[ruby-core:96182] [Ruby master Bug#16288] Segmentation fault with finalizers, threads

Issue #16288 has been updated by davidw (David Welton).


A little update. I have kept looking, a little bit at a time, and have uncovered another piece of what's going on: the difference between Thread.start and Thread.new

Thread.new does not run in a finalizer on its own. Thread.start, however, does run, and can be used to crash the interpreter:

``` ruby
Thread.DEBUG = 1

class Foo
  def initialize
    ObjectSpace.define_finalizer(self, proc do
                                   Foo.foo_finalizer
                                 end)
  end

  def Foo.foo_finalizer
    STDERR.puts "finalizing a Foo"
    Thread.start do
      STDERR.puts "starting ..."
      sleep 5
      STDERR.puts "... finalizing foo thread done"
    end
  end
end

20.times do
  f = Foo.new
end

```


Thread.new has a check, although I don't understand why it runs *after* the thread has been allocated:

``` c
static VALUE
thread_s_new(int argc, VALUE *argv, VALUE klass)
{
    rb_thread_t *th;
    VALUE thread = rb_thread_alloc(klass);

    if (GET_VM()->main_thread->status == THREAD_KILLED)
	rb_raise(rb_eThreadError, "can't alloc thread");

```

Whereas Thread.start has no such check:

``` c
static VALUE
thread_start(VALUE klass, VALUE args)
{
    return thread_create_core(rb_thread_alloc(klass), args, 0);
}

```

As above, care needs to be taken with any potential fixes, because there is currently code out there that depends on, say, Timeout being run in a finalizer, and that utilizes Thread.start.

----------------------------------------
Bug #16288: Segmentation fault with finalizers, threads
https://bugs.ruby-lang.org/issues/16288#change-83059

* Author: davidw (David Welton)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
* ruby -v: ruby 2.6.6p116 (2019-10-02 revision 67825) [x86_64-linux]
* Backport: 2.5: UNKNOWN, 2.6: UNKNOWN
----------------------------------------
Hi,

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.





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