RFC98 text: Build requirements for GDAL 3.9

doc/source/development/rfc/index.rst

@@ -104,3 +104,4 @@ RFC list
     rfc95_standard_int_types
     rfc96_deferred_plugin_loading
     rfc97_feature_and_fielddefn_sealing
+    rfc98_build_requirements_gdal_3_9

doc/source/development/rfc/rfc68_cplusplus11.rst

@@ -256,6 +256,11 @@ Related RFCs:
 -  `Move OTB to
    C++14 <https://wiki.orfeo-toolbox.org/index.php/Request_for_Comments-36:_Move_OTB_to_C%2B%2B14>`__
 
+Amendments
+----------
+
+This RFC is superseded per :ref:`rfc-98`.
+
 Voting history
 --------------
 

doc/source/development/rfc/rfc98_build_requirements_gdal_3_9.rst

@@ -0,0 +1,165 @@
+.. _rfc-98:
+
+=======================================
+RFC 98: Build requirements for GDAL 3.9
+=======================================
+============== =============================================
+Author:        Even Rouault
+Contact:       even.rouault @ spatialys.com
+Started:       2023-Nov-23
+Status:        Adopted, implemented
+Target:        GDAL 3.9
+============== =============================================
+
+Summary
+-------
+
+The document updates :ref:`rfc-68` with the new build requirements for GDAL 3.9:
+
+- C++ >= 17
+- CMake >= 3.16
+- PROJ >= 6.3.1.
+
+The minimum version for the following optional dependencies is also updated:
+
+- Python >= 3.8
+- GEOS >= 3.8
+- Poppler >= 0.86
+- libtiff >= 4.1
+- libcurl >= 7.68
+- libpng >= 1.6.0
+- libsqlite3 >= 3.31
+- libopenjp2 >= 2.3.1
+- libnetcdf >= 4.7 and built with NC4 enabled
+- libhdf5 >= 1.10
+
+Details
+-------
+
+Our past build requirements were C++11, CMake 3.9, and PROJ 6.0.0. All of them
+correspond to now outdated versions of those components, and it makes sense to
+update to more up-to-date versions to be able to clean up code, leverage new
+capabilities and be consistent with the current state of our software environment.
+
+The proposed updates are all compatible with versions of those components
+available by default in the old LTS (Long Term Support) Ubuntu 20.04, which
+corresponds to the oldest environment used by our continuous integration:
+
+- C++17 is the minimum version required by the latest versions of some of our
+  C++ dependencies, including Poppler, PDFium, PoDoFo, TileDB, libarrow-cpp.
+  Ubuntu 20.04 includes GCC 9.4, which supports C++17. While we want to allow
+  C++17 features to build GDAL, for now, we will stick to exposing at most
+  C++11 features in the exported headers of the library to minimize disruption
+  for GDAL C++ users. That might be revisited later.
+  At the time of writing, the C++17 requirement has already been implemented in
+  master / 3.9.0dev.
+
+- CMake 3.16.0 was released in November 2019. Updating to CMake 3.16 enables us
+  to make a number of cleanups in GDAL CMakeLists.txt scripts, and in particular
+  to make it possible to use the
+  [CMAKE_UNITY_BUILD](https://cmake.org/cmake/help/latest/variable/CMAKE_UNITY_BUILD.html)
+  feature. Ubuntu 20.04 includes CMake 3.16.4.
+
+- PROJ 6.3.1 was released in February 2020. Updating to that version implies
+  PROJ >= 6.2 and the availability of PROJJSON output, and PROJ >= 6.3 enables
+  us to remove a few specific code paths in ogrct.cpp. Ubuntu 20.04
+  includes PROJ 6.3.1. Earlier PROJ 6.x versions had a number of annoying issues.
+
+By the time GDAL 3.9 is released (May 2024), all the above requirements
+correspond to versions of the tools/libraries that have been released more than
+4 years earlier.
+
+More generally, we also update requirements for optional dependencies to be
+consistent with the versions available in Ubuntu 20.04, to eliminate code paths
