Feat: Queue read connections OPSQLite

.changeset/kind-cats-check.md

@@ -0,0 +1,5 @@
+---
+'@powersync/op-sqlite': patch
+---
+
+Improved queueing for read connections

packages/powersync-op-sqlite/src/db/OPSqliteAdapter.ts

@@ -35,10 +35,12 @@ export class OPSQLiteDBAdapter extends BaseObserver<DBAdapterListener> implement
 
   protected initialized: Promise<void>;
 
-  protected readConnections: OPSQLiteConnection[] | null;
+  protected readConnections: Array<{ busy: boolean; connection: OPSQLiteConnection }> | null;
 
   protected writeConnection: OPSQLiteConnection | null;
 
+  private readQueue: Array<() => void> = [];
+
   constructor(protected options: OPSQLiteAdapterOptions) {
     super();
     this.name = this.options.name;
@@ -88,7 +90,7 @@ export class OPSQLiteDBAdapter extends BaseObserver<DBAdapterListener> implement
       let dbName = './'.repeat(i + 1) + dbFilename;
       const conn = await this.openConnection(dbName);
       await conn.execute('PRAGMA query_only = true');
-      this.readConnections.push(conn);
+      this.readConnections.push({ busy: false, connection: conn });
     }
   }
 
@@ -145,36 +147,46 @@ export class OPSQLiteDBAdapter extends BaseObserver<DBAdapterListener> implement
   close() {
     this.initialized.then(() => {
       this.writeConnection!.close();
-      this.readConnections!.forEach((c) => c.close());
+      this.readConnections!.forEach((c) => c.connection.close());
     });
   }
 
   async readLock<T>(fn: (tx: OPSQLiteConnection) => Promise<T>, options?: DBLockOptions): Promise<T> {
     await this.initialized;
-    // TODO: Use async queues to handle multiple read connections
-    const sortedConnections = this.readConnections!.map((connection, index) => ({
-      lockKey: `${LockType.READ}-${index}`,
-      connection
-    })).sort((a, b) => {
-      const aBusy = this.locks.isBusy(a.lockKey);
-      const bBusy = this.locks.isBusy(b.lockKey);
-      // Sort by ones which are not busy
-      return aBusy > bBusy ? 1 : 0;
+    return new Promise(async (resolve, reject) => {
+      const execute = async () => {
+        // Find an available connection that is not busy
+        const availableConnection = this.readConnections!.find((conn) => !conn.busy);
+
+        // If we have an available connection, use it
+        if (availableConnection) {
+          availableConnection.busy = true;
+          try {
+            resolve(await fn(availableConnection.connection));
+          } catch (error) {
+            reject(error);
+          } finally {
+            availableConnection.busy = false;
+            // After query execution, process any queued tasks
+            this.processQueue();
+          }
+        } else {
+          // If no available connections, add to the queue
+          this.readQueue.push(execute);
+        }
+      };
+
+      execute();
     });
+  }
 
-    return new Promise(async (resolve, reject) => {
-      try {
-        await this.locks.acquire(
-          sortedConnections[0].lockKey,
-          async () => {
-            resolve(await fn(sortedConnections[0].connection));
-          },
-          { timeout: options?.timeoutMs }
-        );
-      } catch (ex) {
-        reject(ex);
+  private async processQueue(): Promise<void> {
+    if (this.readQueue.length > 0) {
+      const next = this.readQueue.shift();
+      if (next) {
+        next();
       }
-    });
+    }
   }
 
   async writeLock<T>(fn: (tx: OPSQLiteConnection) => Promise<T>, options?: DBLockOptions): Promise<T> {