[PoC] Direct op-sqlite connections

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

@@ -58,6 +58,10 @@ export class OPSQLiteConnection extends BaseObserver<DBAdapterListener> {
     });
   }
 
+  hasUpdates(): boolean {
+    return this.updateBuffer.length > 0;
+  }
+
   flushUpdates() {
     if (!this.updateBuffer.length) {
       return;

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

@@ -1,4 +1,4 @@
-import { ANDROID_DATABASE_PATH, getDylibPath, IOS_LIBRARY_PATH, open, type DB } from '@op-engineering/op-sqlite';
+import { getDylibPath, open, type DB } from '@op-engineering/op-sqlite';
 import { BaseObserver, DBAdapter, DBAdapterListener, DBLockOptions, QueryResult, Transaction } from '@powersync/common';
 import Lock from 'async-lock';
 import { Platform } from 'react-native';
@@ -158,6 +158,10 @@ export class OPSQLiteDBAdapter extends BaseObserver<DBAdapterListener> implement
 
   async readLock<T>(fn: (tx: OPSQLiteConnection) => Promise<T>, options?: DBLockOptions): Promise<T> {
     await this.initialized;
+    if (this.readConnections?.length == 0) {
+      // When opened with no read connections, use the write connection for reads.
+      return this.writeLock(fn, options);
+    }
     return new Promise(async (resolve, reject) => {
       const execute = async () => {
         // Find an available connection that is not busy
@@ -231,11 +235,11 @@ export class OPSQLiteDBAdapter extends BaseObserver<DBAdapterListener> implement
   }
 
   readTransaction<T>(fn: (tx: Transaction) => Promise<T>, options?: DBLockOptions): Promise<T> {
-    return this.readLock((ctx) => this.internalTransaction(ctx, fn));
+    return this.readLock((ctx) => this.internalTransaction(ctx, 'BEGIN', fn));
   }
 
   writeTransaction<T>(fn: (tx: Transaction) => Promise<T>, options?: DBLockOptions): Promise<T> {
-    return this.writeLock((ctx) => this.internalTransaction(ctx, fn));
+    return this.writeLock((ctx) => this.internalTransaction(ctx, 'BEGIN EXCLUSIVE', fn));
   }
 
   getAll<T>(sql: string, parameters?: any[]): Promise<T[]> {
@@ -264,6 +268,7 @@ export class OPSQLiteDBAdapter extends BaseObserver<DBAdapterListener> implement
 
   protected async internalTransaction<T>(
     connection: OPSQLiteConnection,
+    transactionType: 'BEGIN' | 'BEGIN EXCLUSIVE',
     fn: (tx: Transaction) => Promise<T>
   ): Promise<T> {
     let finalized = false;
@@ -282,7 +287,7 @@ export class OPSQLiteDBAdapter extends BaseObserver<DBAdapterListener> implement
       return connection.execute('ROLLBACK');
     };
     try {
-      await connection.execute('BEGIN');
+      await connection.execute(transactionType);
       const result = await fn({
         execute: (query, params) => connection.execute(query, params),
         executeRaw: (query, params) => connection.executeRaw(query, params),

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

@@ -1,12 +1,15 @@
 import { DBAdapter, SQLOpenFactory, SQLOpenOptions } from '@powersync/common';
 import { OPSQLiteDBAdapter } from './OPSqliteAdapter';
 import { DEFAULT_SQLITE_OPTIONS, SqliteOptions } from './SqliteOptions';
+import { OPSQLiteConnection } from './OPSQLiteConnection';
+import { DB } from '@op-engineering/op-sqlite';
 
 export interface OPSQLiteOpenFactoryOptions extends SQLOpenOptions {
   sqliteOptions?: SqliteOptions;
 }
 export class OPSqliteOpenFactory implements SQLOpenFactory {
   private sqliteOptions: Required<SqliteOptions>;
+  private adapters: Set<OPSQLiteDBAdapter> = new Set();
 