+that are no longer exercised by our continuous integration:
+
+- Python >= 3.8: Python 3.7 is already end-of-life (https://devguide.python.org/versions/)
+  Python 3.8 is the minimum version used by our CI
+- GEOS >= 3.8: ensures that MakeValid() is available when GEOS is available,
+  which simplifies the code base and test suite
+- Poppler >= 0.86: removes a lot of #ifdef trickery in the PDF driver
+- libtiff >= 4.1: simplifies a few code paths in the GeoTIFF driver
+- libcurl >= 7.68: removes outdated code paths in CPL networking code
+- libpng >= 1.6.0: removes outdated code paths in the PNG driver
+- libsqlite3 >= 3.31: removes outdated code paths in the SQLite and GPKG drivers
+- libopenjp2 >= 2.3.1: removes outdated code paths in the OpenJPEG driver
+- libnetcdf >= 4.7 built with NC4 support enabled (i.e. libnetcdf built against
+  libhdf5): removes #ifdef code paths in netCDF driver. The netCDF multidimensional
+  code already requires NC4.
+- libhdf5 >= 1.10: removes outdated code paths in the HDF5 driver
+
+C++17 capable compilers
+-----------------------
+
+From https://en.wikipedia.org/wiki/C%2B%2B17, compilers supporting C++17 are:
+
+- GCC >= 8
+- clang >= 5
+- Visual Studio >= 2017 15.8 (MSVC 19.15)
+
+C++14 and C++17 features
+------------------------
+
+Features that can be used in the code base (not an exhaustive list):
+
+- Use of ``std::make_unique<>``  instead of ``cpl::make_unique<>``
+- Use of ``[[fallthrough]]`` instead of ``CPL_FALLTHROUGH``
+- Use of ``[[maybe_unused]]`` instead of ``CPL_UNUSED``
+- Nicer iteration in ``std::map`` with ``for (const auto &[key, value]: my_map )``
+  (more generally "structured binding declarations")
+
+Banned features:
+
+- Use of ``std::filesystem`` (https://en.cppreference.com/w/cpp/filesystem)
+  is not appropriate since our existing :cpp:class:`VSIFilesystemHandler`
+  mechanism has broader support for all our /vsi specific file systems.
+
+Changes in continuous integration
+---------------------------------
+
+Continuous integration is modified to test configurations that have at least
+the new set of build requirements.
+
+Changes in SWIG bindings
+------------------------
+
+None
+
+Backward compatibility
+----------------------
+
+No change in API and ABI
+
+Documentation
+-------------
+
+The Build requirements documentation page will be updated.
+
+Testing
+-------
+
+The existing autotest suite should continue to pass.
+
+Related tickets / pull requests
+-------------------------------
+
+- https://github.com/OSGeo/gdal/pull/8680: CI: Remove Ubuntu 18.04 configurations
+- https://github.com/OSGeo/gdal/issues/8270: Bump to C++17
+- https://github.com/OSGeo/gdal/pull/8687: Switch default C++ standard to C++17
+- https://github.com/OSGeo/gdal/pull/8710: Cxx17 tunings
+- https://github.com/OSGeo/gdal/pull/8716: C++17: use if constexpr() construct
+- https://github.com/OSGeo/gdal/pull/8723: C++17: replace CPL_FALLTHROUGH by standard [[fallthrough]];
+- https://github.com/OSGeo/gdal/pull/8725: C++17: use structured bindings declaration and class template argument deduction for std::pair()
+- https://github.com/OSGeo/gdal/issues/8751: Bumping minimum CMake version to 3.16
+- https://github.com/OSGeo/gdal/issues/8796: Add CI test we can use GDAL public/installed headers with C++11
+- https://github.com/OSGeo/gdal/pull/8804: Make build compatible of -DCMAKE_UNITY_BUILD=ON for faster builds
+
+Related RFCs
+------------
+
+- This RFC supersedes :ref:`rfc-68`
+
+Voting history
+--------------
+
++1 from PSC members JavierJS, KurstS, HowardB, JukkaR and EvenR