gix config refresh and writing

[Sebastian Thiel (Byron)]

Tasks

This section is for Byron only. Models continuing this PR must not add, remove, check, uncheck, rename, or reorder checkboxes here.

• refackiew

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Everything below this line was generated by Codex GPT-5.

Created by Codex on behalf of Byron. Byron will review before this is ready to merge.

Summary

• add Repository::config() and config_mut() accessors that refresh changed configuration files while leaving snapshot-only access free of freshness I/O
• preserve runtime API sections across reloads without duplicating open::Options::config_overrides()
• add SnapshotMut::commit_to_file() to atomically write one exact file designation with lock-first mtime validation, permission preservation, and deleted-file recreation
• reject stale files, forged designations, override-only sources, and invalid snapshots before changing disk or in-memory state

Commits

• 4c759f742e — refresh repository configuration on access, including the platform-specific size guard
• 9bacd89ae1 — write mutable configuration snapshots to disk

Git baseline

Git's config.c clears and reloads repository configuration with includes, and its file mutation path locks <path>.lock, preserves the file mode, then commits by atomic rename. This change adapts those behaviors to long-lived gix::Repository snapshots and adds the requested mtime stale-write guard.

Validation

• GIX_TEST_IGNORE_ARCHIVES=1 cargo test -p gix --test gix repository::config::config_snapshot
• GIX_TEST_IGNORE_ARCHIVES=1 cargo test -p gix --test gix repository::config::config_snapshot::commit_to_file
• GIX_TEST_IGNORE_ARCHIVES=1 cargo test -p gix --test gix repository::size_in_memory -- --exact
• GIX_TEST_IGNORE_ARCHIVES=1 cargo test -p gix --test gix --features async-network-client repository::size_in_memory -- --exact
• cargo clippy -p gix --all-targets -- -D warnings
• cargo deny --workspace --all-features check bans licenses sources
• env GIX_TEST_IGNORE_ARCHIVES=1 just ci-test
• GitHub CI before the Tix-only history cleanup: all 32 checks passed; the resulting tree is unchanged

The required one-time codex review --commit invocation was attempted for each original pre-squash commit, but the review service rejected all four because the account usage limit was exhausted. Independent agent audits were completed for both feature commits and the Windows CI fix.

Reported issue

Original report

While gix::Repository::index() checks if the index file has to be reloaded, the git configuration has no such mechanism. `config_snapshot_mut()` is intentionally a snapshot, and refreshing it is currently impossible.
It would be a start if there was a `config()` or `config_mut()` that returns a snapshot (just like the current accessors), but after checking that none of the included configuration files changed. This information is readily present in section metadata. If any of these files changed according to the modification date, then reload the configuration just like it happens when opening (or reloading) so side-effects are handled.
Make sure that open::Options::config_overrides() aren't duplicated, and that API-only sections aren't lost either.

Write-back follow-up

In a separate commit, let's also add a way to write back changes to any file, based on the in-memory file. This will never write API or env overrides, but writes back any section that matches the desigation metadata. This entails a way to select which single file to write probably by passing section metadata identifying the file, to obtaina lock, and to write changes back to the file if it's not stale based on the mtime should should have. A deleted file is not considered stale.
If the file to write changed after the snapshot was obtained, fail. In that case, the user has to retry with a refreshed file.

Clarification

>  - Capture original file designations and mtime baselines when creating the mutable snapshot.

No, this information is part of what `repo.cache` holds, and the `config_snapshot_mut()` call for instance won't do additional IO. This is only done by `config()` and `config_mut()` respectively.


> - Resolve symlinks, acquire the target’s gix_lock::File, then compare mtimes while holding the lock.

Don't do that, only the local configuration files are (likely) writable and support locks there, and system wide ones probably aren't. But yeah, hold the lock before comparing the mtime of the file to write.

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