chore(swingset): add kernelKeeper.getImporters

This function takes an object kref and returns a list of vatIDs which have
imported that object. As mentioned in #3223, this would be more efficient
with an index.

packages/SwingSet/src/kernel/state/kernelKeeper.js

@@ -788,6 +788,22 @@ export default function makeKernelKeeper(kvStore, streamStore, kernelSlog) {
     return harden(deviceIDs);
   }
 
+  function getImporters(koid) {
+    // TODO maintain an index instead of scanning every single vat
+    const importers = [];
+    function doesImport(vatID) {
+      return getVatKeeper(vatID).importsKernelSlot(koid);
+    }
+    importers.push(...getDynamicVats().filter(doesImport));
+    importers.push(
+      ...getStaticVats()
+        .map(nameAndVatID => nameAndVatID[1])
+        .filter(doesImport),
+    );
+    importers.sort();
+    return importers;
+  }
+
   // used for debugging, and tests. This returns a JSON-serializable object.
   // It includes references to live (mutable) kernel state, so don't mutate
   // the pieces, and be sure to serialize/deserialize before passing it
@@ -891,6 +907,7 @@ export default function makeKernelKeeper(kvStore, streamStore, kernelSlog) {
     addKernelObject,
     ownerOfKernelObject,
     ownerOfKernelDevice,
+    getImporters,
     deleteKernelObject,
 
     addKernelPromise,

packages/SwingSet/src/kernel/state/vatKeeper.js

@@ -118,6 +118,19 @@ export function makeVatKeeper(
     kvStore.set(kernelKey, buildReachableAndVatSlot(false, vatSlot));
   }
 
+  function importsKernelSlot(kernelSlot) {
+    const kernelKey = `${vatID}.c.${kernelSlot}`;
+    const data = kvStore.get(kernelKey);
+    if (data) {
+      const { vatSlot } = parseReachableAndVatSlot(data);
+      const { allocatedByVat } = parseVatSlot(vatSlot);
+      if (!allocatedByVat) {
+        return true;
+      }
+    }
+    return false;
+  }
+
   /**
    * Provide the kernel slot corresponding to a given vat slot, allocating a
    * new one (for exports only) if it doesn't already exist. If we're allowed
@@ -378,6 +391,7 @@ export function makeVatKeeper(
   return harden({
     setSourceAndOptions,
     getSourceAndOptions,
+    importsKernelSlot,
     mapVatSlotToKernelSlot,
     mapKernelSlotToVatSlot,
     getReachableFlag,

packages/SwingSet/test/test-clist.js

@@ -91,3 +91,43 @@ test(`clist reachability`, async t => {
   t.is(vk.mapVatSlotToKernelSlot('o+3', false), 'ko22');
   t.is(s.get(`${vatID}.c.ko22`), '_ o+3');
 });
+
+test('getImporters', async t => {
+  const slog = makeDummySlogger({});
+  const hostStorage = initSimpleSwingStore();
+  const { enhancedCrankBuffer: s } = wrapStorage(hostStorage.kvStore);
+
+  const kk = makeKernelKeeper(s, hostStorage.streamStore, slog);
+  kk.createStartingKernelState('local');
+  const vatID1 = kk.allocateUnusedVatID();
+  kk.addDynamicVatID(vatID1);
+  const vk1 = kk.allocateVatKeeper(vatID1);
+  const vatID2 = kk.allocateUnusedVatID();
+  kk.addDynamicVatID(vatID2);
+  const vk2 = kk.allocateVatKeeper(vatID2);
+  const vatID3 = kk.allocateUnusedVatID();
+  kk.addDynamicVatID(vatID3);
+  const vk3 = kk.allocateVatKeeper(vatID3);
+
+  const kref = kk.addKernelObject('v1', 1);
+  t.deepEqual(kk.getImporters(kref), []);
+
+  const vref1 = vk1.mapKernelSlotToVatSlot(kref);
+  t.deepEqual(kk.getImporters(kref), [vatID1]);
+
+  // add 3 before 2 to check that the result is really sorted
+  const vref3 = vk3.mapKernelSlotToVatSlot(kref);
+  t.deepEqual(kk.getImporters(kref), [vatID1, vatID3]);
+
+  const vref2 = vk2.mapKernelSlotToVatSlot(kref);
+  t.deepEqual(kk.getImporters(kref), [vatID1, vatID2, vatID3]);
+
+  vk3.deleteCListEntry(kref, vref3);
+  t.deepEqual(kk.getImporters(kref), [vatID1, vatID2]);
+
+  vk1.deleteCListEntry(kref, vref1);
+  t.deepEqual(kk.getImporters(kref), [vatID2]);
+
+  vk2.deleteCListEntry(kref, vref2);
+  t.deepEqual(kk.getImporters(kref), []);
+});