Add Python source distribution metadata to source tarball

We'll want to upload the source tarball to PyPI so the complete
corresponding source is available alongside the wheels.  However, it's
infeasible to support PEP 517 source builds with our highly customized
build process.  In practice, `pip install` will only try to do source
builds on platforms where a wheel isn't available.  Configure PEP 517
builds to use meson-python, then have meson-python fail with an error
encouraging users to install OpenSlide from source.

Signed-off-by: Benjamin Gilbert <bgilbert@cs.cmu.edu>

artifacts/postprocess-sdist.py

@@ -31,7 +31,8 @@
 sys.path.insert(0, os.environ['MESON_SOURCE_ROOT'])
 
 from common.argparse import TypedArgs  # noqa: E402
-from common.meson import meson_introspect  # noqa: E402
+from common.meson import meson_introspect, meson_source_root  # noqa: E402
+from common.python import pyproject_to_message  # noqa: E402
 
 
 class Args(TypedArgs):
@@ -46,6 +47,7 @@ class Args(TypedArgs):
 )
 args.parse()
 os.environ['MESONINTROSPECT'] = args.introspect
+src = meson_source_root()
 dest = Path(os.environ['MESON_DIST_ROOT'])
 
 # remove those parts of .github not ignored from .gitattributes
@@ -58,3 +60,12 @@ class Args(TypedArgs):
 except IndexError:
     suffix = ''
 (dest / 'suffix').write_text(suffix + '\n')
+
+# create Python source distribution metadata
+pyproject = (
+    (src / 'artifacts' / 'python' / 'pyproject.in.toml')
+    .read_text()
+    .replace('@version@', version)
+)
+(dest / 'pyproject.toml').write_text(pyproject)
+(dest / 'PKG-INFO').write_bytes(pyproject_to_message(pyproject).as_bytes())

artifacts/python/pyproject.in.toml

@@ -33,3 +33,11 @@ Homepage = "https://openslide.org/"
 # use GitHub Releases page because it has subproject versions
 "Release notes" = "https://github.com/openslide/openslide-bin/releases"
 Repository = "https://github.com/openslide/openslide-bin"
+
+[build-system]
+# The sole purpose of this build is to fail.  See comment in meson.build.
+build-backend = "mesonpy"
+requires = ["meson-python"]
+
+[tool.meson-python.args]
+setup = ['-Dpep517=true']

meson.build

@@ -15,6 +15,35 @@ project(
   ],
 )
 
+if get_option('pep517')
+  # openslide-bin builds OpenSlide and all its dependencies in a controlled
+  # environment:
+  #
+  # - Linux builds are linked against an EL 8 glibc for maximum compatibility.
+  # - macOS builds run Meson twice, for x86_64 and arm64, and then merge the
+  #   results into a universal build.
+  # - Windows builds link against the UCRT, and use a GCC compiled for Win32
+  #   threads to avoid producing a dependency on the winpthreads DLL.
+  #
+  # If we wanted to support building via PEP 517, we'd need to run the
+  # entire build from inside a PEP 517 backend (such as meson-python) which
+  # isn't designed to emit arbitrary artifacts such as bdist archives.  And
+  # even that wouldn't be enough, since the only way to get correct wheel
+  # tags is to fix the tags *after* the PEP 517 build finishes, e.g. with
+  # auditwheel on Linux.
+  #
+  # Since our actual wheel is very simple, and building a wheel isn't the
+  # main purpose of openslide-bin, it's not worth the extra complexity to
+  # support pip builds from source.  And so, while the PyPI source package
+  # contains exactly the bits that built the binaries, those binaries can
+  # only be built by directly invoking our build tooling.
+  #
+  # If `pip install openslide-bin` doesn't find a wheel, either the OS is
+  # too old or it isn't supported (e.g. FreeBSD), and openslide-bin can't
+  # help.
+  error('No openslide-bin wheel is available for your platform. Install OpenSlide from source.')
+endif
+
 system = host_machine.system()
 
 subdir('deps')

meson.options

@@ -15,3 +15,9 @@ option(
   value : false,
   description : 'Enable subprojects for OpenSlide Git main',
 )
+option(
+  'pep517',
+  type : 'boolean',
+  value : false,
+  description : 'PEP 517 build inside meson-python',
+)