Segmentation fault with finalizers, threads
This is a tricky one and I am still working on narrowing it down, but I will report what I have so far.
I compiled a version of 2_6_6 from github: ruby 2.6.6p116 (2019-10-02 revision 67825) [x86_64-linux]
I have a minimal Rails project that uses Mongoid. It crashes with a segmentation fault when rspec runs. The concurrent ruby gem is in some way involved, and I have been posting there: https://github.com/ruby-concurrency/concurrent-ruby/issues/808
However, I think there is a deeper problem - I would not expect a user level script to cause a segmentation fault.
I have been putting a lot of debugging statements in, and turned on Thread.DEBUG, and have noticed some things. I am not experienced with Ruby's internals, so some of these bits of data might be normal or irrelevant:
- The concurrent-ruby gem uses ObjectSpace.define_finalizer to set a finalizer
- That finalizer creates a new Thread
- However, it appears as if that thread is running after the main thread is already dead, so code that expects to reference the main thread crashes, because it's a NULL reference.
I tried the following test code:
class Foo def initialize ObjectSpace.define_finalizer(self, proc do Foo.foo_finalizer end) end def bar puts 'bar' end def Foo.foo_finalizer puts "foo_finalizer" t = Thread.new do puts "Thread reporting for duty" end puts "foo_finalizer thread launched" sleep 5 end end f = Foo.new f.bar f = nil
While trying to develop a simple test case to demonstrate the problem. It triggers rb_raise(rb_eThreadError, "can't alloc thread"); in thread_s_new, because it looks like the main thread has already been marked as 'killed' in this case. When I check the main thread status in thread_s_new with the above code, it reports 'dead'.
When I run my rspec code in the sample Rails project, thread_s_new shows the main thread's status as 'run' even if it should be dead?
I have seen some debugging things that shows some exceptions and thread_join interrupts and so on.
Is it possible that something like this is happening?
Main thread starts doing a cleanup, and gets an exception or something that generates an interrupt, and its KILLED status gets reset to RUNNABLE
Then, in the finalizer, it starts creating a Thread, but at this point the main thread actually does get killed, and when that finalizer thread tries to run it runs into a null reference?
I can provide the Rails sample project if needs be.
Sorry if any of the above isn't clear; I've been staring at the C code for several hours and am a bit cross-eyed!
Thank you for any insights.