Serialize ExifTool work in the worker with a FIFO job queue.

Overlapping parseMetadata calls on one WASM instance caused wrong metadata when multiple files were read in parallel; callers can still postMessage concurrently.

frontend/src/types/workers/exiftoolWorkerJob.ts

@@ -0,0 +1,3 @@
+export type ExiftoolWorkerJob =
+    | { type: 'init' }
+    | { type: 'read-metadata'; id: string; file: File };

frontend/src/workers/exiftoolWorker.ts

@@ -2,11 +2,15 @@ import zeroperlWasmUrl from '@6over3/zeroperl-ts/zeroperl.wasm?url';
 import { parseMetadata } from '@uswriting/exiftool';
 import { EXIFTOOL_WORKER_INIT_JPEG } from '@/consts/exiftoolWorkerInitJpeg';
 import type { ExiftoolWorkerInMessage } from '@/types/workers/exiftoolWorkerInMessage';
+import type { ExiftoolWorkerJob } from '@/types/workers/exiftoolWorkerJob';
 import { resolveErrorMessage } from '@/utils/resolveErrorMessage';
 
 /** Custom fetch function for `@uswriting/exiftool` to load `zeroperl.wasm` from the worker. */
 const exiftoolParseOptions = { fetch: () => fetch(zeroperlWasmUrl) };
 
+const jobQueue: ExiftoolWorkerJob[] = [];
+let drainPromise: Promise<void> | null = null;
+
 let initialized = false;
 let initializeWorkerPromise: Promise<void> | null = null;
 
@@ -20,8 +24,6 @@ let initializeWorkerPromise: Promise<void> | null = null;
  * We run `parseMetadata` on {@link EXIFTOOL_WORKER_INIT_JPEG} so the library performs
  * a real read without needing a user `File`. Output is discarded; sets {@link initialized}.
  *
- * Concurrent callers (init message, read-metadata, future write) share one in-flight warmup.
- *
  * @throws When warmup `parseMetadata` fails (network, WASM, or ExifTool error).
  * @see Main thread `initializeExiftool()` posts `{ type: 'init' }` when idle.
  */
@@ -54,10 +56,36 @@ const initializeWorker = async (): Promise<void> => {
     return initializeWorkerPromise;
 };
 
-self.addEventListener('message', async (event: MessageEvent<ExiftoolWorkerInMessage>) => {
-    const message = event.data;
+const enqueue = (job: ExiftoolWorkerJob): void => {
+    jobQueue.push(job);
 
-    if (message.type === 'init') {
+    if (!drainPromise) {
+        drainPromise = (async () => {
+            try {
+                await drain();
+            } catch {
+                console.error('Failed to drain ExifTool worker');
+            }
+
+            drainPromise = null;
+        })();
+    }
+};
+
+const drain = async (): Promise<void> => {
+    while (jobQueue.length > 0) {
+        const job = jobQueue.shift();
+
+        if (!job) {
+            return;
+        }
+
+        await processJob(job);
+    }
+};
+
+const processJob = async (job: ExiftoolWorkerJob): Promise<void> => {
+    if (job.type === 'init') {
         try {
             await initializeWorker();
 
@@ -72,25 +100,39 @@ self.addEventListener('message', async (event: MessageEvent<ExiftoolWorkerInMess
         return;
     }
 
-    if (message.type === 'read-metadata') {
+    if (job.type === 'read-metadata') {
         try {
             await initializeWorker();
 
-            const result = await parseMetadata(message.file, exiftoolParseOptions);
+            const result = await parseMetadata(job.file, exiftoolParseOptions);
 
             if (!result.success) {
-                self.postMessage({ type: 'read-metadata-error', id: message.id, message: result.error });
+                self.postMessage({ type: 'read-metadata-error', id: job.id, message: result.error });
 
                 return;
             }
 
-            self.postMessage({ type: 'read-metadata-done', id: message.id, output: result.data });
+            self.postMessage({ type: 'read-metadata-done', id: job.id, output: result.data });
         } catch (error) {
             self.postMessage({
                 type: 'read-metadata-error',
-                id: message.id,
+                id: job.id,
                 message: resolveErrorMessage(error, 'Could not read image metadata')
             });
         }
     }
+};
+
+self.addEventListener('message', (event: MessageEvent<ExiftoolWorkerInMessage>) => {
+    const message = event.data;
+
+    if (message.type === 'init') {
+        enqueue({ type: 'init' });
+
+        return;
+    }
+
+    if (message.type === 'read-metadata') {
+        enqueue({ type: 'read-metadata', id: message.id, file: message.file });
+    }
 });