fix: export state-sync snapshot without a DB write-lock

Exporting a state-sync snapshot is a read-only operation, and is
designed to run "in the background", i.e. in parallel with normal
mutating operations. It accomplishes this by opening a read-only
transaction right away, effectively capturing a snapshot of the SQLite
database state, to insulate the export process from ongoing writes by
the execution host.

The cosmic-swingset exporter starts with a query of `host.height`, to
confirm that the database has not already advanced to a new block
before this snapshot/read-transaction can be taken.

Previously, this query worked by using `openSwingStore`, and then
calling `hostStorage.hostKVStore.get('host.height')`. This had two
problems:

* TOCTTOU: the `hostKVStore.get` used a different DB connection (and
  different txn) than the exporter, so it might return a different
  height, negating the accuracy of the consistency check

* read-write txn: `openSwingStore` creates a read-*write* txn, even
  when merely opening the DB (because it might need to create the
  initial tables). This txn is closed right away, before
  `openSwingStore()` returns, so it did not present a threat to
  ongoing operations. But if the exporter was created while the
  ongoing execution side already had its own read-write txn
  open (e.g. while `controller.run()` was running), then it would
  fail, and `makeSwingStoreExporter` would fail with `SQLITE_BUSY`

Instead, we take advantage of the new `swingStoreExporter.getHostKV()`
API, and use *it* to fetch `host.height`. Unlike the normal
swingstore, the swingstore-exporter refrains from creating read-write
transactions entirely. So the cosmic-swingset export code can safely
query the height without fear of getting the wrong value or failing
because of an ongoing write transaction.

We think this should fix the SQLITE_BUSY errors.

refs #8523

packages/cosmic-swingset/src/export-kernel-db.js

@@ -14,7 +14,7 @@ import { makePromiseKit } from '@endo/promise-kit';
 import { Fail, q } from '@agoric/assert';
 import { makeAggregateError } from '@agoric/internal';
 import { makeShutdown } from '@agoric/internal/src/node/shutdown.js';
-import { openSwingStore, makeSwingStoreExporter } from '@agoric/swing-store';
+import { makeSwingStoreExporter } from '@agoric/swing-store';
 
 import { isEntrypoint } from './helpers/is-entrypoint.js';
 import { makeProcessValue } from './helpers/process-value.js';
@@ -144,7 +144,6 @@ export const validateExporterOptions = options => {
  * @param {Pick<import('fs/promises'), 'open' | 'writeFile'>} powers.fs
  * @param {import('path')['resolve']} powers.pathResolve
  * @param {typeof import('@agoric/swing-store')['makeSwingStoreExporter']} [powers.makeSwingStoreExporter]
- * @param {typeof import('@agoric/swing-store')['openSwingStore']} [powers.openSwingStore]
  * @param {null | ((...args: any[]) => void)} [powers.log]
  * @returns {StateSyncExporter}
  */
@@ -154,7 +153,6 @@ export const initiateSwingStoreExport = (
     fs: { open, writeFile },
     pathResolve,
     makeSwingStoreExporter: makeExporter = makeSwingStoreExporter,
-    openSwingStore: openDB = openSwingStore,
     log = console.log,
   },
 ) => {
@@ -183,10 +181,7 @@ export const initiateSwingStoreExport = (
     });
     cleanup.push(async () => swingStoreExporter.close());
 
-    const { hostStorage } = openDB(stateDir);
-
-    savedBlockHeight = Number(hostStorage.kvStore.get('host.height')) || 0;
-    await hostStorage.close();
+    savedBlockHeight = Number(swingStoreExporter.getHostKV('host.height')) || 0;
 
     if (blockHeight) {
       blockHeight === savedBlockHeight ||