chore: refactor the backend (#17)

Reviewed-on: #17

crates/signed_git/src/scan.rs

@@ -0,0 +1,43 @@
+use std::path::{Path, PathBuf};
+
+use ignore::WalkBuilder;
+
+/// Maximum directory nesting depth when scanning for local repositories.
+///
+/// Pathological trees can't stall the scan.
+const SCAN_MAX_DEPTH: usize = 12;
+
+/// Walk `root` recursively and collect the paths of git repositories below it,
+/// honouring `.gitignore` (and `.ignore`) files.
+pub fn find_git_repos(root: &Path) -> Vec<PathBuf> {
+    if !root.is_dir() {
+        return Vec::new();
+    }
+
+    let walker = WalkBuilder::new(root)
+        .max_depth(Some(SCAN_MAX_DEPTH))
+        // Honour `.gitignore` even when the scan root is not itself a repository.
+        .require_git(false)
+        .build();
+
+    let mut repos: Vec<PathBuf> = walker
+        .flatten()
+        .filter(|entry| entry.file_type().is_some_and(|kind| kind.is_dir()))
+        .map(ignore::DirEntry::into_path)
+        .filter(|dir| dir.join(".git").exists())
+        .filter_map(|dir| dir.canonicalize().ok())
+        .collect();
+
+    repos.sort();
+    repos.dedup();
+
+    // A repository nested inside another, like a submodule worktree, is not reported.
+    let mut roots: Vec<PathBuf> = Vec::with_capacity(repos.len());
+    for repo in repos {
+        if !roots.iter().any(|kept| repo.starts_with(kept)) {
+            roots.push(repo);
+        }
+    }
+
+    roots
+}