Wednesday, April 25, 2007

What Would I (Will I?) Change About Ruby

The latest Ruby Blogging Contest hits close to home: What changes would make Ruby into a better language without making it into something that isn't Ruby?

As you might guess, I've got some pretty strong thoughts here. I'm not a heavy Rails user, and I'm not as heavy a Ruby user as I'd like to be. But implementing a Ruby interpreter and now compiler has taught me a few things about what's right and what's wrong with Ruby. I'm not going to complain about performance, whine that the C code is too hard to follow, or even attack C-based extensions. Those may be important issues, but they're all fixable in the long term without breaking anything that works today (or by providing reasonable substitutes). I'm also not going to go into language design ideas...I have mine, you have yours, Matz has his. But my money's on Matz to do the "right thing" with regards to actual language design.

What I'm talking about are a few really important changes to the Ruby runtime, libraries, and ecosystem. Take these as my educated opinions...and don't think too hard about whether I'll be working to change these things in JRuby and in the wider Ruby world.

1. Threading

This more than any other area probably means the most visible changes to Ruby. Ruby currently is green-threaded, as most of you know. JRuby implements native threads mainly because Java uses native threads...we just piggyback off the excellent work of the JVM engineers. And the developing Ruby 1.9, the future successor to the current version 1.8 C implementation, provides something in the middle: native threads with a giant lock, so threads won't run concurrently.

So in general, Ruby is trending toward support for native threads. But there's a problem...some of Ruby's current APIs are impossible to do safely with native threads (and in general, impossible to really do safely with green threads...Ruby just does them anyway). Threading needs to be improved, with support for concurrent execution and removal of operations that prevent that.

Specifically, the following operations and features are inherently unsafe, and are not supported by any mature threaded system:


Killing one thread from another may leave its locks and resources in an unpredictable state. JRuby currently implements this by setting a kill flag on the target thread and waiting for it to die--basically asking the thread to "please die yourself"--but it's not deterministic and the thread could fail to die.


Forcing another thread to raise an exception can have the same effect as kill, since the thread may not expect to handle the given exception and may not be able to release locks or tidy up resources. JRuby handles this similar to kill, by setting a field to contain the exception a target thread should "please raise", but again it's not deterministic and there's no way to guarantee the target thread will raise.


There is no way to deterministically force true concurrent threads to stop and wait for the the current thread, not to mention the horrendous race conditions that can result when locks are involved. As a result of the many critical problems with critical=, it is already slated to be removed in Ruby 1.9/2.0.
In order for Ruby to survive in a parallel-processing era, unsafe threading operations need to go, and any libraries or apps that depend on them need to find new ways to solve these problems. Sorry folks, these aren't my rules. I understand why people like these features...I like them too. But you can't have your concurrency and eat it too.

2. ObjectSpace

ObjectSpace is Ruby's view into the garbage-collected heap. You can use it to iterate over all objects of a particular type, attach finalizers to any object, look an object up by its object ID, and so on. In Ruby, it's a pretty low-cost heap-walker, able to dig up objects matching particular criteria for you on a whim. It sounds like it might be pretty useful, but it's used by very few libraries...and most of those uses can be implemented in other (potentially more efficient) ways.

JRuby implements ObjectSpace by keeping a separate linked list in memory of weak references to created objects. This means that for every ObjectSpace-aware object that's created, a weakref is added to this list. When the object is collected, the weakref removes itself from the list. Walking all objects of a particular type just involves walking that list. Reconstituting an object ID into the object it references is supported by a separate weak list (again, more memory overhead).

There are no plans currently for ObjectSpace to be removed from Ruby in a future version. But there's a addition to being pure overhead in JRuby (which you can turn off completely by using the -O flag), ObjectSpace limits evolving development of the Ruby garbage collector, breaks heap and memory transparency, and poses yet more problems for threading.

There are many issues here. First off, the JRuby thing. By having to add ObjectSpace governors for all objects in the system, JRuby pays a very large penalty. We're forced to do this because the JVM (and most other advanced garbage-collecting VMs) does not allow you to traverse in-memory objects nor retrieve the object that is associated with a given ID. In general this is because the JVM does all sorts of wonderful and magical things with objects and memory behind the scenes, and the ability to ask for all objects of a given type or pull an object based on some ID number at any time cripples many of these tricks.

