Function with_subtree_at

crates/flowtrace-cli/src/git.rs:255–278

Locate a subtree by trace-relative path inside the tree at `sha` and run `f` against it. Returns `Ok(missing)` when the path isn't there (or isn't a tree) — callers pick a sensible default for that case.

(
    root: &Path,
    sha: &str,
    rel: &str,
    missing: R,
    f: impl FnOnce(&Repository, &git2::Tree<'_>) -> Result<R>,
)

/// `f` against it. Returns `Ok(missing)` when the path isn't there (or isn't
/// a tree) — callers pick a sensible default for that case.
fn with_subtree_at<R>(
    root: &Path,
    sha: &str,
    rel: &str,
    missing: R,
    f: impl FnOnce(&Repository, &git2::Tree<'_>) -> Result<R>,
) -> Result<R> {
    let repo = open(root)?;
    let translated = translate(&repo, root, rel);
    let oid = git2::Oid::from_str(sha).context("invalid commit sha")?;
    let commit = repo
        .find_commit(oid)
        .with_context(|| format!("commit {} not found", sha))?;
    let tree = commit.tree()?;
    let entry = match tree.get_path(Path::new(&translated)) {
        Ok(e) => e,
        Err(_) => return Ok(missing),
    };
    let object = entry.to_object(&repo)?;
    let Some(subtree) = object.as_tree() else {
        return Ok(missing);
    };
    f(&repo, subtree)
}

/// For every leaf file under `dir_rel` in the tree at `sha`, return
/// `(filename, blob bytes)`. Reads are batched against a single repo open;