fix: await optional file database saves

src/features/ConfigSync/PendingConfigSaves.ts

@@ -0,0 +1,49 @@
+import { compatGlobal, type CompatTimeoutHandle } from "@vrtmrz/livesync-commonlib/compat/common/coreEnvFunctions";
+
+type PendingSave = {
+    save: () => Promise<void>;
+    completion: Promise<void>;
+    resolve: () => void;
+    reject: (error: unknown) => void;
+    timer?: CompatTimeoutHandle;
+    started: boolean;
+};
+
+/** Owns the quiet-period delay and completion of coalesced customisation saves. */
+export class PendingConfigSaves {
+    private readonly pending = new Map<string, PendingSave>();
+
+    enqueue(key: string, save: () => Promise<void>, whenFlushRequested?: Promise<void>): Promise<void> {
+        let group = this.pending.get(key);
+        if (!group) {
+            let resolve!: () => void;
+            let reject!: (error: unknown) => void;
+            const completion = new Promise<void>((onResolve, onReject) => {
+                resolve = onResolve;
+                reject = onReject;
+            });
+            group = { save, completion, resolve, reject, started: false };
+            this.pending.set(key, group);
+        } else {
+            group.save = save;
+            if (group.timer !== undefined) compatGlobal.clearTimeout(group.timer);
+        }
+        const captured = group;
+        group.timer = compatGlobal.setTimeout(() => this.run(key, captured), 100);
+        // Capture this group: a late flush of an earlier event must not release
+        // a newer group which happens to use the same customisation key.
+        void whenFlushRequested?.then(
+            () => this.run(key, captured),
+            () => {}
+        );
+        return group.completion;
+    }
+
+    private run(key: string, group: PendingSave): void {
+        if (group.started) return;
+        group.started = true;
+        if (group.timer !== undefined) compatGlobal.clearTimeout(group.timer);
+        this.pending.delete(key);
+        void Promise.resolve().then(group.save).then(group.resolve, group.reject);
+    }
+}