Before write-protecting a PageSpace, return bump allocation block to freelist.

runtime/vm/pages.cc

Index: runtime/vm/pages.cc
===================================================================
--- runtime/vm/pages.cc	(revision 45122)
+++ runtime/vm/pages.cc	(working copy)
@@ -508,6 +508,15 @@
 }
 
 
+void PageSpace::AbandonBumpAllocation() {
+  if (bump_top_ < bump_end_) {
+    freelist_[HeapPage::kData].Free(bump_top_, bump_end_ - bump_top_);
+    bump_top_ = 0;
+    bump_end_ = 0;
+  }
+}
+
+
 bool PageSpace::Contains(uword addr) const {
   for (ExclusivePageIterator it(this); !it.Done(); it.Advance()) {
     if (it.page()->Contains(addr)) {
@@ -605,6 +614,10 @@
 
 
 void PageSpace::WriteProtect(bool read_only) {
+  if (read_only) {
+    // Avoid MakeIterable trying to write to the heap.
+    AbandonBumpAllocation();
+  }
   for (ExclusivePageIterator it(this); !it.Done(); it.Advance()) {
     it.page()->WriteProtect(read_only);
   }
@@ -780,9 +793,7 @@
   int64_t mid1 = OS::GetCurrentTimeMicros();
 
   // Abandon the remainder of the bump allocation block.
-  MakeIterable();
-  bump_top_ = 0;
-  bump_end_ = 0;
+  AbandonBumpAllocation();
   // Reset the freelists and setup sweeping.
   freelist_[HeapPage::kData].Reset();
   freelist_[HeapPage::kExecutable].Reset();