Add requires_threadsafe (#353)

See related discussion: TuringLang/Turing.jl#1726 (comment).

Author: torfjelde

Project.toml

@@ -1,6 +1,6 @@
 name = "DynamicPPL"
 uuid = "366bfd00-2699-11ea-058f-f148b4cae6d8"
-version = "0.17.1"
+version = "0.17.2"
 
 [deps]
 AbstractMCMC = "80f14c24-f653-4e6a-9b94-39d6b0f70001"

src/model.jl

@@ -376,6 +376,16 @@ number of `sampler`.
 """
 (model::Model)(args...) = first(evaluate!!(model, args...))
 
+"""
+    use_threadsafe_eval(context::AbstractContext, varinfo::AbstractVarInfo)
+
+Return `true` if evaluation of a model using `context` and `varinfo` should
+wrap `varinfo` in `ThreadSafeVarInfo`, i.e. threadsafe evaluation, and `false` otherwise.
+"""
+function use_threadsafe_eval(context::AbstractContext, varinfo::AbstractVarInfo)
+    return Threads.nthreads() > 1
+end
+
 """
     evaluate!!(model::Model[, rng, varinfo, sampler, context])
 
@@ -388,10 +398,10 @@ The method resets the log joint probability of `varinfo` and increases the evalu
 number of `sampler`.
 """
 function evaluate!!(model::Model, varinfo::AbstractVarInfo, context::AbstractContext)
-    if Threads.nthreads() == 1
-        return evaluate_threadunsafe!!(model, varinfo, context)
+    return if use_threadsafe_eval(context, varinfo)
+        evaluate_threadsafe!!(model, varinfo, context)
     else
-        return evaluate_threadsafe!!(model, varinfo, context)
+        evaluate_threadunsafe!!(model, varinfo, context)
     end
 end