Building from the release tarball fails if nlohmann-json not installed on the system

[westonpace]

Describe your environment

Ubuntu 20.04
Linux pace-desktop 5.11.0-34-generic #36~20.04.1-Ubuntu SMP Fri Aug 27 08:06:32 UTC 2021 x86_64 x86_64 x86_64 GNU/Linux
nlohmann-json3-dev package is NOT installed system wide

Steps to reproduce

• In reality I am trying to add nlohmann-json as a dependency into my own cmake module using ExternalProject and listing the release tarball as the desired version. The below steps however, are simpler, and just pretend I wanted to download and install the release into my system.
• Download release tarball
• Extract release tarball
• cd <extracted_dir>
• mkdir build
• cd build
• cmake ..

What is the expected behavior?
Ideally, cmake would download nlohmann-json using ExternalProject or some other mechanism (or at least have a flag to enable this). Or, nlohmann-json's headers would be included as part of the release tarball. However, that does not seem to be implemented yet, which is fine.

Given this, I would expect to see the standard CMake find_package failure message:

CMake Error at CMakeLists.txt:245 (find_package):
  By not providing "Findnlohmann_json.cmake" in CMAKE_MODULE_PATH this
  project has asked CMake to find a package configuration file provided by
  "nlohmann_json", but CMake did not find one.

  Could not find a package configuration file provided by "nlohmann_json"
  with any of the following names:

    nlohmann_jsonConfig.cmake
    nlohmann_json-config.cmake

  Add the installation prefix of "nlohmann_json" to CMAKE_PREFIX_PATH or set
  "nlohmann_json_DIR" to a directory containing one of the above files.  If
  "nlohmann_json" provides a separate development package or SDK, be sure it
  has been installed.

What is the actual behavior?
What did you see instead?

CMake Error at CMakeLists.txt:256 (add_subdirectory):
  The source directory

    /home/pace/Downloads/opentelemetry-cpp-1.0.0-rc4/third_party/nlohmann-json

  does not contain a CMakeLists.txt file.

Additional context

The problem is that the release tarball contains the nlohmann-json directory but it is empty (likely because it is a git submodule and the CI/build is not including this as part of the tarball).

If I delete that directory then I still don't get the error message I would expect. Instead I get:

  nlohmann_json package was not found.  Please either provide it manually or
  clone with submodules.  To initialize, fetch and checkout any nested
  submodules, you can use the following command:

  git submodule update --init --recursive

This is a little better. I imagine it is a helpful message added for new developers. However, since I am downloading the release tarball, it is more or less meaningless (I didn't clone any git repository, there is no action I can take here). If I mark the package as required then I get the error message I would expect (at which point I clearly realize that I need to provide a system install of the nlohmann-json module and I can resolve that).

This might be opinionated, and I really do appreciate the work that you have done here, but it is rather odd behavior from a CMake module and any deviation from the CMake norm seems to send me down an unfortunate trail of debugging.

[lalitb]

Valid concern. We should improve on the error.

Also, default build of API + SDK + OTLP/gRPC doesn't use JSON, and cmake shouldn't check for this library for default build.