The threading issues are perhaps more important. Imagine if you will a true concurrent VM, with many threads creating objects, maybe one or more threads collecting garbage, and synchronizing all this to guarantee the integrity and efficiency of the heap and garbage collector. There is absolutely no room in this scenario for those multiple threads to request lists of specifically-typed objects at any time, nor to provide an ID and expect its object to be presented to you. These features break encapsulation across threads, they violate security restrictions from thread to thread, and they require whole new levels of locking to ensure that while reading from the heap no other thread produces new objects and no garbage collection occurs. As a result, ObjectSpace harms Ruby by limiting the flexibility of its garbage collecting and threading subsystems, and should be eliminated.

3. $SAFE and tainting

Safe levels are a fairly simple, straightforward way to set a "security level" that governs what operations are possible for a given thread. By setting the safe level to various values, you can limit modification of Object, prevent IO, disallow creation of new methods or classes, and so on. Added to this is the ability to "taint" or "untaint" objects. Tainted objects are considered "unsafe", and so certain security levels will cause errors to be thrown when those objects are passed to safe-only operations.

JRuby has safe level and tainting checks in place, but it's almost assured they're not working correctly. We have never tested them, largely because practically no tests (or perhaps literally no tests) use safe levels or tainting, and we've had *exactly one* bug report relating to safe levels, just a couple weeks ago. And to further kill the possibility of JRuby ever supporting safe levels and tainting correctly, my work tonight to fix some safe level issues revealed that doing so would add a tremendous amount of overhead to almost all critical operations like method creation, module/class mutation, and worst of all, object creation.

At this point, safe levels will probably remain in their current half-implemented state for 1.0, but I think it's almost decided for us that safe levels and tainting will simply not be supported in JRuby. In their place, we'll do two things (which I'd recommend the C implementation consider as well:
  • Recommend that people who really want "safe" environments use an approach like whytheluckystiff's Sandbox, which takes a more JVM-like approach to safety: it runs code in a true sandboxed sub-runtime with only "safe" operations even defined. In other words, not only is it disallowed to load in files or hit the network, it's physically *impossible* to do so. What makes this even better is that Sandbox is already supported in JRuby (gem "javasand") and JRuby out of the box allows a fine granularity of operations to be disabled in new runtimes.
  • Implement safe levels like Java handles security restrictions, which we get to leverage since they're already being checked and enforced at the JVM level. We will not be able to map everything...for obvious reasons, checking tainted strings all the time or limiting class and method creation are unlikely to ever happen, but we can limit those operations that the JVM allows us to limit, like loading remote code, opening sockets, accessing local files, and so on. So it's highly likely JRuby's implementation of safe levels will map to clearly-defined sets of Java security restrictions in the near future.
4. Direction

Ruby is a very free-form community. Matz is the most benevolent dictator I've had the pleasure to work with, and most of the community are true free-thinking artists. It's like the hippie commune of the language world. Peace out, man.

But there's a problem here. Ruby needs guidance beyond VM and language design or the loose meanderings of its more vocal community members. It boils down to a few simple points:
  • Ruby needs a spec. Anyone who believes this isn't true isn't paying attention. Now I'm not talking about a gold-standard legal document signed in blood by Matz and the chief stakeholders of the Ruby community. An officially sponsored, widely supported, and massively publicized community spec would work fine--and probably fit the community and the language better. But something needs to done quickly, since Ruby's "bus number" is dangerously low. A spec is not something to be's a guarantee that Ruby will live on into the future, that alternative implementations (like JRuby) can't intentionally introduce nasty incompatibilities (or at least, that they'd be easy to discover and easy to document), and perhaps most importantly...that the full glory and beauty of Ruby is published forever for all to see and explore, rather than dangerously trapped in very few minds.
  • Ruby needs a non-profit governing body. I'm not necessarily talking about a council of elders here, I'm just talking about some legal entity to which OSS copyrights can be assigned, donations can be made, and from which projects and initiatives can be funded. Maybe this would be RubyCentral, maybe this would be some other (new) organization...I don't know that. But it would be a great help to the community and Ruby's future if there were some official organization that could act as caretaker for Ruby's future. I'm all set to sign over any JRuby copyrights I have to such an organization, to protect the future of Ruby on the JVM just like the future of the C implementation. How about you?
  • Ruby needs you. Granted, this isn't really a change as such. You probably wouldn't be reading this if Ruby didn't already have you. But the Ruby community is at a big point in its risk of losing its identity, being eclipsed by newer projects, or even slipping deep, deep into the trough of disillusionment. What will prevent that happening is the community showing its strong ties, coming together to support official organizations and official documents, and above all, continuing to pour all our hearts into creating newer and better applications and libraries in Ruby, pushing the boundaries of what people think is possible.