Backport #5700: fork {} segfaults during VM cleanup when run inside Fiber

• Status: Closed
• Priority: Normal
• Assignee: -

[normalperson (Eric Wong)]

The issue is very easy to reproduce:

Fiber.new do
  p Process.waitpid2(fork {})
end.resume

Parent process successfully exits, child process segfaults while
it is exiting.

Backtrace is attached (gdb_bt.txt)

MALLOC_CHECK_=3 with GNU libc malloc() implementation detects an
attempt to free invalid pointer (attachment: malloc_check_3.txt)

I can also reproduce this with 1.9.2-p290 and 1.9.3-p0 as well as
latest trunk, so it has been around a while and a fix needs to be
backported.

[normalperson (Eric Wong)]

Eric Wong [e-mail] wrote:

http://redmine.ruby-lang.org/issues/5700

Hello, I would like to see this fixed for the next 1.9.3 release.

I'm pretty sure this is an easy fix for somebody already familiar with
the Fiber/continuation code (unfortunately I am not familiar with it).

[nagachika (Tomoyuki Chikanaga)]

• File bug5700.patch bug5700.patch added

Hmm, it don't seems easy problem for me... Apparently I'm not so familiar with fiber.

Anyway I've written a patch for this issue. I know it's somewhat ad-hoc, but it works.
ko1 and nobu how do you think?

[normalperson (Eric Wong)]

Tomoyuki Chikanaga [e-mail] wrote:

Anyway I've written a patch for this issue. I know it's somewhat ad-hoc, but it works.
ko1 and nobu how do you think?

Thanks for looking into this. This fix works for me, too :>

[nobu (Nobuyoshi Nakada)]

One particular case is OK, two cases would be still OK, but three cases are not exceptional already.
We'll need some standard way to tell if a given object can be discarded immediately or not, I guess.

Anyway, it would be enough for the time being.

[nagachika (Tomoyuki Chikanaga)]

I agree. In fact we don't have to protect all Fibers but only a root Fiber of main Thread.

But can I check-in previous patch for a temporary workaround? The next release of 1.9.3 is almost coming.

[nagachika (Tomoyuki Chikanaga)]

• Tracker changed from Bug to Backport
• Project changed from Ruby master to Backport193
• Category deleted (core)
• Target version deleted (2.0.0)

I've committed at r34629, r34630. Please backport them.

[naruse (Yui NARUSE)]

• Status changed from Open to Closed
• % Done changed from 0 to 100

This issue was solved with changeset r34637.
Eric, thank you for reporting this issue.
Your contribution to Ruby is greatly appreciated.
May Ruby be with you.

---

merge revision(s) 34629,34630:

* cont.c (rb_fiber_reset_root_local_storage): add a new function to
  restore rb_thread_t::local_storage.

* cont.c (rb_obj_is_fiber): add a new function to tell finalizer to
  prevent fibers from destroy.

* gc.c (rb_objspace_call_finalizer): don't sweep fibers at finalizing
  objspace.

* internal.h (rb_fiber_reset_root_local_storage, rb_obj_is_fiber):
  add prototypes.

* vm.c (ruby_vm_destruct): reset main thread's local_storage before
  free main thread. rb_thread_t::local_storage is replaced by fiber's
  local storage when forked from fiber, and it should be already freed
  when the fiber was destroyed.

* test/ruby/test_fiber.rb (test_fork_from_fiber): add test for fork
  from fiber.
  when the fiber was destroyed. [ruby-core:41456] [Bug #5700]