[vm, gc] Require a safepoint for heap iteration.

runtime/vm/clustered_snapshot.cc

Index: runtime/vm/clustered_snapshot.cc
diff --git a/runtime/vm/clustered_snapshot.cc b/runtime/vm/clustered_snapshot.cc
index 6bc3bfe896f83e310fee6d1abebcede823b2c5dd..720e95ae059169e99a44143e0a76d7d8c5b19970 100644
--- a/runtime/vm/clustered_snapshot.cc
+++ b/runtime/vm/clustered_snapshot.cc
@@ -5349,15 +5349,18 @@ FullSnapshotWriter::FullSnapshotWriter(Snapshot::Kind kind,
     // it out as part of the VM isolate snapshot. We first count the number of
     // token streams, allocate an array and then fill it up with the token
     // streams.
-    SnapshotTokenStreamVisitor token_streams_counter(thread());
-    heap()->IterateOldObjects(&token_streams_counter);
-    Dart::vm_isolate()->heap()->IterateOldObjects(&token_streams_counter);
-    intptr_t count = token_streams_counter.count();
-    token_streams_ = Array::New(count, Heap::kOld);
-    SnapshotTokenStreamVisitor script_visitor(thread(), &token_streams_);
-    heap()->IterateOldObjects(&script_visitor);
-    Dart::vm_isolate()->heap()->IterateOldObjects(&script_visitor);
-    ASSERT(script_visitor.count() == count);
+    {
+      HeapIterationScope iteration;
+      SnapshotTokenStreamVisitor token_streams_counter(thread());
+      iteration.IterateObjects(&token_streams_counter);

[danunez, 2017/08/05 17:24:11]

It looks like this version will collect token streams from the new space, since
IterateObjects walks over both spaces. Is this change intended (a fix from the
previous version)?

[rmacnak, 2017/08/07 22:37:50]

Token streams are allocated into old space, which is why the old code works. The
change is mostly to keep symmetry between iterating the two isolates will
avoiding introducing extra every more variants of iteration, e.g.,
IterateVMIsolateOldObjects.

+      iteration.IterateVMIsolateObjects(&token_streams_counter);
+      intptr_t count = token_streams_counter.count();
+      token_streams_ = Array::New(count, Heap::kOld);
+      SnapshotTokenStreamVisitor script_visitor(thread(), &token_streams_);
+      iteration.IterateObjects(&script_visitor);
+      iteration.IterateVMIsolateObjects(&script_visitor);
+      ASSERT(script_visitor.count() == count);
+    }
 
     // Tuck away the current symbol table.
     saved_symbol_table_ = object_store->symbol_table();