refactor(canvas-host): share static file resolver

src/canvas-host/file-resolver.ts

@@ -0,0 +1,50 @@
+import fs from "node:fs/promises";
+import path from "node:path";
+import { SafeOpenError, openFileWithinRoot, type SafeOpenResult } from "../infra/fs-safe.js";
+
+export function normalizeUrlPath(rawPath: string): string {
+  const decoded = decodeURIComponent(rawPath || "/");
+  const normalized = path.posix.normalize(decoded);
+  return normalized.startsWith("/") ? normalized : `/${normalized}`;
+}
+
+export async function resolveFileWithinRoot(
+  rootReal: string,
+  urlPath: string,
+): Promise<SafeOpenResult | null> {
+  const normalized = normalizeUrlPath(urlPath);
+  const rel = normalized.replace(/^\/+/, "");
+  if (rel.split("/").some((p) => p === "..")) {
+    return null;
+  }
+
+  const tryOpen = async (relative: string) => {
+    try {
+      return await openFileWithinRoot({ rootDir: rootReal, relativePath: relative });
+    } catch (err) {
+      if (err instanceof SafeOpenError) {
+        return null;
+      }
+      throw err;
+    }
+  };
+
+  if (normalized.endsWith("/")) {
+    return await tryOpen(path.posix.join(rel, "index.html"));
+  }
+
+  const candidate = path.join(rootReal, rel);
+  try {
+    const st = await fs.lstat(candidate);
+    if (st.isSymbolicLink()) {
+      return null;
+    }
+    if (st.isDirectory()) {
+      return await tryOpen(path.posix.join(rel, "index.html"));
+    }
+  } catch {
+    // ignore
+  }
+
+  return await tryOpen(rel);
+}