auto merge of #5228 : metajack/servo/memory-debug, r=pcwalton

Cycles were being created in the flow tree since absolutely positioned
descendants had pointers to their containing blocks. This adds
WeakFlowRef (based on Weak<T>) and makes these backpointers weak
references. This also harmonizes our custom Arc<T>, FlowRef, to be
consistent with the upstream implementation.

Fixes #4915.

components/layout/flow.rs

@@ -31,7 +31,7 @@ use context::LayoutContext;
 use display_list_builder::DisplayListBuildingResult;
 use floats::Floats;
 use flow_list::{FlowList, FlowListIterator, MutFlowListIterator};
-use flow_ref::FlowRef;
+use flow_ref::{FlowRef, WeakFlowRef};
 use fragment::{Fragment, FragmentBorderBoxIterator, SpecificFragmentInfo};
 use incremental::{self, RECONSTRUCT_FLOW, REFLOW, REFLOW_OUT_OF_FLOW, RestyleDamage};
 use inline::InlineFlow;
@@ -740,7 +740,9 @@ pub struct BaseFlow {
     /// NB: Must be the first element.
     ///
     /// The necessity of this will disappear once we have dynamically-sized types.
-    ref_count: AtomicUint,
+    strong_ref_count: AtomicUint,
+
+    weak_ref_count: AtomicUint,
 
     pub restyle_damage: RestyleDamage,
 
@@ -887,7 +889,8 @@ impl Encodable for BaseFlow {
 #[unsafe_destructor]
 impl Drop for BaseFlow {
     fn drop(&mut self) {
-        if self.ref_count.load(Ordering::SeqCst) != 0 {
+        if self.strong_ref_count.load(Ordering::SeqCst) != 0 &&
+           self.weak_ref_count.load(Ordering::SeqCst) != 0 {
             panic!("Flow destroyed before its ref count hit zero—this is unsafe!")
         }
     }
@@ -948,7 +951,8 @@ impl BaseFlow {
         damage.remove(RECONSTRUCT_FLOW);
 
         BaseFlow {
-            ref_count: AtomicUint::new(1),
+            strong_ref_count: AtomicUint::new(1),
+            weak_ref_count: AtomicUint::new(1),
             restyle_damage: damage,
             children: FlowList::new(),
             intrinsic_inline_sizes: IntrinsicISizes::new(),
@@ -978,8 +982,12 @@ impl BaseFlow {
         self.children.iter_mut()
     }
 
-    pub unsafe fn ref_count<'a>(&'a self) -> &'a AtomicUint {
-        &self.ref_count
+    pub unsafe fn strong_ref_count<'a>(&'a self) -> &'a AtomicUint {
+        &self.strong_ref_count
+    }
+
+    pub unsafe fn weak_ref_count<'a>(&'a self) -> &'a AtomicUint {
+        &self.weak_ref_count
     }
 
     pub fn debug_id(&self) -> uint {
@@ -1333,7 +1341,7 @@ impl MutableOwnedFlowUtils for FlowRef {
 /// FIXME(pcwalton): I think this would be better with a borrow flag instead of `unsafe`.
 pub struct ContainingBlockLink {
     /// The pointer up to the containing block.
-    link: Option<FlowRef>,
+    link: Option<WeakFlowRef>,
 }
 
 impl ContainingBlockLink {
@@ -1344,10 +1352,10 @@ impl ContainingBlockLink {
     }
 
     fn set(&mut self, link: FlowRef) {
-        self.link = Some(link)
+        self.link = Some(link.downgrade())
     }
 
-    pub unsafe fn get<'a>(&'a mut self) -> &'a mut Option<FlowRef> {
+    pub unsafe fn get<'a>(&'a mut self) -> &'a mut Option<WeakFlowRef> {
         &mut self.link
     }
 

components/layout/flow_ref.rs

@@ -4,18 +4,21 @@
 
 //! Reference-counted pointers to flows.
 //!
-//! Eventually, with dynamically sized types in Rust, much of this code will be superfluous.
+//! Eventually, with dynamically sized types in Rust, much of this code will
+//! be superfluous. This design is largely duplicating logic of Arc<T> and
+//! Weak<T>; please see comments there for details.
 
 #![allow(unsafe_blocks)]
 
 use flow::Flow;
 use flow;
 
+use alloc::heap;
 use std::mem;
 use std::ops::{Deref, DerefMut};
 use std::ptr;
 use std::raw;
-use std::sync::atomic::Ordering;
+use std::sync::atomic::{self, Ordering};
 
 #[unsafe_no_drop_flag]
 pub struct FlowRef {
@@ -25,6 +28,14 @@ pub struct FlowRef {
 unsafe impl Send for FlowRef {}
 unsafe impl Sync for FlowRef {}
 
+#[unsafe_no_drop_flag]
+pub struct WeakFlowRef {
+    object: raw::TraitObject,
+}
+
+unsafe impl Send for WeakFlowRef {}
+unsafe impl Sync for WeakFlowRef {}
+
 impl FlowRef {
     pub fn new(mut flow: Box<Flow>) -> FlowRef {
         unsafe {
@@ -37,6 +48,14 @@ impl FlowRef {
             result
         }
     }
+
+    /// Downgrades the FlowRef to a WeakFlowRef.
+    pub fn downgrade(&self) -> WeakFlowRef {
+        unsafe {
+            flow::base(&**self).weak_ref_count().fetch_add(1, Ordering::Relaxed);
+        }
+        WeakFlowRef { object: self.object }
+    }
 }
 
