fix(swingset): retire unreachable orphans

If a kernel object ("koid", the object subset of krefs) is
unreachable, and then becomes orphaned (either because the owning vat
was terminated, or called `syscall.abandonExports`, or was upgraded
and the koid was ephemeral), then retire it immediately.

The argument is that the previously-owning vat can never again talk
about the object, so it can never become reachable again, which is
normally the point at which the owning vat would retire it. But
because the owning vat is dead, it can't retire the koid by itself,
the kernel needs to do the retirement on the vat's behalf.

We now consolidate retirement responsibilities into
processRefcounts(): when terminateVat or syscall.abandonExports use
abandonKernelObjects() to mark a kref as orphaned, it also adds the
kref to maybeFreeKrefs, and then processRefcounts() is responsible for
noticing the kref is both orphaned and unreachable, and then notifying
any importers of its retirement.

I double-checked that cleanupAfterTerminatedVat will always be
followed by a processRefcounts(), by virtue of either being called
from processDeliveryMessage (in the crankResults.terminate clause), or
from within a device invocation syscall (which only happens during a
delivery, so follows the same path). We need this to ensure that any
maybeFreeKrefs created by the cleanup's abandonKernelObjects() will
get processed promptly.

This also changes getObjectRefCount() to tolerate deleted
krefs (i.e. missing `koNN.refCount`) by just returning 0,0. This fixes
a potential kernel panic in the new approach, when a kref is
recognizable by one vat but only reachable by a send-message on the
run-queue, then becomes unreachable as that message is delivered (the
run-queue held the last strong reference), and if the target vat does
syscall.exit during the delivery. The decref pushes the kref onto
maybeFreeKrefs, the terminateVat retires the merely-recognizable
now-orphaned kref, then processRefcounts used getObjectRefCount() to
grab the refcount for the now-retired (and deleted) kref, which
asserted that the koNN.refCount key still existed, which didn't.

This occured in "zoe - secondPriceAuction -- valid input" unit test ,
where the contract did syscall.exit in response to a timer wake()
message sent to a single-use wakeObj.

closes #7212

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

@@ -620,6 +620,9 @@ export default function makeKernelKeeper(
 
   function getObjectRefCount(kernelSlot) {
     const data = kvStore.get(`${kernelSlot}.refCount`);
+    if (!data) {
+      return { reachable: 0, recognizable: 0 };
+    }
     data || Fail`getObjectRefCount(${kernelSlot}) was missing`;
     const [reachable, recognizable] = commaSplit(data).map(Number);
     reachable <= recognizable ||
@@ -949,6 +952,14 @@ export default function makeKernelKeeper(
    *
    */
   function cleanupAfterTerminatedVat(vatID, budget = undefined) {
+    // this is called from terminateVat, which is called from either:
+    // * end of processDeliveryMessage, if crankResults.terminate
+    // * device-vat-admin (when vat-v-a does adminNode.terminateVat)
+    //   (which always happens inside processDeliveryMessage)
+    // so we're always followed by a call to processRefcounts, at
+    // end-of-delivery in processDeliveryMessage, after checking
+    // crankResults.terminate
+
     insistVatID(vatID);
     let cleanups = 0;
     const work = {
@@ -1516,27 +1527,72 @@ export default function makeKernelKeeper(
             deleteKernelPromise(kpid);
           }
         }
+
         if (type === 'object') {
           const { reachable, recognizable } = getObjectRefCount(kref);
           if (reachable === 0) {
-            const ownerVatID = ownerOfKernelObject(kref);
-            if (ownerVatID) {
+            // We avoid ownerOfKernelObject(), which will report
+            // 'undefined' if the owner is dead (and being slowly
+            // deleted). Message delivery should use that, but not us.
+            const ownerKey = `${kref}.owner`;
+            let ownerVatID = kvStore.get(ownerKey);
+            const terminated = terminatedVats.includes(ownerVatID);
+
+            // Some objects that are still owned, but the owning vat
+            // might still alive, or might be terminated and in the
+            // process of being deleted. These two clauses are
+            // mutually exclusive.
+
+            if (ownerVatID && !terminated) {
               const vatKeeper = provideVatKeeper(ownerVatID);
               const isReachable = vatKeeper.getReachableFlag(kref);
               if (isReachable) {
                 // the reachable count is zero, but the vat doesn't realize it
                 actions.add(`${ownerVatID} dropExport ${kref}`);
               }
               if (recognizable === 0) {
-                // TODO: rethink this
+                // TODO: rethink this assert
                 // assert.equal(isReachable, false, `${kref} is reachable but not recognizable`);
                 actions.add(`${ownerVatID} retireExport ${kref}`);
               }
-            } else if (recognizable === 0) {
-              // unreachable, unrecognizable, orphaned: delete the
-              // empty refcount here, since we can't send a GC
-              // action without an ownerVatID
-              deleteKernelObject(kref);
+            } else if (ownerVatID && terminated) {
+              // When we're slowly deleting a vat, and one of its
+              // exports becomes unreferenced, we obviously must not
+              // send dropExports or retireExports into the dead vat.
+              // We fast-forward the abandonment that slow-deletion
+              // would have done, then treat the object as orphaned.
+
+              const { vatSlot } = getReachableAndVatSlot(ownerVatID, kref);
+              // delete directly, not abandonKernelObjects(), which
+              // would re-submit to maybeFreeKrefs
+              kvStore.delete(ownerKey);
+              kvStore.delete(`${ownerVatID}.c.${kref}`);
+              kvStore.delete(`${ownerVatID}.c.${vatSlot}`);
+              // now fall through to the orphaned case
+              ownerVatID = undefined;
+            }
+
+            // Now handle objects which were orphaned. NOTE: this
+            // includes objects which were owned by a terminated (but
+            // not fully deleted) vat, where `ownerVatID` was cleared
+            // in the last line of that previous clause (the
+            // fall-through case). Don't try to change this `if
+            // (!ownerVatID)` into an `else if`: the two clauses are
+            // *not* mutually-exclusive.
+
+            if (!ownerVatID) {
+              // orphaned and unreachable, so retire it
+              if (recognizable) {
+                // there are importers, tell them about the
+                // retirement. retireKernelObjects will also
+                // deleteKernelObject
+                retireKernelObjects([kref]);
+              } else {
+                // just delete, without scanning for importers (since
+                // we know there are none). It would be safe to call
+                // retireKernelObjects instead, just slower.
+                deleteKernelObject(kref);
+              }
             }
           }
         }

