[7oct2023] pybind11 transitive library deps

My original invesgation (below) mistaken. See update cmake headeronly deps.

• cmake version 3.25.3
• pybind11 version ???
• nix build (see https://github.com:rconybea/xo-nix2) Consequences of nix build:
  • Each package installed to a separate directory – no "common swimming pool" like /usr/lib
  • Implies install directory always distinct from any directory containing build inputs
  • Tends to reveal oversights in toolchain, as we'll see below
• pybind library (xo-pyreflect) with dependency on a separate library (xo-reflect), that in turn has secondary dependencies (xo-refcnt, xo-indentlog). Note that xo-indentlog is header-only.

• Expect this cmake script to work:

  find_package(pybind11)
  pybind11_add_module(pyreflect pyreflect.cpp)
  
  find_package(reflect CONFIG REQUIRED)
  target_link_libraries(pyreflect PUBLIC reflect)
  

• Instead, link fails. Link line something like:

  g++ -fPIC ... -o pyreflect.cpython-311-x86_64-linux-gnu.so /path/to/libreflect.so -lrefcnt -lindentlog
  

  Two problems here:

  1. directory containing librefcnt.so isn't on the link line (no -L/path/to/refcnt/dir for example).
  2. libindentlog.so does not exist, since indentlog is header-only

• Looked into intermediate outputs like lib/cmake/reflectTargets.cmake, excerpt:

  set_target_properties(reflect PROPERTIES
      INTERFACE_INCLUDE_DIRECTORIES "${_IMPORT_PREFIX}/include"
      INTERFACE_LINK_LIBRARIES "indentlog;refcnt"
  )
  

  It's not obvious how xo_pyreflect can know that indentlog is header-only, while refcnt isn't (though could presumably extract the relevant libdir from find_package() with some work).

• Recognize that pyreflect link shouldn't need refcnt on the link line, since libreflect.so has a DT_NEEDED entry for it.

  $ readelf -d /path/to/libreflect.so
  
  Dynamic section at offset 0x17860 contains 34 entries:
    Tag        Type                         Name/Value
   0x0000000000000001 (NEEDED)             Shared library: [librefcnt.so.1]
  ...
  

• When building pyreflect, suppress transitive dependencies For example:

  # xo_cxx.cmake
  macro(xo_pybind11_dependency target dep)
      find_package(${dep} CONFIG REQUIRED)
      set_property(TARGET ${dep} PROPERTY INTERFACE_LINK_LIBRARIES "")
      target_link_libraries(${target} PUBLIC ${dep})
  endmacro()
  

  Then in .cmake for pyreflect, something equivalent to:

  pybind11_add_module(pyreflect pyreflect.cpp)
  xo_pybind11_dependency(pyreflect reflect)