feat(xsnap): enable gc() in the start compartment

Modify xsnap.c to add a `gc()` function to the globals of the
initial ("start") Compartment. This function should trigger an immediate,
synchronous, full GC sweep. As a non-standard global, the `gc()` function
will be filtered out of the globals in all child Compartments by SES as
usual.

Note that this changes the snapshot format: heap snapshots written before
this change cannot be read by code after this change. This happens because
`gc()` (which is implemented in C) is a new "callback" (a C function made
available to JS code), which is an "exit" from the reference graph. It must
be recognized during serialization, and re-attached during reload, and xsnap
cannot handle loading snapshots with a different set of exits, even purely
additive changes.

closes #2682
refs #2660
refs #2615

packages/xsnap/src/xsnap.c

@@ -55,7 +55,7 @@ static void fx_Array_prototype_meter(xsMachine* the);
 extern void fx_clearTimer(txMachine* the);
 static void fx_destroyTimer(void* data);
 // static void fx_evalScript(xsMachine* the);
-// static void fx_gc(xsMachine* the);
+static void fx_gc(xsMachine* the);
 // static void fx_isPromiseJobQueueEmpty(xsMachine* the);
 static void fx_markTimer(txMachine* the, void* it, txMarkRoot markRoot);
 static void fx_print(xsMachine* the);
@@ -80,13 +80,13 @@ 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 4
+#define mxSnapshotCallbackCount 5
 txCallback gxSnapshotCallbacks[mxSnapshotCallbackCount] = {
 	fx_issueCommand, // 0
 	fx_Array_prototype_meter, // 1
 	fx_print, // 2
 	fx_setImmediate, // 3
-	// fx_gc,
+	fx_gc, // 4
 	// fx_evalScript,
 	// fx_isPromiseJobQueueEmpty,
 	// fx_setInterval,
@@ -517,7 +517,7 @@ void fxBuildAgent(xsMachine* the)
 	// slot = fxNextHostFunctionProperty(the, slot, fx_clearTimer, 1, xsID("clearInterval"), XS_DONT_ENUM_FLAG);
 	// slot = fxNextHostFunctionProperty(the, slot, fx_clearTimer, 1, xsID("clearTimeout"), XS_DONT_ENUM_FLAG);
 	// slot = fxNextHostFunctionProperty(the, slot, fx_evalScript, 1, xsID("evalScript"), XS_DONT_ENUM_FLAG);
-	// slot = fxNextHostFunctionProperty(the, slot, fx_gc, 1, xsID("gc"), XS_DONT_ENUM_FLAG);
+	slot = fxNextHostFunctionProperty(the, slot, fx_gc, 1, xsID("gc"), XS_DONT_ENUM_FLAG);
 	// slot = fxNextHostFunctionProperty(the, slot, fx_isPromiseJobQueueEmpty, 1, xsID("isPromiseJobQueueEmpty"), XS_DONT_ENUM_FLAG);
 	slot = fxNextHostFunctionProperty(the, slot, fx_print, 1, xsID("print"), XS_DONT_ENUM_FLAG);
 	slot = fxNextHostFunctionProperty(the, slot, fx_setImmediate, 1, xsID("setImmediate"), XS_DONT_ENUM_FLAG);
@@ -870,10 +870,10 @@ void fxPrintUsage()
 // 	mxPullSlot(mxResult);
 // }
 
-// void fx_gc(xsMachine* the)
-// {
-// 	xsCollectGarbage();
-// }
+void fx_gc(xsMachine* the)
+{
+	xsCollectGarbage();
+}
 
 // void fx_isPromiseJobQueueEmpty(txMachine* the)
 // {

packages/xsnap/test/test-xsnap.js

@@ -196,6 +196,23 @@ test('print - start compartment only', async t => {
   t.deepEqual(['no print in Compartment'], opts.messages);
 });
 
+test('gc - start compartment only', async t => {
+  const opts = options();
+  const vat = xsnap(opts);
+  await vat.evaluate(`
+    gc();
+    const send = it => issueCommand(ArrayBuffer.fromString(it));
+    print(123);
+    try {
+      (new Compartment()).evalate('gc()');
+    } catch (_err) {
+      send('no gc in Compartment');
+    }
+  `);
+  await vat.close();
+  t.deepEqual(['no gc in Compartment'], opts.messages);
+});
+
 test('run script until idle', async t => {
   const opts = options();
   const vat = xsnap(opts);