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

@@ -303,6 +303,18 @@ export default function makeKernelKeeper(kvStore, streamStore, kernelSlog) {
     return owner;
   }
 
+  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(nvID => nvID[1]).filter(doesImport));
+    importers.sort();
+    return importers;
+  }
+
   function deleteKernelObject(koid) {
     kvStore.delete(`${koid}.owner`);
     kvStore.delete(`${koid}.refCount`);
@@ -1013,6 +1025,7 @@ export default function makeKernelKeeper(kvStore, streamStore, kernelSlog) {
     addKernelObject,
     ownerOfKernelObject,
     ownerOfKernelDevice,
+    getImporters,
     deleteKernelObject,
 
     addKernelPromise,

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

@@ -145,6 +145,19 @@ export function makeVatKeeper(
     }
   }
 
+  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
@@ -422,6 +435,7 @@ export function makeVatKeeper(
   return harden({
     setSourceAndOptions,
     getSourceAndOptions,
+    importsKernelSlot,
     mapVatSlotToKernelSlot,
     mapKernelSlotToVatSlot,
     getReachableFlag,