Trac #30580: sage_setup: Remove import-time dependency (setup_requires) on pkgconfig, numpy

Just loading `src/setup.py` already pulls in Cython, `numpy`, and
`pkgconfig` via `sage_setup`, so these distributions would have to be
declared as `[build_system] requires` in `src/pyproject.toml` (ex
`setup_requires`)

By moving some computations from import-time to runtime, we get rid of
this early dependency on `pkgconfig`, `numpy`.  (They are, of course,
still required for building the package.)

This makes `setup.py sdist` work using a Python that does not have
`numpy `or `pkgconfig` installed.  To test (with a system python that
has `Cython`):
{{{
  $ (cd build/pkgs/sagelib/src && python3 -u setup.py --no-user-cfg
sdist)
}}}

(We also reduce the load-time dependency on Cython; however, we do not
address the whole load-time dependency of `setup.py` on `Cython` (via
`sage_setup.find`, which uses `open_source_file` and `is_package_dir`)
in this ticket. This is best done after #28925.)

URL: https://trac.sagemath.org/30580
Reported by: mkoeppe
Ticket author(s): Matthias Koeppe
Reviewer(s): Dima Pasechnik

Author: Release Manager

build/pkgs/sagelib/src/setup.py

@@ -57,18 +57,20 @@
 print("Discovering Python/Cython source code....")
 t = time.time()
 
-distributions = ['']
-
 from sage_setup.optional_extension import is_package_installed_and_updated
 
-optional_packages_with_extensions = ['mcqd', 'bliss', 'tdlib', 'primecount',
-                                     'coxeter3', 'fes', 'sirocco', 'meataxe']
-
-distributions += ['sage-{}'.format(pkg)
-                  for pkg in optional_packages_with_extensions
-                  if is_package_installed_and_updated(pkg)]
-
-log.warn('distributions = {0}'.format(distributions))
+if sdist:
+    # No need to compute distributions.  This avoids a dependency on Cython
+    # just to make an sdist.
+    distributions = None
+else:
+    distributions = ['']
+    optional_packages_with_extensions = ['mcqd', 'bliss', 'tdlib', 'primecount',
+                                         'coxeter3', 'fes', 'sirocco', 'meataxe']
+    distributions += ['sage-{}'.format(pkg)
+                      for pkg in optional_packages_with_extensions
+                      if is_package_installed_and_updated(pkg)]
+    log.warn('distributions = {0}'.format(distributions))
 
 from sage_setup.find import find_python_sources
 python_packages, python_modules, cython_modules = find_python_sources(

src/sage_setup/command/sage_build_cython.py

@@ -13,14 +13,10 @@
 from distutils.errors import (DistutilsModuleError,
                               DistutilsOptionError)
 
-from Cython.Build.Dependencies import default_create_extension
 from sage_setup.util import stable_uniq
 from sage_setup.find import find_extra_files
-from sage_setup.library_order import library_order
 from sage_setup.cython_options import compiler_directives, compile_time_env_variables
 
-from sage.env import (cython_aliases, sage_include_directories)
-
 # Do not put all, but only the most common libraries and their headers
 # (that are likely to change on an upgrade) here:
 # [At least at the moment. Make sure the headers aren't copied with "-p",
@@ -58,13 +54,6 @@
 if subprocess.call("""$CC --version | grep -i 'gcc.* 4[.]8' >/dev/null """, shell=True) == 0:
     extra_compile_args.append('-fno-tree-copyrename')
 
-# Search for dependencies in the source tree and add to the list of include directories
-include_dirs = sage_include_directories(use_sources=True)
-
-# Look for libraries only in what is configured already through distutils
-# and environment variables
-library_dirs = []
-
 class sage_build_cython(Command):
     name = 'build_cython'
     description = "compile Cython extensions into C/C++ extensions"
@@ -211,6 +200,13 @@ def run(self):
         Call ``cythonize()`` to replace the ``ext_modules`` with the
         extensions containing Cython-generated C code.
         """
+        from sage.env import (cython_aliases, sage_include_directories)
+        # Set variables used in self.create_extension
+        from ..library_order import library_order
+        self.library_order = library_order
+        # Search for dependencies in the source tree and add to the list of include directories
+        self.sage_include_dirs = sage_include_directories(use_sources=True)
+
         from Cython.Build import cythonize
         import Cython.Compiler.Options
 
@@ -281,7 +277,7 @@ def create_extension(self, template, kwds):
         # Libraries: sort them
         libs = kwds.get('libraries', [])
         kwds['libraries'] = sorted(set(libs),
-                key=lambda lib: library_order.get(lib, 0))
+                key=lambda lib: self.library_order.get(lib, 0))
 
         # Dependencies: add setup.py and lib_headers
         depends = kwds.get('depends', []) + [self.distribution.script_name]
@@ -322,11 +318,13 @@ def create_extension(self, template, kwds):
         kwds['extra_link_args'] = stable_uniq(ldflags)
 
         # Process library_dirs
-        lib_dirs = kwds.get('library_dirs', []) + library_dirs
+        lib_dirs = kwds.get('library_dirs', [])
         kwds['library_dirs'] = stable_uniq(lib_dirs)
 
         # Process include_dirs
-        inc_dirs = kwds.get('include_dirs', []) + include_dirs + [self.build_dir]
+        inc_dirs = kwds.get('include_dirs', []) + self.sage_include_dirs + [self.build_dir]
         kwds['include_dirs'] = stable_uniq(inc_dirs)
 
+        from Cython.Build.Dependencies import default_create_extension
+
         return default_create_extension(template, kwds)