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.

---
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- [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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1 files changed, 24 insertions, 9 deletions

diff --git a/components/script/dom/window.rs b/components/script/dom/window.rs
index 7dd8d71e9c8..37bf817ed7a 100644
--- a/components/script/dom/window.rs
+++ b/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();