On Wed, Apr 17, 2024 at 12:42:42PM +0100, Daniel P. Berrangé wrote:

On Tue, Apr 16, 2024 at 10:22:32PM +0100, Richard W.M. Jones wrote:

...

This has not worked for a long time because of a fundamental bug in how Ruby's garbage collector interacts with pthread stacks. There has been a bug open for a long time (over 14 years) which has never been fixed:

https://redmine.ruby-lang.org/issues/2294#note-34

As that discussion is rather hard to follow, the basic problem is this:

When you embed Ruby you must tell the interpreter where "the stack" is. You do this by calling RUBY_INIT_STACK early on (which we do in load(), on the main thread). This marks the top of stack.

From time to time Ruby's garbage collector will scan the stack starting at the current stack pointer, and going up til it reaches the previously marked top of stack. It scans this memory looking for roots etc.

This strategy fails completely if you have threads. In a thread (using a different stack entirely) it scans randomly across the heap, resulting in weird behaviour at best, but more usually straight crashes.

Urgh, horrible.

...

We've had a Ruby plugin for a long time, but it broke with Ruby 1.9, when Ruby changed from some kind of cooperative threading to using native threads (and therefore separate stacks).

The tests have been disabled since early 2018 (nbdkit < 1.2). I re-enabled the tests just now (Ruby 3.3) but it's broken in the same way as ever.

The only changes that have happened since the tests were disabled are mechanical ones / project-wide cleanups.

It currently crashes in open().

It seems highly unlikely that anyone is using this plugin.

IIUC, the '--threads' arg is just a per-client limit. So if there are multiple clients, you'll still get many threads using ruby even with "--threads = 1".

Yes, and in fact you'll get an extra thread even with one client. Using a background thread outside of nbdkit's control and funnelling requests through that (see my follow up) seems to be the only way.

If spawning nbdkit with --single though, you'll get a separate process per client, and then with --threads=1, you could can sure that only 1 thread is calling into ruby.

True, although --single is of very limited use. Basically it forces you to use xinetd or systemd or libnbd's nbd_connect_command.

Is there a way a plugin can declare that it is non-threadsafe, and thus allow nbdkit to refuse to run it without --single and --threads=1, being set ?

It's possible, but the use case would be so limiting that I'm not sure if the cure is better than the disease.

I guess that will complicate your test suite though unless you can skip all tests which rely on multi-connections and threads.

Rich.

With regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|

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