fix: add vatDecRef to kernel, offer to liveslots

In the future, when liveslots implements GC and discovers that a Presence has
ceased to be referenced by the user-level vat code, it will call this
function to decrement the kernel's reference count for the imported object's
c-list entry.

`vatDecRef` might be called at any time (although always in its own turn).
The kernel will eventually add the decref information to a queue, to be
processed between cranks. For now, the kernel only records the information if
an option was set to enable it (for future unit tests).

Most of this patch is the kernel-worker protocol wiring to allow
child-process vat workers to deliver the decref back up to the kernel
process.

Liveslots does not use this yet. A future patch will switch it on.

refs #1872

packages/SwingSet/src/controller.js

@@ -46,7 +46,12 @@ export async function makeSwingsetController(
   insistStorageAPI(hostStorage);
 
   // build console early so we can add console.log to diagnose early problems
-  const { verbose, debugPrefix = '', slogFile } = runtimeOptions;
+  const {
+    verbose,
+    debugPrefix = '',
+    slogFile,
+    testTrackDecref,
+  } = runtimeOptions;
   if (typeof Compartment === 'undefined') {
     throw Error('SES must be installed before calling makeSwingsetController');
   }
@@ -188,7 +193,7 @@ export async function makeSwingsetController(
     FinalizationRegistry,
   };
 
-  const kernelOptions = { verbose };
+  const kernelOptions = { verbose, testTrackDecref };
   const kernel = buildKernel(kernelEndowments, deviceEndowments, kernelOptions);
 
   if (runtimeOptions.verbose) {
@@ -280,8 +285,9 @@ export async function buildVatController(
     verbose,
     kernelBundles,
     debugPrefix,
+    testTrackDecref,
   } = runtimeOptions;
-  const actualRuntimeOptions = { verbose, debugPrefix };
+  const actualRuntimeOptions = { verbose, debugPrefix, testTrackDecref };
   const initializationOptions = { verbose, kernelBundles };
   let bootstrapResult;
   if (!swingsetIsInitialized(hostStorage)) {

packages/SwingSet/src/kernel/kernel.js

@@ -103,7 +103,7 @@ export default function buildKernel(
     FinalizationRegistry,
   } = kernelEndowments;
   deviceEndowments = { ...deviceEndowments }; // copy so we can modify
-  const { verbose } = kernelOptions;
+  const { verbose, testTrackDecref = false } = kernelOptions;
   const logStartup = verbose ? console.debug : () => 0;
 
   insistStorageAPI(hostStorage);
@@ -144,6 +144,17 @@ export default function buildKernel(
     return kernelSlog.vatConsole(vatID, origConsole);
   }
 
+  const pendingDecrefs = [];
+  function decref(vatID, vref, count) {
+    assert(ephemeral.vats.has(vatID), `unknown vatID ${vatID}`);
+    assert(count > 0, `bad count ${count}`);
+    // TODO: decrement the clist import counter by 'count', then GC if zero
+    if (testTrackDecref) {
+      console.log(`kernel decref [${vatID}].${vref} -= ${count}`);
+      pendingDecrefs.push({ vatID, vref, count });
+    }
+  }
+
   // runQueue entries are {type, vatID, more..}. 'more' depends on type:
   // * deliver: target, msg
   // * notifyFulfillToData/notifyFulfillToPresence/notifyReject:
@@ -531,7 +542,7 @@ export default function buildKernel(
     }
   }
 
-  const gcTools = harden({ WeakRef, FinalizationRegistry });
+  const gcTools = harden({ WeakRef, FinalizationRegistry, decref });
   const vatManagerFactory = makeVatManagerFactory({
     allVatPowers,
     kernelKeeper,
@@ -955,7 +966,7 @@ export default function buildKernel(
       // a time, so any log() calls that were interleaved during their
       // original execution will be sorted by vat in the replace). Logs are
       // not kept in the persistent state, only in ephemeral state.
-      return { log: ephemeral.log, ...kernelKeeper.dump() };
+      return { log: ephemeral.log, pendingDecrefs, ...kernelKeeper.dump() };
     },
     kdebugEnable,
 

packages/SwingSet/src/kernel/vatManager/factory.js

@@ -30,18 +30,21 @@ export function makeVatManagerFactory({
     makeNodeWorker,
     kernelKeeper,
     testLog: allVatPowers.testLog,
+    decref: gcTools.decref,
   });
 
   const nodeSubprocessFactory = makeNodeSubprocessFactory({
     startSubprocessWorker: startSubprocessWorkerNode,
     kernelKeeper,
     testLog: allVatPowers.testLog,
+    decref: gcTools.decref,
   });
 
