ProbProg: Making trace an operand

[sbrantq]

No description provided.

[sbrantq]

@wsmoses So I added GC.@preservings to fix the deadlock issue with the ConcreteRArray that specifies rng.seed (for context GC triggered by EnzymeJaXMapSymbol'd functions somehow tries to free only(rng.seed.data).buffer):

Reactant.jl/src/probprog/Modeling.jl

Lines 283 to 285 in 9bd1dee

	seed_buffer = only(rng.seed.data).buffer
	GC.@preserve seed_buffer constraint begin
	trace, _, _ = compiled_fn(rng, constraint_ptr, args...)

Also GC triggered by mapped functions in src/probprog/FFI.jl can cause jl_gc_wait_for_the_world to wait forever for the main Julia thread which is waiting during Base.convert to Julia Array, like this one

Reactant.jl/src/probprog/Modeling.jl

Line 292 in 9bd1dee

trace = unsafe_pointer_to_objref(Ptr{Any}(Array(trace)[1]))

So I added

Reactant.jl/src/probprog/Modeling.jl

Lines 287 to 289 in 9bd1dee

	while !isready(trace)
	yield()
	end

(Adding trivial Base.isready for ConcretePJRTArray and ConcreteIFRTArray).

Does that sound like an ok-ish fix, or am i missing something?

[codecov]

Codecov Report

❌ Patch coverage is 4.17537% with 459 lines in your changes missing coverage. Please review.
✅ Project coverage is 41.72%. Comparing base (5d0ba5a) to head (20b9b9c).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/probprog/Modeling.jl	0.00%	163 Missing ⚠️
src/probprog/FFI.jl	10.06%	143 Missing ⚠️
src/probprog/Display.jl	0.00%	58 Missing ⚠️
src/probprog/Types.jl	0.00%	44 Missing ⚠️
src/probprog/Inference.jl	0.00%	35 Missing ⚠️
src/Compiler.jl	17.64%	14 Missing ⚠️
src/Types.jl	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1444      +/-   ##
==========================================
- Coverage   42.69%   41.72%   -0.98%     
==========================================
  Files         123      129       +6     
  Lines       21807    22286     +479     
==========================================
- Hits         9310     9298      -12     
- Misses      12497    12988     +491     

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