fix(db): tryHardRead's replica leg doesn't fail over to the primary

MySQL's read() retries on the primary after a load-shed or transient
replica error, so routing tryHardRead through it read the primary twice
on that path. The replica leg now reads the replica alone.

src/backend/clients/database/DatabaseClient.ts

@@ -119,6 +119,17 @@ export class AbstractDatabaseClient extends PuterClient {
         return false;
     }
 
+    /**
+     * The replica leg of `tryHardRead`. Override when `read()` falls back to
+     * the primary by itself, or a replica failure reads the primary twice.
+     */
+    protected replicaRead(
+        query: string,
+        params: unknown[],
+    ): Promise<Record<string, unknown>[]> {
+        return this.read(query, params);
+    }
+
     /**
      * Like `read()`, but an empty or failed replica read falls back to the
      * primary, so replication lag can't hide a row that was just written. The
@@ -134,7 +145,7 @@ export class AbstractDatabaseClient extends PuterClient {
 
         const primary = settle(this.pread(query, params));
         try {
-            const rows = await this.read(query, params);
+            const rows = await this.replicaRead(query, params);
             if (rows.length > 0) return rows;
         } catch {
             // replica failed — fall through to primary

src/backend/clients/database/MySQLDatabaseClient.test.ts

@@ -619,6 +619,21 @@ describe('MySQLDatabaseClient — tryHardRead', () => {
             { from: 'primary' },
         ]);
     });
+
+    it('reads the primary once when the replica fails in a way read() fails over on', async () => {
+        const replica = {
+            execute: vi.fn().mockRejectedValue(codedError('dbBatchFailed')),
+        };
+        const primary = {
+            execute: vi.fn().mockResolvedValue([[{ from: 'primary' }]]),
+        };
+        const client = makeClient({ replica, primary });
+
+        await expect(client.tryHardRead('SELECT 1')).resolves.toEqual([
+            { from: 'primary' },
+        ]);
+        expect(primary.execute).toHaveBeenCalledTimes(1);
+    });
 });
 
 describe('MySQLDatabaseClient — migrations', () => {

src/backend/clients/database/MySQLDatabaseClient.ts

@@ -205,6 +205,16 @@ export class MySQLDatabaseClient extends AbstractDatabaseClient {
         return this.configuration === Configuration.REPLICA;
     }
 
+    // No failover: `tryHardRead` already has a primary read in flight.
+    protected override async replicaRead(
+        query: string,
+        params: unknown[],
+    ): Promise<Record<string, unknown>[]> {
+        const result = await this.dbReplica.execute(query, params);
+        if (!result) return [];
+        return (result[0] as Record<string, unknown>[]) ?? [];
+    }
+
     // ------------------------------------------------------------------
     // Pool management
     // ------------------------------------------------------------------