feat(xsnap): FFI to enable/disable metering (#3480)

API:

```js
    const noLimit = 0;

    const limit = currentMeterLimit();
    const before = resetMeter(noLimit, 0);
    try {
      // unmetered work...
    } finally {
      resetMeter(limit, before);
    }
```

fixes #3457

packages/xsnap/src/xsnap.c

@@ -63,6 +63,8 @@ static void fx_setImmediate(txMachine* the);
 // static void fx_setTimeout(txMachine* the);
 static void fx_setTimer(txMachine* the, txNumber interval, txBoolean repeat);
 static void fx_setTimerCallback(txJob* job);
+static void fx_currentMeterLimit(xsMachine* the);
+static void fx_resetMeter(xsMachine* the);
 
 static void fxFulfillModuleFile(txMachine* the);
 static void fxRejectModuleFile(txMachine* the);
@@ -79,13 +81,15 @@ static char* fxWriteNetStringError(int code);
 // The order of the callbacks materially affects how they are introduced to
 // code that runs from a snapshot, so must be consistent in the face of
 // upgrade.
-#define mxSnapshotCallbackCount 5
+#define mxSnapshotCallbackCount 7
 txCallback gxSnapshotCallbacks[mxSnapshotCallbackCount] = {
 	fx_issueCommand, // 0
 	fx_print, // 1
 	fx_setImmediate, // 2
 	fx_gc, // 3
 	fx_performance_now, // 4
+	fx_currentMeterLimit, // 5
+	fx_resetMeter, // 6
 	// fx_evalScript,
 	// fx_isPromiseJobQueueEmpty,
 	// fx_setInterval,
@@ -559,6 +563,8 @@ void fxBuildAgent(xsMachine* the)
 	slot = fxNextSlotProperty(the, slot, the->stack, xsID("performance"), XS_DONT_ENUM_FLAG);
 	mxPop();
 
+	slot = fxNextHostFunctionProperty(the, slot, fx_currentMeterLimit, 1, xsID("currentMeterLimit"), XS_DONT_ENUM_FLAG);
+	slot = fxNextHostFunctionProperty(the, slot, fx_resetMeter, 1, xsID("resetMeter"), XS_DONT_ENUM_FLAG);
 	// mxPush(mxObjectPrototype);
 	// fxNextHostFunctionProperty(the, fxLastProperty(the, fxNewObjectInstance(the)), fx_print, 1, xsID("log"), XS_DONT_ENUM_FLAG);
 	// slot = fxNextSlotProperty(the, slot, the->stack, xsID("console"), XS_DONT_ENUM_FLAG);
@@ -907,6 +913,26 @@ void fx_print(xsMachine* the)
 		fflush(stdout);
 }
 
+static void fx_currentMeterLimit(xsMachine* the)
+{
+#if mxMetering
+	xsResult = xsInteger(gxCurrentMeter);
+#endif
+}
+
+static void fx_resetMeter(xsMachine* the)
+{
+#if mxMetering
+	xsIntegerValue argc = xsToInteger(xsArgc);
+	if (argc < 2) {
+		xsTypeError("expected newMeterLimit, newMeterIndex");
+	}
+	xsResult = xsInteger(the->meterIndex);
+	gxCurrentMeter = xsToInteger(xsArg(0));
+	the->meterIndex = xsToInteger(xsArg(1));
+#endif
+}
+
 // void fx_setInterval(txMachine* the)
 // {
 // 	fx_setTimer(the, fxToNumber(the, mxArgv(1)), 1);

packages/xsnap/test/test-xs-perf.js

@@ -133,3 +133,49 @@ test('high resolution timer', async t => {
   t.log({ milliseconds, date: new Date(milliseconds) });
   t.is('number', typeof milliseconds);
 });
+
+test('metering can be switched off / on at run-time', async t => {
+  const opts = options(io);
+  const vat = xsnap(opts);
+  t.teardown(() => vat.terminate());
+  const { meterUsage: { compute: noUnMeteredCompute } } = await vat.evaluate(`
+      for (let work=0; work < 1000; work++) {}
+      const limit = currentMeterLimit();
+      const before = resetMeter(0, 0);
+      try {
+        // nothing
+      } finally {
+        resetMeter(limit, before);
+      }
+      for (let work=0; work < 1000; work++) {}
+    `);
+  const { meterUsage: { compute: someUnMeteredCompute } } = await vat.evaluate(`
+    for (let work=0; work < 1000; work++) {}
+    const limit = currentMeterLimit();
+    const before = resetMeter(0, 0);
+    try {
+      for (let work=0; work < 2000; work++) {}
+    } finally {
+      resetMeter(limit, before);
+    }
+    for (let work=0; work < 1000; work++) {}
+  `);
+  t.log({ noUnMeteredCompute, someUnMeteredCompute });
+  t.is(noUnMeteredCompute, someUnMeteredCompute);
+});
+
+test('metering switch - start compartment only', async t => {
+  const opts = options(io);
+  const vat = xsnap(opts);
+  await vat.evaluate(`
+    const send = it => issueCommand(ArrayBuffer.fromString(it));
+    resetMeter(0, 0);
+    try {
+      (new Compartment()).evalate('resetMeter(0, 0)');
+    } catch (_err) {
+      send('no meteringSwitch in Compartment');
+    }
+  `);
+  await vat.close();
+  t.deepEqual(['no meteringSwitch in Compartment'], opts.messages);
+});