Use simpler scheme for SVGSMILElement dependents cycle breaking

While flag-waving may be considered somewhat ugly, it's quicker (no
hash lookups) and slightly more flexible (then providing access to the
tracking set) if you want check for reentrance earlier (which is what we
want to do).

Bug: 998526
Change-Id: I91f9df93879e4446bd5908513cd8a4226c657bd7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1831906Reviewed-by: Stephen Chenney <schenney@chromium.org>
Commit-Queue: Fredrik Söderquist <fs@opera.com>
Cr-Commit-Position: refs/heads/master@{#701464}

third_party/blink/renderer/core/svg/animation/svg_smil_element.cc

@@ -27,6 +27,7 @@
 
 #include <algorithm>
 
+#include "base/auto_reset.h"
 #include "third_party/blink/public/platform/task_type.h"
 #include "third_party/blink/renderer/bindings/core/v8/script_event_listener.h"
 #include "third_party/blink/renderer/core/dom/document.h"
@@ -198,8 +199,8 @@ SVGSMILElement::SVGSMILElement(const QualifiedName& tag_name, Document& doc)
       cached_repeat_count_(SMILRepeatCount::Invalid()),
       cached_min_(kInvalidCachedTime),
       cached_max_(kInvalidCachedTime),
-      interval_has_changed_(false) {
-}
+      interval_has_changed_(false),
+      is_notifying_dependents_(false) {}
 
 SVGSMILElement::~SVGSMILElement() = default;
 
@@ -1136,23 +1137,14 @@ void SVGSMILElement::NotifyDependentsOnRepeat(unsigned repeat_nr,
 }
 
 void SVGSMILElement::NotifyDependents(const NotifyDependentsInfo& info) {
-  // |loopBreaker| is used to avoid infinite recursions which may be caused by:
-  // |notifyDependentsIntervalChanged| -> |createInstanceTimesFromSyncBase| ->
-  // |add{Begin,End}Time| -> |{begin,end}TimeChanged| ->
-  // |notifyDependentsIntervalChanged|
-  //
-  // As the set here records SVGSMILElements on the stack, it is acceptable to
-  // use a HashSet of untraced heap references -- any conservative GC which
-  // strikes before unwinding will find these elements on the stack.
-  DEFINE_STATIC_LOCAL(HashSet<UntracedMember<SVGSMILElement>>, loop_breaker,
-                      ());
-  if (!loop_breaker.insert(this).is_new_entry)
+  // Avoid infinite recursion which may be caused by:
+  // |NotifyDependents| -> |CreateInstanceTimesFromSyncBase| ->
+  // |AddInstanceTime| -> |InstanceListChanged| -> |NotifyDependents|
+  if (is_notifying_dependents_)
     return;
-
+  base::AutoReset<bool> reentrancy_guard(&is_notifying_dependents_, true);
   for (SVGSMILElement* element : sync_base_dependents_)
     element->CreateInstanceTimesFromSyncBase(this, info);
-
-  loop_breaker.erase(this);
 }
 
 void SVGSMILElement::CreateInstanceTimesFromSyncBase(

third_party/blink/renderer/core/svg/animation/svg_smil_element.h

@@ -278,6 +278,7 @@ class CORE_EXPORT SVGSMILElement : public SVGElement, public SVGTests {
   mutable SMILTime cached_max_;
 
   bool interval_has_changed_;
+  bool is_notifying_dependents_;
 
   friend class ConditionEventListener;
 };