   const xsWorkerFactory = makeNodeSubprocessFactory({
     startSubprocessWorker: startSubprocessWorkerXS,
     kernelKeeper,
     testLog: allVatPowers.testLog,
+    decref: gcTools.decref,
   });
 
   function validateManagerOptions(managerOptions) {

packages/SwingSet/src/kernel/vatManager/localVatManager.js

@@ -17,6 +17,7 @@ export function makeLocalVatManagerFactory(tools) {
   } = tools;
 
   const { makeGetMeter, refillAllMeters, stopGlobalMeter } = meterManager;
+  const { WeakRef, FinalizationRegistry } = gcTools;
   const baseVP = {
     Remotable: allVatPowers.Remotable,
     getInterfaceOf: allVatPowers.getInterfaceOf,
@@ -105,10 +106,14 @@ export function makeLocalVatManagerFactory(tools) {
       vatParameters,
       testLog: allVatPowers.testLog,
     });
+    function vatDecref(vref, count) {
+      gcTools.decref(vatID, vref, count);
+    }
+    const lsgc = harden({ WeakRef, FinalizationRegistry, vatDecref });
 
     // we might or might not use this, depending upon whether the vat exports
     // 'buildRootObject' or a default 'setup' function
-    const ls = makeLiveSlots(syscall, vatID, vatPowers, vatParameters, gcTools);
+    const ls = makeLiveSlots(syscall, vatID, vatPowers, vatParameters, lsgc);
 
     let meterRecord = null;
     if (metered) {

packages/SwingSet/src/kernel/vatManager/nodeWorker.js

@@ -23,7 +23,7 @@ function parentLog(first, ...args) {
 }
 
 export function makeNodeWorkerVatManagerFactory(tools) {
-  const { makeNodeWorker, kernelKeeper, testLog } = tools;
+  const { makeNodeWorker, kernelKeeper, testLog, decref } = tools;
 
   function createFromBundle(vatID, bundle, managerOptions) {
     const { vatParameters } = managerOptions;
@@ -68,6 +68,10 @@ export function makeNodeWorkerVatManagerFactory(tools) {
       doSyscall(vatSyscallObject);
     }
 
+    function vatDecref(vref, count) {
+      decref(vatID, vref, count);
+    }
+
     // start the worker and establish a connection
 
     const { promise: workerP, resolve: gotWorker } = makePromiseKit();
@@ -107,6 +111,9 @@ export function makeNodeWorkerVatManagerFactory(tools) {
           const deliveryResult = args;
           resolve(deliveryResult);
         }
+      } else if (type === 'decref') {
+        const [vref, count] = args;
+        vatDecref(vref, count);
       } else {
         parentLog(`unrecognized uplink message ${type}`);
       }

packages/SwingSet/src/kernel/vatManager/worker-subprocess-node.js

@@ -24,7 +24,7 @@ function parentLog(first, ...args) {
 }
 
 export function makeNodeSubprocessFactory(tools) {
-  const { startSubprocessWorker, kernelKeeper, testLog } = tools;
+  const { startSubprocessWorker, kernelKeeper, testLog, decref } = tools;
 
   function createFromBundle(vatID, bundle, managerOptions) {
     const { vatParameters } = managerOptions;
@@ -71,6 +71,10 @@ export function makeNodeSubprocessFactory(tools) {
       doSyscall(vatSyscallObject);
     }
 
+    function vatDecref(vref, count) {
+      decref(vatID, vref, count);
+    }
+
     // start the worker and establish a connection
     const { fromChild, toChild, terminate, done } = startSubprocessWorker();
 
@@ -109,6 +113,9 @@ export function makeNodeSubprocessFactory(tools) {
           const deliveryResult = args;
           resolve(deliveryResult);
         }
+      } else if (type === 'decref') {
+        const [vref, count] = args;
+        vatDecref(vref, count);
       } else {
         parentLog(`unrecognized uplink message ${type}`);
       }

packages/SwingSet/test/test-vpid-liveslots.js

@@ -191,7 +191,8 @@ function resolutionOf(vpid, mode, targets) {
 }
 
 function makeDispatch(syscall, build) {
-  const gcTools = harden({ WeakRef, FinalizationRegistry });
+  function vatDecref() {}
+  const gcTools = harden({ WeakRef, FinalizationRegistry, vatDecref });
   const { setBuildRootObject, dispatch } = makeLiveSlots(
     syscall,
     'vatA',