Show chain of references in Ractor errors

[etiennebarrie]

Improve the messages of exceptions raised by the Ractor implementation.

When an object fails to be made shareable with Ractor.make_shareable or when an unshareable object is accessed through module constants or module instance variables, the error message now includes the chain of references that leads to the unshareable value.

[eightbitraptor]

Why are we duplicating the chain_append logic here?

[etiennebarrie]

I don't know what's the best way to share code here. Should I make the function non-static, prefix it somehow and add it to "ractor_core.h"?

[etiennebarrie]

I moved it to an inline function in ractor_core.h.

[eightbitraptor]

The chain approach makes sense to me, but I'm finding the error message a little hard to parse - it's not immediately obvious to me from the message what objects the ivars are attached to, or where the hash values are coming from. Is it worth using rb_inspect or rb_obj_as_string here or is the performance an issue?

[etiennebarrie]

It's not a performance issue, it's more that the output gets really messy really fast.

Even in the simple case of just instance variables, it will cause a huge wall of text with a lot of repetition:

class A; attr_accessor :b; end
class B; attr_accessor :c; end
class C; attr_accessor :d; end
class D; attr_accessor :e; end

a = A.new
a.b = b = B.new
b.c = c = C.new
c.d = d = D.new
d.e = ->{}

Ractor.make_shareable a

../../test.rb:12:in 'Ractor.make_shareable': Proc's self is not shareable: #<Proc:0x0000000103731150 ../../test.rb:10 (lambda)> (Ractor::IsolationError)
  from block self main
  from instance variable @e of #<D:0x0000000103731210 @e=#<Proc:0x0000000103731150 ../../test.rb:10 (lambda)>>
  from instance variable @d of #<C:0x0000000103731300 @d=#<D:0x0000000103731210 @e=#<Proc:0x0000000103731150 ../../test.rb:10 (lambda)>>>
  from instance variable @c of #<B:0x00000001037313f0 @c=#<C:0x0000000103731300 @d=#<D:0x0000000103731210 @e=#<Proc:0x0000000103731150 ../../test.rb:10 (lambda)>>>>
  from instance variable @b of #<A:0x00000001037314b0 @b=#<B:0x00000001037313f0 @c=#<C:0x0000000103731300 @d=#<D:0x0000000103731210 @e=#<Proc:0x0000000103731150 ../../test.rb:10 (lambda)>>>>>
	from ../../test.rb:12:in '<main>'

My first approach always had the object under consideration in addition to the "reference" and it wasn't super readable.

Using this branch I've noticed I usually just need the last line anyway.