Infra: unify git root discovery

src/infra/git-root.ts

@@ -0,0 +1,72 @@
+import fs from "node:fs";
+import path from "node:path";
+
+export const DEFAULT_GIT_DISCOVERY_MAX_DEPTH = 12;
+
+function walkUpFrom<T>(
+  startDir: string,
+  opts: { maxDepth?: number },
+  resolveAtDir: (dir: string) => T | null | undefined,
+): T | null {
+  let current = path.resolve(startDir);
+  const maxDepth = opts.maxDepth ?? DEFAULT_GIT_DISCOVERY_MAX_DEPTH;
+  for (let i = 0; i < maxDepth; i += 1) {
+    const resolved = resolveAtDir(current);
+    if (resolved !== null && resolved !== undefined) {
+      return resolved;
+    }
+    const parent = path.dirname(current);
+    if (parent === current) {
+      break;
+    }
+    current = parent;
+  }
+  return null;
+}
+
+function hasGitMarker(repoRoot: string): boolean {
+  const gitPath = path.join(repoRoot, ".git");
+  try {
+    const stat = fs.statSync(gitPath);
+    return stat.isDirectory() || stat.isFile();
+  } catch {
+    return false;
+  }
+}
+
+export function findGitRoot(startDir: string, opts: { maxDepth?: number } = {}): string | null {
+  // A `.git` file counts as a repo marker even if it is not a valid gitdir pointer.
+  return walkUpFrom(startDir, opts, (repoRoot) => (hasGitMarker(repoRoot) ? repoRoot : null));
+}
+
+function resolveGitDirFromMarker(repoRoot: string): string | null {
+  const gitPath = path.join(repoRoot, ".git");
+  try {
+    const stat = fs.statSync(gitPath);
+    if (stat.isDirectory()) {
+      return gitPath;
+    }
+    if (!stat.isFile()) {
+      return null;
+    }
+    const raw = fs.readFileSync(gitPath, "utf-8");
+    const match = raw.match(/gitdir:\s*(.+)/i);
+    if (!match?.[1]) {
+      return null;
+    }
+    return path.resolve(repoRoot, match[1].trim());
+  } catch {
+    return null;
+  }
+}
+
+export function resolveGitHeadPath(
+  startDir: string,
+  opts: { maxDepth?: number } = {},
+): string | null {
+  // Stricter than findGitRoot: keep walking until a resolvable git dir is found.
+  return walkUpFrom(startDir, opts, (repoRoot) => {
+    const gitDir = resolveGitDirFromMarker(repoRoot);
+    return gitDir ? path.join(gitDir, "HEAD") : null;
+  });
+}