Feature #19627: Ensure the VM is alive before accessing objspace in C API

• Status: Closed
• Assignee: -
• Target version: -

[ianks (Ian Ker-Seymer)]

Currently, there is no supported way to check if the Ruby VM has been shut down. There are a couple of workarounds that I know of:

1. Setup an exit handler which sets a global variable (like I did in https://github.com/tmm1/stackprof/pull/200).
2. In Ruby 2.4 thru 3.2, you could check the hidden ruby_vm_global_ptr != NULL.

For my use, in rb-sys there is a feature to automatically report Rust memory allocations via rb_gc_adjust_memory_usage. However, there is a bug that can cause processes to deadlock and/or segfault if a Rust destructor gets called after the VM quits (which can happen in Rust background threads). Currently, I don’t think there’s a way to safely make this feature work. (https://github.com/oxidize-rb/rb-sys/pull/205)

It would be nice to have an official way to know if the Ruby VM is available, basically just a ruby_current_vm_ptr != null. Thoughts?

[mdalessio (Mike Dalessio)]

I reported a related issue in https://bugs.ruby-lang.org/issues/19580 which I fixed with https://github.com/ruby/ruby/pull/7663

Maybe you want to explore a similar fix which will short-circuit functions like rb_gc_adjust_memory_usage instead of forcing callers to figure out if it's safe?

[ianks (Ian Ker-Seymer)]

That’s a good idea. I’ll put a patch up for that.

So far I’ve encountered this issue this issue in rb_gc_adjust_memory_usage and rb_during_gc, so I still think having the ability to check for a Ruby VM is important for writing robust code. At the very least, it would provide an escape hatch in case there are undiscovered edge cases lurking.

[ianks (Ian Ker-Seymer)]

For documentations sake, he's a real world reproduction of this issue that happens when calling TLS destructors. In Ruby < 3.3-dev, I'm able to avoid this by checking the hidden ruby_current_vm_ptr symbol so the code won't segfault after VM shutdown.

-- C level backtrace information -------------------------------------------
/home/runner/.rubies/ruby-head/lib/libruby.so.3.3(rb_print_backtrace+0xd) [0x7f360aab3a3f] /home/runner/work/ruby-dev-builder/ruby-dev-builder/vm_dump.c:785
/home/runner/.rubies/ruby-head/lib/libruby.so.3.3(rb_vm_bugreport) /home/runner/work/ruby-dev-builder/ruby-dev-builder/vm_dump.c:1101
/home/runner/.rubies/ruby-head/lib/libruby.so.3.3(rb_bug_for_fatal_signal+0xf4) [0x7f360a8b8c94] /home/runner/work/ruby-dev-builder/ruby-dev-builder/error.c:814
/home/runner/.rubies/ruby-head/lib/libruby.so.3.3(sigsegv+0x4d) [0x7f360a9feecd] /home/runner/work/ruby-dev-builder/ruby-dev-builder/signal.c:964
/lib/x86_64-linux-gnu/libc.so.6(0x7f360a413520) [0x7f360a413520]
/home/runner/.rubies/ruby-head/lib/libruby.so.3.3(rb_gc_adjust_memory_usage+0xe) [0x7f360a8dedbe] /home/runner/work/ruby-dev-builder/ruby-dev-builder/gc.c:12743
/lib/x86_64-linux-gnu/libc.so.6(__call_tls_dtors+0x3f) [0x7f360a416d9f]
/lib/x86_64-linux-gnu/libc.so.6(0x7f360a4165c9) [0x7f360a4165c9]
/lib/x86_64-linux-gnu/libc.so.6(on_exit+0x20) [0x7f360a416610]
/lib/x86_64-linux-gnu/libc.so.6(0x7f360a3fad97) [0x7f360a3fad97]
/lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0x80) [0x7f360a3fae40]
[0x55e16aa391d5]

[ianks (Ian Ker-Seymer)]

• Status changed from Open to Closed

Applied in changeset git|2f9f44f077a53b14aa1fbd43111955251750d31f.

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Ensure the VM is alive before accessing objspace in C API (Feature #19627)

[Feature #19627]

[byroot (Jean Boussier)]

• Subject changed from Add a way to detect if the Ruby VM is running in C API to Ensure the VM is alive before accessing objspace in C API