   constructor(protected options: OPSQLiteOpenFactoryOptions) {
     this.sqliteOptions = {
@@ -16,10 +19,85 @@ export class OPSqliteOpenFactory implements SQLOpenFactory {
   }
 
   openDB(): DBAdapter {
-    return new OPSQLiteDBAdapter({
+    const adapter = new OPSQLiteDBAdapter({
       name: this.options.dbFilename,
       dbLocation: this.options.dbLocation,
       sqliteOptions: this.sqliteOptions
     });
+    this.adapters.add(adapter);
+    (adapter as any).abortController.signal.addEventListener('abort', () => {
+      this.adapters.delete(adapter);
+    });
+
+    return adapter;
+  }
+
+  /**
+   * Opens a direct op-sqlite DB connection. This can be used concurrently with PowerSyncDatabase.
+   *
+   * This can be used to execute synchronous queries, or to access other op-sqlite functionality directly.
+   *
+   * Update notifications are propagated to any other PowerSyncDatabase opened with this factory.
+   *
+   * If a write statement or transaction is currently open on any of the other adapters, any
+   * write statements on this connection will block until the others are done. This may create a deadlock,
+   * since this also blocks the JavaScript thread. For that reason, do any write statements in a
+   * writeLock() on the PowerSyncDatabase.
+   *
+   * Read statements can execute concurrently with write statements, so does not have the same risk.
+   *
+   * This is not recommended for most use cases, as synchronous queries block the JavaScript thread,
+   * and the code is not portable to other platforms.
+   */
+  async openDirectConnection(): Promise<DB> {
+    const adapter = new OPSQLiteDBAdapter({
+      name: this.options.dbFilename,
+      dbLocation: this.options.dbLocation,
+      sqliteOptions: {
+        ...this.sqliteOptions,
+        readConnections: 0,
+        // Configure the BUSY_TIMEOUT to be very short, since this is a direct connection.
+        // In general, we should not wait for a lock when using any synchronous queries,
+        // since any locks won't be released while we lock the JS thread.
+        lockTimeoutMs: 50
+      }
+    });
+    await (adapter as any).initialized;
+
+    const connection = (adapter as any).writeConnection as OPSQLiteConnection;
+    connection.registerListener({
+      tablesUpdated: (updateNotification) => {
+        // Pass on to all other adapters.
+        this.adapters.forEach((adapter) => {
+          adapter.iterateListeners((listener) => {
+            listener.tablesUpdated?.(updateNotification);
+          });
+        });
+      }
+    });
+    const database = (connection as any).DB as DB;
+
+    database.commitHook(() => {
+      // This is effectively a "pre-commit" hook, so changes may not actually reflect yet.
+      // To make sure the changes reflect, we first get start a new write transaction (not just a
+      // write lock, since we need to get a lock on the actual SQLite file).
+      const firstAdapter = [...this.adapters][0];
+      if (firstAdapter != null && connection.hasUpdates()) {
+        firstAdapter
+          .writeLock(async (tx) => {
+            // Slightly less overhead than writeTransaction().
+            await tx.execute('BEGIN EXCLUSIVE; ROLLBACK;');
+          })
+          .catch((e) => {
+            // Ignore
+          })
+          .finally(() => {
+            // This triggers the listeners registered above
+            connection.flushUpdates();
+          });
+      }
+    });
+
+    return database;
   }
 }

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

@@ -53,6 +53,11 @@ export interface SqliteOptions {
     path: string;
     entryPoint?: string;
   }>;
+
+  /**
+   * Number of read-only connections to use. Defaults to 5.
+   */
+  readConnections?: number;
 }
 
 export enum TemporaryStorageOption {
@@ -89,5 +94,6 @@ export const DEFAULT_SQLITE_OPTIONS: Required<SqliteOptions> = {
   temporaryStorage: TemporaryStorageOption.MEMORY,
   lockTimeoutMs: 30000,
   encryptionKey: null,
+  readConnections: 5,
   extensions: []
 };