topic can clearly be seen.
That is, as far as I know, quite an old piece of code which I guess was introduced specifically to support JRuby and other concurrent ruby implementations.
Today, we don't officially support JRuby and other non-MRI rubies anymore, mostly because I think we lack the necessary knowledge to do so, but we just left that untouched in the hope it would at least help.
I'm not really sure how to tackle this, if you or @eregon could point us in the right direction I'd be happy to gain some knowledge and have a look 🤓
iMacTia
on 30 Oct 2019
This is also a bug on CRuby. I can submit a PR.
ioquatix
on 30 Oct 2019
I tried one solution in #1074. I have another idea that involves converting the MiddlewareRegistry module into a class. So instead of this:
It could be something like:
module Faraday
class Response
self.middleware_registry = MiddlewareRegistry.new(File.expand_path('response', __dir__),
raise_error: [:RaiseError, 'raise_error'],
logger: [:Logger, 'logger'],
)
This doesn't rely on class lifecycle hooks. It's pretty clear that MiddlewareRegistry is being instantiated here when the ruby is being interpreted.
class MiddlewareRegistry
def initialize
@mutex = Monitor.new
end
end
technoweenie
on 13 Nov 2019
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Most helpful comment
@ioquatix
So
||=is like:To make it not override in MRI we'd need:
Of course that has a cost as it's an extra check before assigning.
And the block would still execute multiple times, trying to fix that automatically would likely lead to deadlock and high overhead, so that part is best left to user code.