Prevent data loss from read errors and concurrent writes

readCards now throws on fetch/parse failures instead of returning []
so a broken read can never trigger an overwrite of valid data.
Added a write lock to serialize card mutations within a single instance.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

apps/api/src/index.ts

@@ -10,6 +10,7 @@ import {
   readCards,
   saveImage,
   storageMode,
+  withWriteLock,
   writeCards
 } from "./store";
 
@@ -251,16 +252,18 @@ const app = new Elysia()
     }
 
     try {
-      const form = await request.formData();
+      await withWriteLock(async () => {
-      const cards = await readCards();
+        const form = await request.formData();
-      const id = String(form.get("id") || "");
+        const cards = await readCards();
-      const existing = cards.find((card) => card.id === id);
+        const id = String(form.get("id") || "");
-      const nextCard = await cardFromForm(form, existing);
+        const existing = cards.find((card) => card.id === id);
-      const nextCards = existing
+        const nextCard = await cardFromForm(form, existing);
-        ? cards.map((card) => (card.id === existing.id ? nextCard : card))
+        const nextCards = existing
-        : [nextCard, ...cards];
+          ? cards.map((card) => (card.id === existing.id ? nextCard : card))
+          : [nextCard, ...cards];
 
-      await writeCards(nextCards);
+        await writeCards(nextCards);
+      });
       return redirect("/admin");
     } catch (error) {
       return renderAdminWithMessage(error instanceof Error ? error.message : "Could not save card.");
@@ -278,16 +281,18 @@ const app = new Elysia()
     }
 
     try {
-      const form = await request.formData();
+      await withWriteLock(async () => {
-      const id = String(form.get("id") || "");
+        const form = await request.formData();
-      const cards = await readCards();
+        const id = String(form.get("id") || "");
-      const existing = cards.find((card) => card.id === id);
+        const cards = await readCards();
+        const existing = cards.find((card) => card.id === id);
 
-      if (existing?.imageUrl) {
+        if (existing?.imageUrl) {
-        await deleteRemoteImage(existing.imageUrl);
+          await deleteRemoteImage(existing.imageUrl);
-      }
+        }
 
-      await writeCards(cards.filter((card) => card.id !== id));
+        await writeCards(cards.filter((card) => card.id !== id));
+      });
       return redirect("/admin");
     } catch (error) {
       return renderAdminWithMessage(error instanceof Error ? error.message : "Could not delete card.");

apps/api/src/store.ts

@@ -20,30 +20,35 @@ const localCardsPath = path.join(localDataDir, "cards.json");
 const hasBlobToken = () => Boolean(process.env.BLOB_READ_WRITE_TOKEN?.trim());
 const isVercel = () => process.env.VERCEL === "1";
 
+let writeLock: Promise<void> = Promise.resolve();
+
+export function withWriteLock<T>(fn: () => Promise<T>): Promise<T> {
+  const next = writeLock.then(fn, fn);
+  writeLock = next.then(() => {}, () => {});
+  return next;
+}
+
 export const storageMode = hasBlobToken() ? "Vercel Blob" : "Local file";
 
 export async function readCards(): Promise<BoardCard[]> {
   if (hasBlobToken()) {
-    try {
+    const { blobs } = await list({ prefix: dataBlobPath, limit: 1 });
-      const { blobs } = await list({ prefix: dataBlobPath, limit: 1 });
+    if (blobs.length === 0) return [];
-      if (blobs.length === 0) return [];
 
-      const res = await fetch(blobs[0].url);
+    const res = await fetch(blobs[0].url);
-      if (!res.ok) return [];
+    if (!res.ok) throw new Error(`Blob read failed: ${res.status}`);
 
-      const parsed = (await res.json()) as BoardCard[];
+    const parsed = (await res.json()) as BoardCard[];
-      return Array.isArray(parsed) ? parsed : [];
+    return Array.isArray(parsed) ? parsed : [];
-    } catch {
-      return [];
-    }
   }
 
   try {
     const text = await readFile(localCardsPath, "utf8");
     const parsed = JSON.parse(text) as BoardCard[];
     return Array.isArray(parsed) ? parsed : [];
-  } catch {
+  } catch (err: any) {
-    return [];
+    if (err?.code === "ENOENT") return [];
+    throw err;
   }
 }