Auto merge of #14312 - asajeffrey:script-discard-documents, r=cbrewster

Implement discarding Document objects to reclaim space.

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This PR implements document discarding. Active documents are kept alive strongly, but inactive documents are only kept alive weakly. When a document is GCd, it is marked as discarded, and if it is every reactivated, a reload of the URL is triggered.

Note that this PR is pretty aggressive about discarding, and can any inactive document (other than those being kept alive by other same-origin pipelines). We might want to damp it down a bit.

Also note that this interacts with browser.html in that the reloading triggered by reactivating a document triggers mozbrowser events.

To test this, I added a `-Zdiscard-inactive-documents` debug flag, which discards all inactive documents, even ones which are reachable through other same-origin pipelines.

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #14262.
- [X] These changes do not require tests because we should be able to use the existing tests with `-Zdiscard-inactive-documents`.

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components/script/dom/window.rs

@@ -901,18 +901,24 @@ impl Window {
     }
 
     pub fn clear_js_runtime(&self) {
+        // We tear down the active document, which causes all the attached
+        // nodes to dispose of their layout data. This messages the layout
+        // thread, informing it that it can safely free the memory.
         self.Document().upcast::<Node>().teardown();
 
-        // The above code may not catch all DOM objects
-        // (e.g. DOM objects removed from the tree that haven't
-        // been collected yet). Forcing a GC here means that
-        // those DOM objects will be able to call dispose()
-        // to free their layout data before the layout thread
-        // exits. Without this, those remaining objects try to
-        // send a message to free their layout data to the
-        // layout thread when the script thread is dropped,
-        // which causes a panic!
+        // The above code may not catch all DOM objects (e.g. DOM
+        // objects removed from the tree that haven't been collected
+        // yet). There should not be any such DOM nodes with layout
+        // data, but if there are, then when they are dropped, they
+        // will attempt to send a message to the closed layout thread.
+        // This causes memory safety issues, because the DOM node uses
+        // the layout channel from its window, and the window has
+        // already been GC'd.  For nodes which do not have a live
+        // pointer, we can avoid this by GCing now:
         self.Gc();
+        // but there may still be nodes being kept alive by user
+        // script.
+        // TODO: ensure that this doesn't happen!
 
         self.current_state.set(WindowState::Zombie);
         *self.js_runtime.borrow_mut() = None;
@@ -1445,6 +1451,15 @@ impl Window {
         None
     }
 
+    pub fn freeze(&self) {
+        self.upcast::<GlobalScope>().suspend();
+        // A hint to the JS runtime that now would be a good time to
+        // GC any unreachable objects generated by user script,
+        // or unattached DOM nodes. Attached DOM nodes can't be GCd yet,
+        // as the document might be thawed later.
+        self.Gc();
+    }
+
     pub fn thaw(&self) {
         self.upcast::<GlobalScope>().resume();
 

components/script/script_thread.rs

@@ -1346,7 +1346,7 @@ impl ScriptThread {
     fn handle_freeze_msg(&self, id: PipelineId) {
         let window = self.documents.borrow().find_window(id);
         if let Some(window) = window {
-            window.upcast::<GlobalScope>().suspend();
+            window.freeze();
             return;
         }
         let mut loads = self.incomplete_loads.borrow_mut();

components/script/timers.rs

@@ -228,18 +228,20 @@ impl OneshotTimers {
     }
 
     pub fn suspend(&self) {
-        assert!(self.suspended_since.get().is_none());
+        // Suspend is idempotent: do nothing if the timers are already suspended.
+        if self.suspended_since.get().is_some() {
+            return warn!("Suspending an already suspended timer.");
+        }
 
         self.suspended_since.set(Some(precise_time_ms()));
         self.invalidate_expected_event_id();
     }
 
     pub fn resume(&self) {
-        assert!(self.suspended_since.get().is_some());
-
+        // Suspend is idempotent: do nothing if the timers are already suspended.
         let additional_offset = match self.suspended_since.get() {
             Some(suspended_since) => precise_time_ms() - suspended_since,
-            None => panic!("Timers are not suspended.")
+            None => return warn!("Resuming an already resumed timer."),
         };
 
         self.suspension_offset.set(self.suspension_offset.get() + additional_offset);