build(tox): let the gix env's tests download packages

`tox -e gix` runs the test suite through `test/run-local.py`, which sets
`PIP_NO_INDEX=1` so that nothing is downloaded during the run, and
points `PIP_FIND_LINKS` at a hand-filled `.cache/gix-wheels` directory.
But `test_installation` installs `./smmap`, `./gitdb`, and GitPython
into a fresh virtual environment, and pip needs `setuptools` from the
package index to build them. So without that directory, as on a fresh
clone, the test fails with "Could not find a version that satisfies
the requirement setuptools>=40.8.0 (from versions: none)". tox itself
has already installed the environment's dependencies from the index by
then, so the restriction only stops this test. The environment and the
runner were added in 212573b (gitpython-developers#2274); a commit message in that PR's
earlier history noted that tox was not available to run it.

Have `run-local.py` keep a `PIP_NO_INDEX` that is already set, as it
already does for `PIP_FIND_LINKS`, and set `PIP_NO_INDEX` to `0` in the
`gix` environment unless it is already set. A set value, including an
empty one, is passed on unchanged, so pip treats it as it would
anywhere else; pip takes an empty value, like `0`, as allowing the
index. Running `run-local.py` directly stays offline by default, and
`PIP_NO_INDEX=1 tox -e gix` still runs offline, though tox then also
installs the environment without the index. The environment keeps the
runner's private Git configuration, Git daemon port, and pytest
temporary directory.

Having the environment run pytest directly, like the other test
environments, would also fix this, with a smaller diff, but would
change more about how the environment runs. Document the new behavior
in `doc/gix-backend.md`.

With tox 4.22.0 and 4.67.1, `tox config -e gix` resolves `PIP_NO_INDEX`
to `0` when it is unset, `1` when it is `1`, and empty when it is empty.
With tox 4.67.1:

- `tox -e gix -- test/test_installation.py` passed.
- With `PIP_NO_INDEX=1`, the test failed as before, and in a new
  environment, tox's own installation of its dependencies failed for
  lack of an index.
- Run directly with the environment's Python, `run-local.py` failed the
  test as before when `PIP_NO_INDEX` was unset, and passed when it was
  empty.

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

Author: EliahKagan

doc/gix-backend.md

@@ -182,12 +182,20 @@ the test aliases directly, matching CI without introducing config includes.
 
 The runner uses local version tags, creates an isolated Git configuration,
 prepares the historical test fixture through cached Bash scripts, gives
-pytest a separate temporary root for each run, and disables package-index
-access. Cleanup of these isolated directories is best-effort, so a locked
-leftover cannot change pytest's exit status. Tests use local repositories,
-including the tutorial example. The suite needs loopback sockets for its Git daemon and
-permission to inspect its own child processes. It does not need a remote Git
-server. Missing local tags or packages are errors, not invitations to download.
+pytest a separate temporary root for each run, and by default disables
+package-index access. Cleanup of these isolated directories is best-effort,
+so a locked leftover cannot change pytest's exit status. Tests use local
+repositories, including the tutorial example. The suite needs loopback
+sockets for its Git daemon and permission to inspect its own child
+processes. It does not need a remote Git server. Missing local tags are
+errors, not invitations to download, and by default so are missing
+packages.
+
+The runner keeps `PIP_NO_INDEX` if it is already set, and pip treats `0` as
+allowing the package index. `tox -e gix` sets it to `0` unless it is set, so
+its installation test can download build dependencies. To run that
+environment offline, set `PIP_NO_INDEX=1`, which tox also applies when
+installing the environment, and point `PIP_FIND_LINKS` at the wheels.
 
 The ignored `.cache/gix-wheels` directory contains the published GixPython wheel and
 the build/test dependencies available on this machine. To recreate an

test/run-local.py

@@ -26,7 +26,7 @@ def main():
             **os.environ,
             "GIT_CONFIG_NOSYSTEM": "1",
             "GIT_CONFIG_GLOBAL": str(config),
-            "PIP_NO_INDEX": "1",
+            "PIP_NO_INDEX": os.environ.get("PIP_NO_INDEX", "1"),
             "PIP_DISABLE_PIP_VERSION_CHECK": "1",
             "PIP_FIND_LINKS": os.environ.get("PIP_FIND_LINKS", str(root / ".cache/gix-wheels")),
             "UV_OFFLINE": "1",

tox.ini

@@ -33,7 +33,10 @@ set_env =
 commands = ruff check .
 
 [testenv:gix]
-set_env = GIT_PYTHON_BACKEND=gix
+# Let tests download packages, as `test/run-local.py` otherwise forbids, unless `PIP_NO_INDEX` is already set.
+set_env =
+    GIT_PYTHON_BACKEND=gix
+    PIP_NO_INDEX={env:PIP_NO_INDEX:0}
 description = Run the suite with released GixPython and CLI fallback
 base_python = python3
 package_env = .pkg-gix