US07-02: Harden API Authorization and Path Boundaries (#84)

frontend/js/api.js

@@ -2,14 +2,43 @@
 // cancellation. Every method accepts an optional { signal } from cancellable().
 export const BASE = "/api/v1";
 
+// The API refuses every request without the session cookie, and every mutation
+// without this token echoed back. The token is readable only same-origin, which is
+// what makes it proof that the caller is this app and not another page.
+let csrfToken = null;
+
+async function session() {
+  if (csrfToken === null) {
+    const response = await fetch(BASE + "/session", { credentials: "same-origin" });
+    const body = await response.json().catch(() => null);
+    csrfToken = (body && body.csrf_token) || null;
+  }
+  return csrfToken || "";
+}
+
+async function send(path, { signal, ...options }) {
+  return fetch(BASE + path, {
+    credentials: "same-origin",
+    signal,
+    ...options,
+    headers: {
+      "Content-Type": "application/json",
+      "X-CSRF-Token": await session(),
+      ...(options.headers || {}),
+    },
+  });
+}
+
 async function request(path, { signal, ...options } = {}) {
   let response;
   try {
-    response = await fetch(BASE + path, {
-      headers: { "Content-Type": "application/json" },
-      signal,
-      ...options,
-    });
+    response = await send(path, { signal, ...options });
+    // A restarted server issues a new session; re-bootstrap once rather than
+    // stranding an open tab on 401.
+    if (response.status === 401) {
+      csrfToken = null;
+      response = await send(path, { signal, ...options });
+    }
   } catch (error) {
     // A caller-cancelled fetch is not a failure; tag it so views can ignore it.
     if (error.name === "AbortError") {