When defining singleton structs, don't reallocate the ->instance field (#37598)

Otherwise, we end up with two singletons for the type, which
confuses many different parts of the runtime system. This
happens quite frequently when using Revise, which likes to
redefine types.

Author: Keno

src/datatype.c

@@ -224,8 +224,11 @@ STATIC_INLINE int jl_is_datatype_make_singleton(jl_datatype_t *d)
 STATIC_INLINE void jl_maybe_allocate_singleton_instance(jl_datatype_t *st)
 {
     if (jl_is_datatype_make_singleton(st)) {
-        st->instance = jl_gc_alloc(jl_get_ptls_states(), 0, st);
-        jl_gc_wb(st, st->instance);
+        // It's possible for st to already have an ->instance if it was redefined
+        if (!st->instance) {
+            st->instance = jl_gc_alloc(jl_get_ptls_states(), 0, st);
+            jl_gc_wb(st, st->instance);
+        }
     }
 }
 

test/core.jl

@@ -7397,6 +7397,14 @@ let v = NullableHomogeneousPointerImmutable(),
     @test r[] == 3
 end
 
+struct RedefinedSingleton
+end
+redefined_singleton_ref = Ref{Any}(RedefinedSingleton())
+struct RedefinedSingleton
+end
+cmp_refs(a::Ref{Any}) = a[] === RedefinedSingleton()
+@test cmp_refs(redefined_singleton_ref)
+
 struct PointerNopadding{T}
     a::Symbol
     b::T