feat: add exporter.getHostKV() API

Add a new API to the exporter, `exporter.getHostKV(key)`, so that the
cosmic-swingset state-sync exporter process can query `host.height`
without accidentally creating a read-write transaction too.

refs #8523

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

@@ -145,6 +145,9 @@ export const performStateSyncImport = async (
   // Represent the data in `exportDir` as a SwingSetExporter object.
   /** @type {import('@agoric/swing-store').SwingStoreExporter} */
   const exporter = harden({
+    getHostKV(_key) {
+      return undefined;
+    },
     async *getExportData() {
       log?.('importing export data');
       const exportData = createReadStream(

packages/swing-store/src/exporter.js

@@ -36,6 +36,11 @@ import { validateArtifactMode } from './internal.js';
  * the concurrent activity of other swingStore instances, the data representing
  * the commit point will stay consistent and available.
  *
+ * @property {(key: string) => string | undefined} getHostKV
+ *
+ * Retrieve a value from the "host" portion of the kvStore, just like
+ * hostStorage.hostKVStore.get() would do.
+ *
  * @property {() => AnyIterableIterator<KVPair>} getExportData
  *
  * Get a full copy of the first-stage export data (key-value pairs) from the
@@ -112,6 +117,33 @@ export function makeSwingStoreExporter(dirPath, options = {}) {
     assertComplete(internal, artifactMode);
   }
 
+  const sqlKVGet = db.prepare(`
+    SELECT value
+    FROM kvStore
+    WHERE key = ?
+  `);
+  sqlKVGet.pluck(true);
+
+  /**
+   * Obtain the value stored for a given host key. This is for the
+   * benefit of clients who need to briefly query the DB to ensure
+   * they are exporting the right thing, and need to avoid modifying
+   * anything (or creating a read-write DB lock) in the process.
+   *
+   * @param {string} key  The key whose value is sought.
+   *
+   * @returns {string | undefined} the (string) value for the given key, or
+   *    undefined if there is no such value.
+   *
+   * @throws if key is not a string, or the key is not in the host
+   * section
+   */
+  function getHostKV(key) {
+    typeof key === 'string' || Fail`key must be a string`;
+    getKeyType(key) === 'host' || Fail`getHostKV requires host keys`;
+    return sqlKVGet.get(key);
+  }
+
   const sqlGetAllKVData = db.prepare(`
     SELECT key, value
     FROM kvStore
@@ -173,6 +205,7 @@ export function makeSwingStoreExporter(dirPath, options = {}) {
   }
 
   return harden({
+    getHostKV,
     getExportData,
     getArtifactNames,
     getArtifact,

packages/swing-store/test/test-bundles.js

@@ -107,6 +107,9 @@ test('b0 import', async t => {
   const idA = makeB0ID(b0A);
   const nameA = `bundle.${idA}`;
   const exporter = {
+    getHostKV(_key) {
+      return undefined;
+    },
     async *getExportData() {
       yield /** @type {const} */ ([nameA, idA]);
     },
@@ -135,6 +138,9 @@ test('b0 bad import', async t => {
   const idA = makeB0ID(b0A);
   const nameA = `bundle.${idA}`;
   const exporter = {
+    getHostKV(_key) {
+      return undefined;
+    },
     async *getExportData() {
       yield /** @type {const} */ ([nameA, idA]);
     },

packages/swing-store/test/test-export.js

@@ -36,6 +36,8 @@ const exportTest = test.macro(async (t, mode) => {
   const ss1 = initSwingStore(dbDir, options);
   const ks = ss1.kernelStorage;
 
+  ss1.hostStorage.kvStore.set('host.h1', 'hostvalue1');
+
   // build a DB with four spans (one in an old incarnation, two
   // historical but current incarnation, only one inUse) and two
   // snapshots (only one inUSe)
@@ -88,6 +90,13 @@ const exportTest = test.macro(async (t, mode) => {
   }
   const exporter = makeSwingStoreExporter(dbDir, { artifactMode });
 
+  // hostKV
+  t.is(exporter.getHostKV('host.h1'), 'hostvalue1');
+  t.is(exporter.getHostKV('host.hmissing'), undefined);
+  t.throws(() => exporter.getHostKV('nonhost'), {
+    message: 'getHostKV requires host keys',
+  });
+
   // exportData
   {
     const exportData = new Map();

packages/swing-store/test/test-import.js

@@ -54,6 +54,9 @@ function convert(orig) {
  */
 export function makeExporter(exportData, artifacts) {
   return {
+    getHostKV(_key) {
+      return undefined;
+    },
     async *getExportData() {
       for (const [key, value] of exportData.entries()) {
         /** @type { KVPair } */