Fix context chaining to prevent memory leak (issue 7681).

runtime/vm/flow_graph_builder.cc

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "vm/flow_graph_builder.h"
 
 #include "vm/ast_printer.h"
 #include "vm/code_descriptors.h"
 #include "vm/dart_entry.h"
 #include "vm/flags.h"
@@ skipping 2604 matching lines @@
   if (num_context_variables > 0) {
     InlineBailout("EffectGraphVisitor::VisitSequenceNode (context)");
     // The loop local scope declares variables that are captured.
     // Allocate and chain a new context.
     // Allocate context computation (uses current CTX)
     Value* allocated_context =
         Bind(new AllocateContextInstr(node->token_pos(),
                                       num_context_variables));
 
     // If this node_sequence is the body of the function being compiled, and if
-    // this function is not a closure, do not link the current context as the
-    // parent of the newly allocated context, as it is not accessible. Instead,
-    // save it in a pre-allocated variable and restore it on exit.
+    // this function allocates context variables, but none of its enclosing
+    // functions do, the context on entry is not linked as parent of the
+    // allocated context but saved on entry and restored on exit as to prevent
+    // memory leaks.
+    // In this case, the parser pre-allocates a variable to save the context.
     if (MustSaveRestoreContext(node)) {
       Value* current_context = Bind(new CurrentContextInstr());
       Do(BuildStoreTemp(*owner()->parsed_function().saved_context_var(),
                         current_context));
       Value* null_context = Bind(new ConstantInstr(Object::ZoneHandle()));
       AddInstruction(new StoreContextInstr(null_context));
     }
 
     AddInstruction(new ChainContextInstr(allocated_context));
     owner()->set_context_level(scope->context_level());
@@ skipping 430 matching lines @@
   intptr_t len = OS::SNPrint(NULL, 0, kFormat, function_name, reason) + 1;
   char* chars = Isolate::Current()->current_zone()->Alloc<char>(len);
   OS::SNPrint(chars, len, kFormat, function_name, reason);
   const Error& error = Error::Handle(
       LanguageError::New(String::Handle(String::New(chars))));
   Isolate::Current()->long_jump_base()->Jump(1, error);
 }
 
 
 }  // namespace dart