packages/SwingSet/test/abandon-export.test.js

@@ -77,7 +77,7 @@ const makeTestVat = async (t, kernel, name, kernelKeeper) => {
     //
     // The new hack is to inject a `{ type: 'negated-gc-action' }`
     // onto the run-queue, which is handled by processDeliveryMessage
-    // but doesn't actually delivery anything.
+    // but doesn't actually deliver anything.
     //
     // A safer approach would be to define the vat's dispatch() to
     // listen for some messages that trigger each of the syscalls we

packages/SwingSet/test/gc-kernel-orphan.test.js

@@ -257,7 +257,7 @@ for (const cause of ['abandon', 'terminate']) {
 // the fix was to change kernelKeeper.getRefCounts to handle a missing
 // koNN.refCounts by just returning 0,0
 
-test('termination plus maybeFreeKrefs', async t => {
+test('termination plus maybeFreeKrefs - dropped', async t => {
   const { kernel, kvStore } = await makeKernel();
   await kernel.start();
 
@@ -267,6 +267,23 @@ test('termination plus maybeFreeKrefs', async t => {
   // it (but doesn't retire it), vatB will self-terminate upon
   // receiving that message, creating two places that try to retire it
 
+  // The order of events will be:
+  // * 'terminate' translated, drops reachable to zero, adds to maybeFreeKrefs
+  // * 'terminate' delivered, delivery marks vat for termination
+  // * post-delivery crankResults.terminate check marks vat as terminated
+  //   (but slow-deletion means nothing is deleted on that crank)
+  // * post-delivery does processRefCounts()
+  //   * that processes ko22/billy, sees 0,1, owner=v2, v2 is terminated
+  //     so it orphans ko22 (removes from vatB c-list and clears .owner)
+  //     and falls through to (owner=undefined) case
+  //     which sees recognizable=1 and retires the object
+  //      (i.e. pushes retireImport gcAction and deletes .owner and .refCounts)
+  // * next crank starts cleanup, walks c-list, orphans ko21/bob
+  //   which adds ko21 to maybeFreeKrefs
+  // * post-cleanup processRefCounts() does ko21, sees 1,1, owner=undefined
+  //   does nothing, since vatA still holds an (orphaned) reference
+  // * cleanup finishes
+
   // Our two root objects (alice and bob) are pinned so they don't
   // disappear while the test is talking to them, so vatB exports
   // "billy".
@@ -339,4 +356,108 @@ test('termination plus maybeFreeKrefs', async t => {
 
