Mark Ox::sax_parse as Ractor-safe and add a Ractor-based Ox::Sax example.

[okeeblow]

For #277

I've been experimenting with Ractors since upgrading to Ruby 3.0 for #275 but quickly ran into Ractor::UnsafeError when trying to call Ox::sax_parse in anything except the main Ractor.

Per Ruby's Ractor C extension documentation (link below):
"By default, all C extensions are recognized as Ractor-unsafe. If C extension becomes Ractor-safe,
the extension should call rb_ext_ractor_safe(true) at the Init_ function and all defined method marked as Ractor-safe. Ractor-unsafe C-methods only been called from main-ractor. If non-main ractor calls it, then Ractor::UnsafeError is raised."

I don't like to open seemingly-large feature requests like this without making some attempt at it myself first,
and luckily it seems like Ox::sax_parse Just Works™ since I marked it Ractor-safe, even with the class_cache. Confirming this safety, making any remaining changes to Ox::Sax, and expanding this to the non-Sax parts of Ox are all unfortunately out of my depth as a n00b C coder, so I would appreciate if you could take this over if it interests you. I am happy with just Sax support since I have no current need for marshalling, but I imagine other Ox users wouldn't be satisfied if stratified.

In this commit:

• Enable rb_ext_ractor_safe preprocessor macro via have_func in extconf.rb.
• Mark Init_Ox and ox_sax_parse as Ractor -safe.
• Add a new Ractor-based Ox::Sax example exercising both parallel and serial Ox::Sax handler Ractors to parse data from shared-mime-info XML files many users likely already have on their systems.

Official Ractor info:

• "Ractor: a proposal for a new concurrent abstraction without thread-safety issues": https://bugs.ruby-lang.org/issues/17100 (Ractor: a proposal for new concurrent abstraction without thread-safety issues ruby/ruby#3365)
• Ruby's official Ractor documentation: https://docs.ruby-lang.org/en/master/doc/ractor_md.html
• "A way to mark C extensions as thread-safe, Ractor-safe, or unsafe": https://bugs.ruby-lang.org/issues/17307 (rb_ext_ractor_safe() to declare ractor-safe ext ruby/ruby#3824)
• Ruby's C Extension Ractor documention covering rb_ext_ractor_safe: https://docs.ruby-lang.org/en/master/doc/extension_rdoc.html#label-Appendix+F.+Ractor+support
• A Ractor C Extension from the creator of Ractor that might serve as a useful example: https://github.com/ko1/ractor-tvar

Blogs:

• "Ractors: Multi-Core Parallel Processing Comes to Ruby 3": https://www.ruby3.dev/ruby-3-fundamentals/2021/01/27/ractors-multi-core-parallel-processing-in-ruby-3/
• "Ruby Ractor Experiments: Safe async communication" :https://ivoanjo.me/blog/2021/02/14/ractor-experiments-safe-async/
• "Playing with Ruby Ractors": https://billy-ruffian.co.uk/playing-with-ruby-ractors/
• "How Fast are Ractors?": https://www.fastruby.io/blog/ruby/performance/how-fast-are-ractors.html (https://github.com/noahgibbs/ractor_basic_benchmarks/tree/main/benchmarks)

Before this change:

[okeeblow@emi#CHECKING-YOU-OUT] time ./bin/checking-you-out ~/224031-dot-jpg
Received /home/okeeblow/.local/share/mime/packages/user-extension-rsrc.xml
/home/okeeblow/Works/DistorteD/CHECKING-YOU-OUT/lib/checking-you-out/ghost_revival/mr_mime.rb:375:in `sax_parse': ractor unsafe method called from not main ractor (Ractor::UnsafeError)
        from /home/okeeblow/Works/DistorteD/CHECKING-YOU-OUT/lib/checking-you-out/ghost_revival/mr_mime.rb:375:in `block in open'
        from /home/okeeblow/Works/DistorteD/CHECKING-YOU-OUT/lib/checking-you-out/ghost_revival/mr_mime.rb:331:in `open'
        from /home/okeeblow/Works/DistorteD/CHECKING-YOU-OUT/lib/checking-you-out/ghost_revival/mr_mime.rb:331:in `open'
        from /home/okeeblow/Works/DistorteD/CHECKING-YOU-OUT/lib/checking-you-out/ghost_revival.rb:24:in `block (2 levels) in remember_me'
        from /home/okeeblow/Works/DistorteD/CHECKING-YOU-OUT/lib/checking-you-out/ghost_revival.rb:19:in `loop'
        from /home/okeeblow/Works/DistorteD/CHECKING-YOU-OUT/lib/checking-you-out/ghost_revival.rb:19:in `block in remember_me'
<internal:ractor>:583:in `send': The incoming-port is already closed (Ractor::ClosedError)
        from /home/okeeblow/Works/DistorteD/CHECKING-YOU-OUT/lib/checking-you-out/ghost_revival.rb:86:in `block in extended'
        from /home/okeeblow/Works/DistorteD/CHECKING-YOU-OUT/lib/checking-you-out/ghost_revival.rb:63:in `each'
        from /home/okeeblow/Works/DistorteD/CHECKING-YOU-OUT/lib/checking-you-out/ghost_revival.rb:63:in `each_with_object'
        from /home/okeeblow/Works/DistorteD/CHECKING-YOU-OUT/lib/checking-you-out/ghost_revival.rb:63:in `extended'
        from /home/okeeblow/Works/DistorteD/CHECKING-YOU-OUT/lib/checking-you-out/inner_spirit.rb:216:in `extend'
        from /home/okeeblow/Works/DistorteD/CHECKING-YOU-OUT/lib/checking-you-out/inner_spirit.rb:216:in `<top (required)>'
        from /home/okeeblow/Works/DistorteD/CHECKING-YOU-OUT/lib/checking-you-out.rb:10:in `require_relative'
        from /home/okeeblow/Works/DistorteD/CHECKING-YOU-OUT/lib/checking-you-out.rb:10:in `<top (required)>'
        from ./bin/checking-you-out:3:in `require_relative'
        from ./bin/checking-you-out:3:in `<main>'
./bin/checking-you-out ~/224031-dot-jpg  0.12s user 0.05s system 100% cpu 0.168 total

