Fix IDBMirrorVFS corrupting the database on rollback

src/examples/IDBMirrorVFS.js

@@ -354,7 +354,10 @@ export class IDBMirrorVFS extends FacadeVFS {
           file.blocks.set(0, newBlock);
           block = newBlock;
         }
-        block.set(pData, iOffset);
+        // pData is a Uint8ArrayProxy, which has no indexed access, so set()
+        // would read undefined at every index and store zeroes. subarray()
+        // returns a real Uint8Array over the same bytes.
+        block.set(pData.subarray(), iOffset);
         file.blockSize = Math.max(file.blockSize, iOffset + pData.byteLength);
       }
       return VFS.SQLITE_OK;

test/IDBMirrorVFS.test.js

@@ -4,6 +4,7 @@ import { vfs_xAccess } from "./vfs_xAccess.js";
 import { vfs_xClose } from "./vfs_xClose.js";
 import { vfs_xRead } from "./vfs_xRead.js";
 import { vfs_xWrite } from "./vfs_xWrite.js";
+import { vfs_rollback } from "./vfs_rollback.js";
 
 const CONFIG = 'IDBMirrorVFS';
 const BUILDS = ['asyncify', 'jspi'];
@@ -22,6 +23,7 @@ describe(CONFIG, function() {
       vfs_xClose(context);
       vfs_xRead(context);
       vfs_xWrite(context);
+      vfs_rollback(context);
     });
   }
 });

test/vfs_rollback.js

@@ -0,0 +1,47 @@
+import * as Comlink from 'comlink';
+
+/**
+ * A rollback of a transaction large enough to spill the page cache. SQLite
+ * then journals the pages it writes, and stamps the journal header once they
+ * are on disk; the rollback reads that header back to undo them.
+ * @param {import('./TestContext.js').TestContext} context
+ */
+export function vfs_rollback(context) {
+  describe('vfs_rollback', function() {
+    let proxy, sqlite3, db;
+    beforeEach(async function() {
+      proxy = await context.create();
+      sqlite3 = proxy.sqlite3;
+      db = await sqlite3.open_v2('rollback-test');
+    });
+
+    afterEach(async function() {
+      await sqlite3.close(db);
+      await context.destroy(proxy);
+    });
+
+    it('should undo a transaction that spilled the page cache', async function() {
+      await sqlite3.exec(db, 'CREATE TABLE t(x)');
+      await sqlite3.exec(db, "INSERT INTO t VALUES ('before')");
+
+      // Large enough that SQLite must write pages before the commit, which
+      // is what makes it journal them.
+      await sqlite3.exec(db, 'BEGIN');
+      await sqlite3.exec(db, `
+        INSERT INTO t WITH RECURSIVE c(x) AS
+          (SELECT 1 UNION ALL SELECT x + 1 FROM c WHERE x < 200000)
+        SELECT x FROM c`);
+      await sqlite3.exec(db, 'ROLLBACK');
+
+      const integrity = [];
+      await sqlite3.exec(db, 'PRAGMA integrity_check',
+        Comlink.proxy(row => { integrity.push(row[0]); }));
+      expect(integrity).toEqual(['ok']);
+
+      const rows = [];
+      await sqlite3.exec(db, 'SELECT x FROM t',
+        Comlink.proxy(row => { rows.push(row[0]); }));
+      expect(rows).toEqual(['before']);
+    });
+  });
+}