exclude_vcs on default_build_sdist() breaks plugin signature checks #1444

What changed

#1417 added a new parameter to the public default_build_sdist() hook:

def default_build_sdist(
    ctx, extra_environ, req, version, sdist_root_dir, build_env, build_dir,
    *,
    exclude_vcs: bool = False,  # new
) -> pathlib.Path:

Fromager sets exclude_vcs=True only for packages with source: and build_sdist: tarball, so .git stays out of the sdist. This change is on new-resolver-config only, not on main. It would ship with #1414.

Builds still work

Fromager never passes exclude_vcs to plugin build_sdist hooks. A plugin hook without the new parameter still works, and so does a plugin that calls default_build_sdist() directly.

Plugin tests break

Plugin projects often test that each hook has the same parameter names as fromager's default, to catch hooks that drift from the API:

default = inspect.signature(fromager.sources.default_build_sdist)
plugin = inspect.signature(my_plugin.build_sdist)
assert set(default.parameters) == set(plugin.parameters)

After upgrading, this test fails for every build_sdist hook, because the default now has exclude_vcs and the plugin doesn't. So upgrading fromager means changing either every hook or the test, even though nothing is wrong.

Why it matters

#1414 should not break existing plugins. This doesn't break builds, but it does break plugin test suites that follow this pattern.

Also: VCS directories removed from non-Git archives

The configured tarball path passes exclude_vcs=True for every package, not only Git checkouts. So pypi-sdist, pypi-download, *-tag-download and hook-sdist archives also lose any .git, .hg, .svn or .bzr directories they contain (for example in test fixtures). The same version then gives a different sdist with and without source:.

Only Git checkouts should drop VCS directories. Reported in #1414 (comment).

Options

1. Keep the public signature unchanged. Move exclude_vcs into a private helper used by both default_build_sdist() and the configured tarball path, and set it only when the source is a Git checkout. Plugins and their tests need no changes.
2. Keep the new parameter. Document it in the 0.103.0 notes, tell plugin authors to update their hooks or tests, and still set it only for Git checkouts.

Option 1 is a small change, avoids changing a public hook, and fixes both problems.