🛡️ Expose is_deep_immutable for lib devs checks.

Author: iago-lito

src/Networks/Networks.jl

@@ -79,6 +79,12 @@ to the closures used in the API, as it would collapse its logic.
 Julia unfortunately has no straightforward mechanism
 to automatically enforce that it cannot happen.
 
+In particular,
+using views to access individual pieces of data *can* leak references.
+Only allow it if the data is "deeply" immutable in the sense
+of the exposed `is_deep_immutable(::Type)` method.
+Don't produce views into mutable data that could otherwise break the COW-pattern.
+
 [COW]: https://en.wikipedia.org/wiki/Copy-on-write
 [RC]: https://en.wikipedia.org/wiki/Reference_counting
 [Boxing]: https://en.wikipedia.org/wiki/Boxing_(computer_programming)

src/Networks/views.jl

@@ -42,6 +42,28 @@ Base.setproperty!(::View, ::Symbol, _) = throw("View fields are private.")
 Base.deepcopy(::View) = throw("Deepcopying the view would break its logic.")
 Base.copy(v::View) = v # There is no use in a copy.
 
+"""
+True if a reference leaked from a view cannot be used to break the COW-pattern.
+This is not checked on construction of every view,
+and Networks users are responsible to only emit views for these types.
+"""
+function is_deep_immutable(T::Type)
+    # Special-case these: julia marks them as 'mutable'
+    # but user can't do anything to mutate them, right?
+    T isa String && return true
+    T isa Symbol && return true
+    if T isa Union
+        (; a, b) = T
+        is_deep_immutable(a) && is_deep_immutable(b)
+    elseif T isa DataType
+        ismutabletype(T) && return false
+        all(I.map(is_deep_immutable, fieldtypes(T)))
+    else
+        throw("Deep mutability analysis unimplemented for $T.")
+    end
+end
+export is_deep_immutable
+
 # Forward to underlying entry.
 n_networks(v::View) = n_networks(entry(v))
 

src/views.jl

@@ -39,7 +39,7 @@ struct NodesView{T}
 end
 S = NodesView # "Self"
 Base.length(v::S) = length(view(v))
-Base.getindex(v::S, i) = getindex(view(v), i) # WARN: leak if entry is mutable?
+Base.getindex(v::S, i) = getindex(view(v), i)
 Base.setindex!(v::S, x, i) = setindex!(view(v), x, i)
 nodes_view(m::Model, class::Symbol, data::Symbol) =
     NodesView(m, N.nodes_view(m._value, class, data), data)