RObust client saving

frontend/sync-client/src/persistence/client-metadata-store.ts

@@ -0,0 +1,55 @@
+import type { StoredClient } from "../sync-client";
+import type { StoredDatabase } from "./database";
+import type { MetadataPersistenceProvider } from "./metadata-persistence-provider";
+import { Lock } from "../utils/data-structures/locks";
+
+// Serialize partial updates to the shared settings, database and notification store
+export class ClientMetadataStore {
+    private readonly lock = new Lock();
+    private reloadBeforeSave = false;
+
+    private constructor(
+        public stored: StoredClient,
+        private readonly persistence: MetadataPersistenceProvider<StoredClient>
+    ) { }
+
+    public static async load(
+        persistence: MetadataPersistenceProvider<StoredClient>
+    ): Promise<ClientMetadataStore> {
+        return new ClientMetadataStore(
+            (await persistence.load()) ?? {},
+            persistence
+        );
+    }
+
+    public async save(update: StoredClient): Promise<void> {
+        await this.lock.withLock(async () => {
+            if (this.reloadBeforeSave) {
+                await this.reload();
+            }
+
+            const next = { ...this.stored, ...structuredClone(update) };
+            try {
+                await this.persistence.save(next);
+                this.stored = next;
+            } catch (error) {
+                // A failed save may have committed. Reload before another update
+                // so settings or notifications cannot roll the database back.
+                this.reloadBeforeSave = true;
+                throw error;
+            }
+        });
+    }
+
+    public async reloadDatabase(): Promise<StoredDatabase | undefined> {
+        return this.lock.withLock(async () => {
+            await this.reload();
+            return this.stored.database;
+        });
+    }
+
+    private async reload(): Promise<void> {
+        this.stored = (await this.persistence.load()) ?? {};
+        this.reloadBeforeSave = false;
+    }
+}