   t.is(kvStore.get(`${billyKref}.owner`), undefined);
   t.is(kvStore.get(`${billyKref}.refCounts`), undefined);
+  t.is(kvStore.get(`${vatA}.c.${billyKref}`), undefined);
+  t.is(kvStore.get(`${vatA}.c.${vrefs.billyForA}`), undefined);
+  t.is(kvStore.get(`${vatB}.c.${billyKref}`), undefined);
+  t.is(kvStore.get(`${vatB}.c.${vrefs.billyForB}`), undefined);
+});
+
+// like above, but the object doesn't remain recognizable
+test('termination plus maybeFreeKrefs - retired', async t => {
+  const { kernel, kvStore } = await makeKernel();
+  await kernel.start();
+
+  const vrefs = {}; // track vrefs within vats
+
+  // vatB exports an object to vatA, vatA sends it a message and drops
+  // and retires it, vatB will self-terminate upon receiving that
+  // message. The order of events will be:
+  // * 'terminate' translated, drops refcount to zero, adds to maybeFreeKrefs
+  // * 'terminate' delivered, delivery marks vat for termination
+  // * post-delivery crankResults.terminate check marks vat as terminated
+  //   (but slow-deletion means nothing is deleted on that crank)
+  // * post-delivery does processRefCounts()
+  //   * that processes ko22/billy, sees 0,0, owner=v2, v2 is terminated
+  //     so it orphans ko22 (removes from vatB c-list and clears .owner)
+  //     and falls through to (owner=undefined) case
+  //     which sees recognizable=0 and deletes the object (just .refCount now)
+  // * next crank starts cleanup, walks c-list, orphans ko21/bob
+  //   which adds ko21 to maybeFreeKrefs
+  // * post-cleanup processRefCounts() does ko21, sees 1,1, owner=undefined
+  //   does nothing, since vatA still holds an (orphaned) reference
+  // * cleanup finishes
+
+  vrefs.aliceForA = 'o+100';
+  vrefs.bobForB = 'o+200';
+  vrefs.billyForB = 'o+201';
+  let billyKref;
+
+  let vatA;
+  function setupA(syscall, _state, _helpers, _vatPowers) {
+    function dispatch(vd) {
+      if (vd[0] === 'startVat') {
+        return; // skip startVat
+      }
+      console.log(`deliverA:`, JSON.stringify(vd));
+      if (vd[0] === 'message') {
+        const methargs = kunser(vd[2].methargs);
+        const [method] = methargs;
+        if (method === 'call-billy') {
+          t.is(vd[2].methargs.slots.length, 1);
+          vrefs.billyForA = vd[2].methargs.slots[0];
+          t.is(vrefs.billyForA, 'o-50'); // probably
+          billyKref = kvStore.get(`${vatA}.c.${vrefs.billyForA}`);
+          syscall.send(
+            vrefs.billyForA,
+            kser(['terminate', [kslot(vrefs.billyForA, 'billy-A')]]),
+          );
+          syscall.dropImports([vrefs.billyForA]);
+          syscall.retireImports([vrefs.billyForA]);
+        }
+      }
+    }
+    return dispatch;
+  }
+  await kernel.createTestVat('vatA', setupA);
+  vatA = kernel.vatNameToID('vatA');
+  const alice = kernel.addExport(vatA, vrefs.aliceForA);
+
+  function setupB(syscall, _state, _helpers, _vatPowers) {
+    function dispatch(vd) {
+      if (vd[0] === 'startVat') {
+        return; // skip startVat
+      }
+      console.log(`deliverB:`, JSON.stringify(vd));
+      if (vd[0] === 'message') {
+        const [method] = kunser(vd[2].methargs);
+        if (method === 'init') {
+          vrefs.aliceForB = vd[2].methargs.slots[0];
+          syscall.send(
+            vrefs.aliceForB,
+            kser(['call-billy', [kslot(vrefs.billyForB, 'billy-B')]]),
+          );
+        }
+        if (method === 'terminate') {
+          t.is(vd[2].methargs.slots.length, 1);
+          assert.equal(vd[2].methargs.slots[0], vrefs.billyForB);
+          syscall.exit(false, kser('reason'));
+        }
+      }
+    }
+    return dispatch;
+  }
+  await kernel.createTestVat('vatB', setupB);
+  const vatB = kernel.vatNameToID('vatB');
+  const bob = kernel.addExport(vatB, vrefs.bobForB);
+
+  // this triggers everything, the bug was a kernel crash
+  kernel.queueToKref(bob, 'init', [kslot(alice, 'alice')], 'none');
+  await kernel.run();
+
+  t.is(kvStore.get(`${billyKref}.owner`), undefined);
+  t.is(kvStore.get(`${billyKref}.refCounts`), undefined);
+  t.is(kvStore.get(`${vatA}.c.${billyKref}`), undefined);
+  t.is(kvStore.get(`${vatA}.c.${vrefs.billyForA}`), undefined);
+  t.is(kvStore.get(`${vatB}.c.${billyKref}`), undefined);
+  t.is(kvStore.get(`${vatB}.c.${vrefs.billyForB}`), undefined);
 });