My new Ox::Sax Ractor example script's usage:

[okeeblow@emi#ox] ./examples/sax_ractor.rb
Please provide the path to a `shared-mime-info` XML package and some media-type query arguments (e.g. 'image/jpeg')

[okeeblow@emi#ox] ./examples/sax_ractor.rb /usr/share/mime/packages/freedesktop.org.xml
Please provide some media-type query arguments (e.g. 'image/jpeg')

Finding all-extant types:

[okeeblow@emi#ox] ./examples/sax_ractor.rb /usr/share/mime/packages/freedesktop.org.xml image/jpeg font/ttf application/xhtml+xml image/x-pict
Parallel Ractors
["Worker 0 gave us JPEG 影像 (image/jpeg) [.jpeg,.jpg,.jpe]",
 "Worker 1 gave us TrueType 字型 (font/ttf) [.ttf]",
 "Worker 2 gave us XHTML 網頁 (application/xhtml+xml) [.xhtml,.xht]",
 "Worker 3 gave us Macintosh Quickdraw/PICT 繪圖 (image/x-pict) [.pct,.pict,.pict1,.pict2]"]

Serial Ractor
"ONLY ONE OX gave us [#<CYO JPEG 影像 (image/jpeg) [.jpeg,.jpg,.jpe]>, #<CYO TrueType 字型 (font/ttf) [.ttf]>, #<CYO XHTML 網頁 (application/xhtml+xml) [.xhtml,.xht]>, #<CYO Macintosh Quickdraw/PICT 繪圖 (image/x-pict) [.pct,.pict,.pict1,.pict2]>]"

…and not finding invalid ones:

[okeeblow@emi#ox] ./examples/sax_ractor.rb /usr/share/mime/packages/freedesktop.org.xml lol/rofl fart/butt image/jpeg
Parallel Ractors
["Worker 0 gave us nothing",
 "Worker 1 gave us nothing",
 "Worker 2 gave us JPEG 影像 (image/jpeg) [.jpeg,.jpg,.jpe]"]

Serial Ractor
"ONLY ONE OX gave us [nil, nil, #<CYO JPEG 影像 (image/jpeg) [.jpeg,.jpg,.jpe]>]"

Unit tests pass.

[ohler55]

Let me do a little reading on actor before I merge this.

[ohler55]

After a quick scan of Ox code it looks like the only part that needs some work to be ractor (thread) safe is the cache code. Adding a mutex to that should take care of that though.

From the ractor information it sounds like rb_ext_ractor_safe need to be called in Init_ox() as you have done and every other method function. Do you know how much overhead there is in the rb_ext_ractor_safe() call? If it is expensive is there a way to check to see if the code is running in a ractor?

[okeeblow]

rb_ext_ractor_safe doesn't appear to be expensive at all, just setting a flag as a hoop to jump through to make sure extension authors have tested it. Here's the relevant part of the commit that added the function showing it just looks for the flag and bails out if unset: ruby/ruby@a58c322#diff-f8c174347e6ea8889b5036064a1ff4fe5e7c53a821befa9bdc5ccbf17800a649R2472-L2472

[ohler55]

The check internal to ruby is fine. I was wondering more about the calling of rb_ext_ractor_safe() itself. It does need to find the ractor for the current thread and set the flag. I guess the only way to find out is to run some benchmarks.

I'm inclined to merge this MR and then make a second pass to finish it before doing a release. Let me look more carefully this evening.

[okeeblow]

That check is in the part I linked, not in rb_ext_ractor_safe itself, which just sets the flag: ruby/ruby@a58c322#diff-cb1f5822bd6dce7e2bc5e5ae0927004df17afe0034ed9a69ab34c50beed12034R1016

[ohler55]

Yes, I got that. I was not concerned about that as there is nothing I can do about that. My concern was over whether rb_ext_ractor_safe() was expensive to call or not. If it is I will have Ox keep track of whether or not a check should be made before calling it.

[okeeblow]

Ah I get what you mean. I don't have enough experience with threads in Ruby to have a sense for how expensive GET_THREAD() is in any other context v(._. )v

[ohler55]

I'll try some benchmarks. Might take a day or two though.

[okeeblow]

Take all the time you like. Thanks for your attention to these Ruby 3 features :)

[ohler55]

Are you part of the Ruby support team?

[okeeblow]

Nope, just a happy Ox user with some specific goals for my Ox-using library

[ohler55]

A data point for you. When parsing with the new parse on Oj placing a mutex around the cache (string cache so it's used more) takes the performance for 68% of the old to 81% so a significant drop in performance. I think there will need to be a check against the cache option to determine if the code is actually ractor_safe.

[okeeblow]

I see. I have a lot to learn in that regard so I appreciate your patience with my amateur C changes :)

[ohler55]

We are all learning. You've taught me a bit about Ractor. Sharing what we knew help us all.