When updating to the newest JSON library version (beginning 3.8.0), nix fails to build

[OmnipotentEntity]

Describe the bug

Nix fails to build with the following error message when using a version of nlohmann's json library that includes the binary type feature.

builder for '/nix/store/cd3a1sxaq9idl129bvgbgrazpfnpp792-nix-3.0pre20200829_f156513.drv' failed with exit code 2; last 10 log lines:
                   from src/libexpr/json-to-value.cc:4:
  /nix/store/a15crh4aa98283pmx0gxzchgi39547qf-nlohmann_json-3.9.1/include/nlohmann/detail/input/binary_reader.hpp:289:113: error: 'nlohmann::json_sax<nlohmann::basic_json<> >' is not an accessible base of 'nlohmann::detail::binary_reader<nlohmann::basic_json<>, nlohmann::detail::iterator_input_adapter<__gnu_cxx::__normal_iterator<const char*, std::__cxx11::basic_string<char> > >, nix::JSONSax>::json_sax_t' {aka 'nix::JSONSax'}
    289 |                 return get_number<std::int32_t, true>(input_format_t::bson, len) && get_bson_binary(len, value) && sax->binary(value);
    CXX    src/nix/repl.o
    CXX    src/libexpr/nixexpr.o
    CXX    src/nix/registry.o
    CXX    src/nix/profile.o
    CXX    src/nix/ping-store.o
  make: *** [mk/patterns.mk:3: src/libexpr/json-to-value.o] Error 1
  make: *** Waiting for unfinished jobs....

Steps To Reproduce

1. nix run -I nixpkgs=channel:nixos-unstable nixpkgs.nixpkgs-review -c nixpkgs-review pr 97266

Expected behavior

Builds

nix-env --version output

nix-env (Nix) 2.3.6

Additional context

This was caused by changes to the Sax class in version 3.8.0 to accommodate a new internal type called "binary" which handles binary input/output from binary filetypes (such as CBOR and BSON). You'll need to implement a binary method in the nix::JSONSax class. I do not think you are currently using binary types, so you can just implement a "do nothing" version safely. There are examples of this in the nlohmann JSON tests.

[matthewbauer]

I just had to add bool binary(binary_t& val) for this to work with 3.8.0. I bet more stuff changed with 3.9.0 though.

obsidiansystems@d2f52cb#diff-d00c80d9496f26e308e2d07d1ee91a0f

[OmnipotentEntity]

You'll definitely want a return true in that function. Otherwise, if you somehow call it you'll trigger UB.

Thanks for giving me an easy thing to test against though!

[matthewbauer]

You'll definitely want a return true in that function. Otherwise, if you somehow call it you'll trigger UB.

Thanks for giving me an easy thing to test against though!

Yeah - although throw might be better - we can't really do much with binary data here.