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/swing-store/src/exporter.js

@@ -112,6 +112,33 @@ 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`;
+    key.startsWith('host.') || Fail`key must start with host.`;
+    return sqlKVGet.get(key);
+  }
+
   const sqlGetAllKVData = db.prepare(`
     SELECT key, value
     FROM kvStore
@@ -173,6 +200,7 @@ export function makeSwingStoreExporter(dirPath, options = {}) {
   }
 
   return harden({
+    getHostKV,
     getExportData,
     getArtifactNames,
     getArtifact,

packages/swing-store/test/exports.js

@@ -76,6 +76,9 @@ export function buildData() {
  */
 export function makeExporter(exportData, artifacts) {
   return {
+    getHostKV(_key) {
+      return undefined;
+    },
     async *getExportData() {
       for (const [key, value] of exportData.entries()) {
         /** @type { import('../src/exporter.js').KVPair } */

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

@@ -34,6 +34,8 @@ 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)
@@ -86,6 +88,10 @@ 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);
+
   // exportData
   {
     const exportData = new Map();