 impl<'a> Deref for FlowRef {
@@ -62,19 +81,41 @@ impl Drop for FlowRef {
             if self.object.vtable.is_null() {
                 return
             }
-            if flow::base(&**self).ref_count().fetch_sub(1, Ordering::SeqCst) > 1 {
+            if flow::base(&**self).strong_ref_count().fetch_sub(1, Ordering::Release) != 1 {
                 return
             }
+            atomic::fence(Ordering::Acquire);
+
+            // Normally we'd call the underlying Drop logic but not free the
+            // allocation, but this is almost impossible without DST in
+            // Rust. Instead we make a fake trait object to run the drop logic
+            // on.
             let flow_ref: FlowRef = mem::replace(self, FlowRef {
                 object: raw::TraitObject {
                     vtable: ptr::null_mut(),
                     data: ptr::null_mut(),
                 }
             });
-            drop(mem::transmute::<raw::TraitObject, Box<Flow>>(flow_ref.object));
+
+            let vtable: &[usize; 3] = mem::transmute::<*mut (), &[usize; 3]>(flow_ref.object.vtable);
+            let object_size = vtable[1];
+            let object_align = vtable[2];
+
+            let fake_data = heap::allocate(object_size, object_align);
+            ptr::copy_memory(fake_data,
+                             flow_ref.object.data as *const u8,
+                             object_size);
+
+            let fake_box = raw::TraitObject { vtable: flow_ref.object.vtable, data: fake_data as *mut () };
+            let fake_flow = mem::transmute::<raw::TraitObject, Box<Flow>>(fake_box);
+            drop(fake_flow);
+
+            if flow::base(&*flow_ref).weak_ref_count().fetch_sub(1, Ordering::Release) == 1 {
+                atomic::fence(Ordering::Acquire);
+                heap::deallocate(flow_ref.object.data as *mut u8, object_size, object_align);
+            }
+
             mem::forget(flow_ref);
-            self.object.vtable = ptr::null_mut();
-            self.object.data = ptr::null_mut();
         }
     }
 }
@@ -82,7 +123,7 @@ impl Drop for FlowRef {
 impl Clone for FlowRef {
     fn clone(&self) -> FlowRef {
         unsafe {
-            drop(flow::base(&**self).ref_count().fetch_add(1, Ordering::SeqCst));
+            let _ = flow::base(&**self).strong_ref_count().fetch_add(1, Ordering::Relaxed);
             FlowRef {
                 object: raw::TraitObject {
                     vtable: self.object.vtable,
@@ -92,3 +133,78 @@ impl Clone for FlowRef {
         }
     }
 }
+
+impl WeakFlowRef {
+    /// Upgrades a WeakFlowRef to a FlowRef.
+    pub fn upgrade(&self) -> Option<FlowRef> {
+        unsafe {
+            let object = flow::base(&**self);
+            // We use a CAS loop to increment the strong count instead of a
+            // fetch_add because once the count hits 0 is must never be above
+            // 0.
+            loop {
+                let n = object.strong_ref_count().load(Ordering::SeqCst);
+                if n == 0 { return None }
+                let old = object.strong_ref_count().compare_and_swap(n, n + 1, Ordering::SeqCst);
+                if old == n {
+                    return Some(FlowRef { object: self.object })
+                }
+            }
+        }
+    }
+}
+
+impl<'a> Deref for WeakFlowRef {
+    type Target = Flow + 'a;
+    fn deref(&self) -> &(Flow + 'a) {
+        unsafe {
+            mem::transmute_copy::<raw::TraitObject, &(Flow + 'a)>(&self.object)
+        }
+    }
+}
+
+impl DerefMut for WeakFlowRef {
+    fn deref_mut<'a>(&mut self) -> &mut (Flow + 'a) {
+        unsafe {
+            mem::transmute_copy::<raw::TraitObject, &mut (Flow + 'a)>(&self.object)
+        }
+    }
+}
+
+impl Clone for WeakFlowRef {
+    fn clone(&self) -> WeakFlowRef {
+        unsafe {
+            flow::base(&**self).weak_ref_count().fetch_add(1, Ordering::Relaxed);
+        }
+        WeakFlowRef { object: self. object }
+    }
+}
+
+impl Drop for WeakFlowRef {
+    fn drop(&mut self) {
+        unsafe {
+            if self.object.vtable.is_null() {
+                return
+            }
+
+            if flow::base(&**self).weak_ref_count().fetch_sub(1, Ordering::Release) == 1 {
+                atomic::fence(Ordering::Acquire);
+
+                // This dance deallocates the Box<Flow> without running its
+                // drop glue. The drop glue is run when the last strong
+                // reference is released.
+                let weak_ref: WeakFlowRef = mem::replace(self, WeakFlowRef {
+                    object: raw::TraitObject {
+                        vtable: ptr::null_mut(),
+                        data: ptr::null_mut(),
+                    }
+                });
+                let vtable: &[usize; 3] = mem::transmute::<*mut (), &[usize; 3]>(weak_ref.object.vtable);
+                let object_size = vtable[1];
+                let object_align = vtable[2];
+                heap::deallocate(weak_ref.object.data as *mut u8, object_size, object_align);
+                mem::forget(weak_ref);
+            }
+        }
+    }
+}

components/layout/layout_task.rs

@@ -804,8 +804,12 @@ impl LayoutTask {
         }
 
         if needs_reflow {
-            self.try_get_layout_root(*node).map(
-                |mut flow| LayoutTask::reflow_all_nodes(&mut *flow));
+            match self.try_get_layout_root(*node) {
+                None => {}
+                Some(mut flow) => {
+                    LayoutTask::reflow_all_nodes(&mut *flow);
+                }
+            }
         }
 
         // Create a layout context for use throughout the following passes.

components/layout/lib.rs

@@ -49,6 +49,7 @@ extern crate util;
 extern crate string_cache_macros;
 extern crate string_cache;
 
+extern crate alloc;
 extern crate collections;
 extern crate encoding;
 extern crate libc;