Include Extension.depends in manifests and sdists by default (#4000)

newsfragments/4000.feature.rst

@@ -0,0 +1,2 @@
+Automatically add files listed in ``Extension.depends`` to sdists,
+as long as they are contained in the project directory -- by :user:`RuRo`

setuptools/command/build_ext.py

@@ -4,6 +4,7 @@
 from importlib.machinery import EXTENSION_SUFFIXES
 from importlib.util import cache_from_source as _compiled_file_name
 from typing import Dict, Iterator, List, Tuple
+from pathlib import Path
 
 from distutils.command.build_ext import build_ext as _du_build_ext
 from distutils.ccompiler import new_compiler
@@ -261,6 +262,47 @@ def links_to_dynamic(self, ext):
         pkg = '.'.join(ext._full_name.split('.')[:-1] + [''])
         return any(pkg + libname in libnames for libname in ext.libraries)
 
+    def get_source_files(self) -> List[str]:
+        return [*_build_ext.get_source_files(self), *self._get_internal_depends()]
+
+    def _get_internal_depends(self) -> Iterator[str]:
+        """Yield ``ext.depends`` that are contained by the project directory"""
+        project_root = Path(self.distribution.src_root or os.curdir).resolve()
+        depends = (dep for ext in self.extensions for dep in ext.depends)
+
+        def skip(orig_path: str, reason: str) -> None:
+            log.info(
+                "dependency %s won't be automatically "
+                "included in the manifest: the path %s",
+                orig_path,
+                reason,
+            )
+
+        for dep in depends:
+            path = Path(dep)
+
+            if path.is_absolute():
+                skip(dep, "must be relative")
+                continue
+
+            if ".." in path.parts:
+                skip(dep, "can't have `..` segments")
+                continue
+
+            try:
+                resolved = (project_root / path).resolve(strict=True)
+            except OSError:
+                skip(dep, "doesn't exist")
+                continue
+
+            try:
+                resolved.relative_to(project_root)
+            except ValueError:
+                skip(dep, "must be inside the project root")
+                continue
+
+            yield path.as_posix()
+
     def get_outputs(self) -> List[str]:
         if self.inplace:
             return list(self.get_output_mapping().keys())