fix: dedupe concurrent watermark rendering

scripts/test-media-watermark-policy.mjs

@@ -111,6 +111,14 @@ await runTest('image watermark renderer visibly changes raster media', async ()
   assert.equal(metadata.height, 360);
 });
 
+await runTest('watermark renderer dedupes concurrent generation for the same media', () => {
+  const source = read('src/lib/media-watermark.ts');
+
+  assert.match(source, /inflightWatermarkJobs/);
+  assert.match(source, /inflightWatermarkJobs\.get\(outputKey\)/);
+  assert.match(source, /inflightWatermarkJobs\.delete\(outputKey\)/);
+});
+
 await runTest('local storage route uses watermark access instead of exposing raw object URLs by default', () => {
   const source = read('src/app/api/local-storage/[...path]/route.ts');
 

src/lib/media-watermark.ts

@@ -13,6 +13,11 @@ import {
 const WATERMARK_LOGO_PATH = path.join(process.cwd(), 'public', 'watermark', 'miaojing-watermark-logo.png');
 const WATERMARK_TEXT = 'MIAOJING AI';
 const WATERMARK_OPACITY = 0.5;
+const inflightWatermarkJobs = new Map<string, Promise<{
+  key: string;
+  buffer: Buffer;
+  contentType: string;
+}>>();
 
 export async function serveWatermarkedStorageFile(key: string, contentType: string): Promise<{
   key: string;
@@ -59,7 +64,23 @@ async function createWatermarkedFile(key: string, contentType: string): Promise<
       contentType,
     };
   }
+  const inflight = inflightWatermarkJobs.get(outputKey);
+  if (inflight) return inflight;
 
+  const job = createWatermarkedFileUncached(key, contentType, outputKey);
+  inflightWatermarkJobs.set(outputKey, job);
+  try {
+    return await job;
+  } finally {
+    inflightWatermarkJobs.delete(outputKey);
+  }
+}
+
+async function createWatermarkedFileUncached(key: string, contentType: string, outputKey: string): Promise<{
+  key: string;
+  buffer: Buffer;
+  contentType: string;
+}> {
   const kind = getMediaKindFromContentType(key, contentType);
   if (kind === 'image') {
     const input = await localStorage.readFileAsync(key);