UndefRefError: access to undefined reference

[SamuelBadr]

Starting in 0.5.2, this errors:

julia> using JLD2

julia> struct Foo <: Function
           x::Int
       end

julia> foo = Foo(2)
(::Foo) (generic function with 0 methods)

julia> jldsave(tempname(); foo)
ERROR: UndefRefError: access to undefined reference
Stacktrace:
  [1] getproperty
    @ ./Base.jl:43 [inlined]
  [2] h5convert!(out::JLD2.IndirectPointer, ::JLD2.OnDiskRepresentation{…}, f::JLD2.JLDFile{…}, T::DataType, wsession::JLD2.JLDWriteSession{…})
    @ JLD2 ~/.julia/packages/JLD2/JyzIz/src/data/writing_datatypes.jl:448
  [3] write_data(io::JLD2.MmapIO, f::JLD2.JLDFile{…}, data::Type, odr::JLD2.OnDiskRepresentation{…}, ::JLD2.HasReferences, wsession::JLD2.JLDWriteSession{…})
    @ JLD2 ~/.julia/packages/JLD2/JyzIz/src/dataio.jl:96
  [4] write_attribute(io::JLD2.MmapIO, f::JLD2.JLDFile{…}, attr::JLD2.WrittenAttribute{…}, wsession::JLD2.JLDWriteSession{…})
    @ JLD2 ~/.julia/packages/JLD2/JyzIz/src/attributes.jl:47
  [5] write_header_message(io::JLD2.MmapIO, f::JLD2.JLDFile{…}, msg::JLD2.WrittenAttribute{…}, wsession::JLD2.JLDWriteSession{…})
    @ JLD2 ~/.julia/packages/JLD2/JyzIz/src/explicit_datasets.jl:233
  [6] write_header_message
    @ ~/.julia/packages/JLD2/JyzIz/src/explicit_datasets.jl:232 [inlined]
  [7] commit(f::JLD2.JLDFile{JLD2.MmapIO}, dt::JLD2.H5Datatype, attrs::Tuple{JLD2.WrittenAttribute{…}})
    @ JLD2 ~/.julia/packages/JLD2/JyzIz/src/datatypes.jl:302
  [8] commit(::JLD2.JLDFile{JLD2.MmapIO}, ::Any, ::DataType, ::DataType)
    @ JLD2 ~/.julia/packages/JLD2/JyzIz/src/data/writing_datatypes.jl:248
  [9] commit_compound(f::JLD2.JLDFile{JLD2.MmapIO}, names::Vector{Symbol}, writtenas::DataType, readas::Type)
    @ JLD2 ~/.julia/packages/JLD2/JyzIz/src/data/writing_datatypes.jl:209
 [10] h5type(f::JLD2.JLDFile{JLD2.MmapIO}, writtenas::Any, x::Any)
    @ JLD2 ~/.julia/packages/JLD2/JyzIz/src/data/writing_datatypes.jl:136
 [11] h5type(f::JLD2.JLDFile{JLD2.MmapIO}, x::Any)
    @ JLD2 ~/.julia/packages/JLD2/JyzIz/src/data/writing_datatypes.jl:141
 [12] write_dataset(f::JLD2.JLDFile{JLD2.MmapIO}, x::Any, wsession::JLD2.JLDWriteSession{Dict{UInt64, JLD2.RelOffset}})
    @ JLD2 ~/.julia/packages/JLD2/JyzIz/src/datasets.jl:389
 [13] write(g::JLD2.Group{…}, name::String, obj::Function, wsession::JLD2.JLDWriteSession{…}; compress::Nothing)
    @ JLD2 ~/.julia/packages/JLD2/JyzIz/src/compression.jl:87
 [14] write(f::JLD2.JLDFile{JLD2.MmapIO}, name::String, obj::Any, wsession::JLD2.JLDWriteSession{Dict{…}}; compress::Nothing)
    @ JLD2 ~/.julia/packages/JLD2/JyzIz/src/compression.jl:71
 [15] write
    @ ~/.julia/packages/JLD2/JyzIz/src/compression.jl:71 [inlined]
 [16] jldsave(filename::String, compress::Bool, iotype::Type{JLD2.MmapIO}; kwargs...)
    @ JLD2 ~/.julia/packages/JLD2/JyzIz/src/loadsave.jl:290
 [17] jldsave
    @ ~/.julia/packages/JLD2/JyzIz/src/loadsave.jl:283 [inlined]
 [18] top-level scope
    @ REPL[4]:1
Some type information was truncated. Use `show(err)` to see complete types.

This is because the instance property can't be accessed for all function types, c.f. https://discourse.julialang.org/t/how-to-get-a-function-instance-from-a-function-type/79585.

In 0.5.1, this used to throw warnings

julia> jldsave(tempname(); foo)
┌ Warning: Attempting to store Foo.
│  Function types cannot be propertly stored in JLD2 files.
│  Loading may yield unexpected results.
└ @ JLD2 ~/.julia/packages/JLD2/JHhTf/src/data/writing_datatypes.jl:631
┌ Warning: Attempting to store Foo.
│  Function types cannot be propertly stored in JLD2 files.
│  Loading may yield unexpected results.
└ @ JLD2 ~/.julia/packages/JLD2/JHhTf/src/data/writing_datatypes.jl:631

[JonasIsensee]

Ah, thanks for the report.
Sorry about that....

Do you happen to know of a more reliable way to retrieve the name of a function?

[SamuelBadr]

I think Base.isgensym(Symbol(T)) does the trick. I can make a PR :)

[JonasIsensee]

I think Base.isgensym(Symbol(T)) does the trick. I can make a PR :)

Awesome!
Please do :)

Since you also found such a nice example, you could add that as a test?

I'd add it at the bottom of test/loadsave.jl in the same style as the ones above