Bug 928476 - Add telemetry to measure cross-global adopts. r=mrbkap,nfroyd

dom/bindings/BindingUtils.cpp

@@ -1659,14 +1659,19 @@ ReparentWrapper(JSContext* aCx, JS::Handle<JSObject*> aObjArg)
 
   JSAutoCompartment oldAc(aCx, oldParent);
 
-  if (js::GetObjectCompartment(oldParent) ==
-      js::GetObjectCompartment(newParent)) {
+  JSCompartment* oldCompartment = js::GetObjectCompartment(oldParent);
+  JSCompartment* newCompartment = js::GetObjectCompartment(newParent);
+  if (oldCompartment == newCompartment) {
     if (!JS_SetParent(aCx, aObj, newParent)) {
       MOZ_CRASH();
     }
     return NS_OK;
   }
 
+  // Telemetry.
+  xpc::RecordDonatedNode(oldCompartment);
+  xpc::RecordAdoptedNode(newCompartment);
+
   nsISupports* native = UnwrapDOMObjectToISupports(aObj);
   if (!native) {
     return NS_OK;

js/xpconnect/src/XPCJSRuntime.cpp

@@ -261,9 +261,32 @@ public:
 
 namespace xpc {
 
+static uint32_t kLivingAdopters = 0;
+
+void
+RecordAdoptedNode(JSCompartment *c)
+{
+    CompartmentPrivate *priv = EnsureCompartmentPrivate(c);
+    if (!priv->adoptedNode) {
+        priv->adoptedNode = true;
+        ++kLivingAdopters;
+    }
+}
+
+void
+RecordDonatedNode(JSCompartment *c)
+{
+    EnsureCompartmentPrivate(c)->donatedNode = true;
+}
+
 CompartmentPrivate::~CompartmentPrivate()
 {
     MOZ_COUNT_DTOR(xpc::CompartmentPrivate);
+
+    Telemetry::Accumulate(Telemetry::COMPARTMENT_ADOPTED_NODE, adoptedNode);
+    Telemetry::Accumulate(Telemetry::COMPARTMENT_DONATED_NODE, donatedNode);
+    if (adoptedNode)
+        --kLivingAdopters;
 }
 
 bool CompartmentPrivate::TryParseLocationURI()
@@ -653,6 +676,13 @@ XPCJSRuntime::GCSliceCallback(JSRuntime *rt,
 void
 XPCJSRuntime::CustomGCCallback(JSGCStatus status)
 {
+    // Record kLivingAdopters once per GC to approximate time sampling during
+    // periods of activity.
+    if (status == JSGC_BEGIN) {
+        Telemetry::Accumulate(Telemetry::COMPARTMENT_LIVING_ADOPTERS,
+                              kLivingAdopters);
+    }
+
     nsTArray<xpcGCCallback> callbacks(extraGCCallbacks);
     for (uint32_t i = 0; i < callbacks.Length(); ++i)
         callbacks[i](status);

js/xpconnect/src/xpcprivate.h

@@ -3744,6 +3744,8 @@ public:
     CompartmentPrivate()
         : wantXrays(false)
         , universalXPConnectEnabled(false)
+        , adoptedNode(false)
+        , donatedNode(false)
         , scope(nullptr)
         , locationWasParsed(false)
     {
@@ -3760,6 +3762,10 @@ public:
     // the old scoping rules of enablePrivilege). Using it is inherently unsafe.
     bool universalXPConnectEnabled;
 
+    // for telemetry. See bug 928476.
+    bool adoptedNode;
+    bool donatedNode;
+
     // Our XPCWrappedNativeScope. This is non-null if and only if this is an
     // XPConnect compartment.
     XPCWrappedNativeScope *scope;

js/xpconnect/src/xpcpublic.h

@@ -359,6 +359,12 @@ SystemErrorReporterExternal(JSContext *cx, const char *message,
 NS_EXPORT_(void)
 SimulateActivityCallback(bool aActive);
 
+void
+RecordAdoptedNode(JSCompartment *c);
+
+void
+RecordDonatedNode(JSCompartment *c);
+
 } // namespace xpc
 
 namespace mozilla {

toolkit/components/telemetry/Histograms.json

@@ -28,6 +28,19 @@
     "n_values": 21,
     "description": "Maximum number of concurrent threads reached during a given download session"
   },
+  "COMPARTMENT_DONATED_NODE": {
+    "kind": "boolean",
+    "description": "When a compartment is destroyed, we record whether one of its nodes was ever adopted into another compartment"
+  },
+  "COMPARTMENT_ADOPTED_NODE": {
+    "kind": "boolean",
+    "description": "When a compartment is destroyed, we record whether it had ever adopted a node from another compartment"
+  },
+  "COMPARTMENT_LIVING_ADOPTERS": {
+    "kind": "enumerated",
+    "n_values": 10,
+    "description": "The number of living compartments for which COMPARTMENT_ADOPTED_NODE is true"
+  },
   "CYCLE_COLLECTOR": {
     "kind": "exponential",
     "high": "10000",