New Zygote context in every call to AD.pullback_function (#77)

* New Zygote context in every call to `AD.pullback_function`

* Make fix more modular

Project.toml

@@ -1,7 +1,7 @@
 name = "AbstractDifferentiation"
 uuid = "c29ec348-61ec-40c8-8164-b8c60e9d9f3d"
 authors = ["Mohamed Tarek <mohamed82008@gmail.com> and contributors"]
-version = "0.5.0"
+version = "0.5.1"
 
 [deps]
 ChainRulesCore = "d360d2e6-b24c-11e9-a2a3-2a2ae2dbcce4"

ext/AbstractDifferentiationChainRulesCoreExt.jl

@@ -4,7 +4,7 @@ import AbstractDifferentiation as AD
 using ChainRulesCore: ChainRulesCore
 
 AD.@primitive function pullback_function(ba::AD.ReverseRuleConfigBackend, f, xs...)
-    _, back = ChainRulesCore.rrule_via_ad(ba.ruleconfig, f, xs...)
+    _, back = ChainRulesCore.rrule_via_ad(AD.ruleconfig(ba), f, xs...)
     pullback(vs) = Base.tail(back(vs))
     pullback(vs::Tuple{Any}) = Base.tail(back(first(vs)))
     return pullback

ext/AbstractDifferentiationZygoteExt.jl

@@ -9,4 +9,9 @@ end
 
 AD.ZygoteBackend() = AD.ReverseRuleConfigBackend(Zygote.ZygoteRuleConfig())
 
+# Context should not persist between different AD calls: fixes #69
+function AD.ruleconfig(::AD.ReverseRuleConfigBackend{<:Zygote.ZygoteRuleConfig})
+    return Zygote.ZygoteRuleConfig()
+end
+
 end # module

src/backends.jl

@@ -61,6 +61,11 @@ struct ReverseRuleConfigBackend{RC} <: AbstractReverseMode
     ruleconfig::RC
 end
 
+# internal function for extracting the rule config
+# falls back to returning the wrapped `ruleconfig` but can be specialized
+# e.g., for Zygote to fix #69
+ruleconfig(ba::ReverseRuleConfigBackend) = ba.ruleconfig
+
 """
     ZygoteBackend()
 

test/ruleconfig.jl

@@ -30,4 +30,26 @@ using Zygote
             test_lazy_jacobians(backend)
         end
     end
+
+    # issue #69
+    @testset "Zygote context" begin
+        ad = AD.ZygoteBackend()
+
+        # example in #69: context is not mutated
+        @test ad.ruleconfig.context.cache === nothing
+        @test AD.derivative(ad, exp, 1.0) === (exp(1.0),)
+        @test ad.ruleconfig.context.cache === nothing
+        @test AD.derivative(ad, exp, 1.0) === (exp(1.0),)
+        @test ad.ruleconfig.context.cache === nothing
+
+        # Jacobian computation still works
+        # https://github.com/JuliaDiff/AbstractDifferentiation.jl/pull/70#issuecomment-1449481724
+        function f(x, a)
+           r = Ref(x)
+           r[] = r[] + r[]
+           r[] = r[] * a
+           r[]
+        end
+        @test AD.jacobian(ad, f, [1, 2, 3], 3) == ([6.0 0.0 0.0; 0.0 6.0 0.0; 0.0 0.0 6.0], [2.0, 4.0, 6.0])
+    end
 end