[Libguestfs] Re: [PATCH nbdkit] Remove nbdkit-ruby-plugin and Ruby language support completely

There is an alternative to removing Ruby, but I don't think it's very attractive to us. However here it is for your information ... We could run every Ruby API call from a single thread. We would start a background thread (when exactly to do this is complex - more below). The background thread would be the only one that uses RUBY_INIT_STACK. nbdkit would make plugin requests through its threads, and these would be enqueued and processed serially by the background thread. eg. The .pread method would enqueue a 'RUBY_READ' message which the background thread would pick up and process by calling into the Ruby plugin using Ruby APIs. The result would be sent back to the nbdkit thread which was doing the .pread, which would return the result. (We use a similar technique for different reasons in VDDK and curl plugins.) Since all Ruby APIs are called on a single thread, presumably the Ruby GC would only ever run on this same thread, and the stack problem would not occur (even though this is not the nbdkit main thread). I've not actually tested this, but it seems a reasonable assumption. We would probably want to create the background thread early, not least because plugins usually want to handle .config and other early methods. But if we create the background thread in .load, then it would be deleted when we fork (which is the purpose of the .after_fork method - to create background threads after nbdkit has forked). If we create the background thread in .after_fork, then it cannot handle .config etc, unless those were queued up, but then error handling is difficult. Creating a background thread + Ruby interpreter in .load and then creating a second background thread + Ruby interpreter in .after_fork would requiring copying interpreter state from one thread to another somehow. We could also ban forking when using the Ruby plugin, but that disables some important features such as '--run'. As you can see, when you get into the details this is not a very attractive option. Therefore I think deleting the Ruby plugin is the best plan of action for us. If people really want to write nbdkit plugins in Ruby, my suggestion would be to use nbdkit-sh-plugin, and perhaps some sort of "FastCGI"-type mechanism to ensure that the Ruby interpreter is running in a separate process entirely from nbdkit. Rich. -- Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones Read my programming and virtualization blog: http://rwmj.wordpress.com virt-builder quickly builds VMs from scratch http://libguestfs.org/virt-builder.1.html