Add 'address' vm service message that finds heap objects.

runtime/vm/service.cc

Index: runtime/vm/service.cc
===================================================================
--- runtime/vm/service.cc	(revision 34144)
+++ runtime/vm/service.cc	(working copy)
@@ -5,6 +5,7 @@
 #include "vm/service.h"
 
 #include "include/dart_api.h"
+#include "platform/globals.h"
 
 #include "vm/compiler.h"
 #include "vm/coverage.h"
@@ -1450,9 +1451,47 @@
 }
 
 
+class ContainsAddressVisitor : public FindObjectVisitor {
+ public:
+  ContainsAddressVisitor(Isolate* isolate, uword addr)
+      : FindObjectVisitor(isolate), addr_(addr) { }
+  virtual ~ContainsAddressVisitor() { }
+
+  virtual uword filter_addr() const { return addr_; }

[turnidge, 2014/03/20 17:01:19]

Is this accessor used?

[koda, 2014/03/20 20:25:37]

Yes, it's called by the super class (FindObjectVisitor::VisitRange).

+
+  virtual bool FindObject(RawObject* obj) const {
+    uword obj_begin = RawObject::ToAddr(obj);
+    uword obj_end = obj_begin + obj->Size();
+    return obj_begin <= addr_ && addr_ < obj_end;
+  }
+ private:
+  uword addr_;
+};
+
+
+static bool HandleAddress(Isolate* isolate, JSONStream* js) {
+  uword addr = 0;
+  if (js->num_arguments() != 2 ||
+      !GetUnsignedIntegerId(js->GetArgument(1), &addr, 16)) {
+    static const uword kExampleAddr = static_cast<uword>(kIntptrMax / 7);
+    PrintError(js, "Must specify address: address/" Px ".", kExampleAddr);
+    return true;
+  }
+  Object& object = Object::Handle(isolate);
+  {
+    NoGCScope no_gc;
+    ContainsAddressVisitor visitor(isolate, addr);
+    object = isolate->heap()->FindObject(&visitor);
+  }
+  object.PrintToJSONStream(js, true);

[turnidge, 2014/03/20 17:01:19]

Printing the null object isn't the same as printing the null instance (perhaps
it should be).  Consider checking for null and using Object::null_instance()
like we do at the end of HandleClassesEval.

[turnidge, 2014/03/20 17:08:25]

Actually returning the null instance here would be wrong, wouldn't it?  What's
really going on is that the object has been moved or collected if we can't find
it.

Look for PrintPseudoNull in this file -- we use it to tag objects that are
expired (handle out of date) or collected.  Not sure what to mark these -- maybe
expired.

[koda, 2014/03/20 18:36:26]

Note that we might find *any* heap object, not just subclasses of Instance (for
example, we could find RawInstructions or even a FreeListElement).

Also, note that the queried address might *never* have been occupied by any heap
object, so neither "expired" nor "collected" feels entirely appropriate.

Therefore, I chose Object::null(). Let me know what you think.

[turnidge, 2014/03/20 20:34:54]

I guess it depends on what the UI expects.  We have a general convention that
all json responses from the vmservice are tagged with a type -- I think this
would be the only exception, which makes me pause.

One option is to make a new PseudoNull type -- something like "notfound".

If you want to leave it as "null" for now, go ahead.  We can change it later if
we need to.

[koda, 2014/03/20 20:44:19]

For most objects, obj.PrintToJSONStream does tag the response with a type (my
unit test checks for '@String' and 'null').

It's true that some internal VM classes (e.g., Stackmap) just add an empty
object to the stream. We should probably fix that, and make it a requirement
that any override of PrintToJSONStream at least populates type and size.

+  return true;
+}
+
+
 static IsolateMessageHandlerEntry isolate_handlers[] = {
   { "_echo", HandleIsolateEcho },
   { "", HandleIsolate },
+  { "address", HandleAddress },
   { "allocationprofile", HandleAllocationProfile },
   { "classes", HandleClasses },
   { "code", HandleCode },