[squeak-dev] Debugger bug: disappearing contents

Frank Shearar frank.shearar at angband.za.org
Sun Nov 21 15:47:23 UTC 2010

On 2010/11/13 17:35, Frank Shearar wrote:
> Hi,
> I've been chasing down a strange bug these past few days:
> http://bugs.squeak.org/view.php?id=7569
> The executive summary is this: when viewing a method in the debugger
> that refers to an instvar deleted after the Debugger instantiated, the
> CodePane shows the correct source for a fraction of a second, and then
> goes blank.
> What I'd expected to see was the source of the method rendered as per
> usual, and the reference to the missing instvar coloured red, to
> indicate a problem. (Just like what you'd see if you opened up a Browser
> on the method.)
> The CodePane goes blank because the Debugger's contents instvar is
> nilled out.
> CodeHolder>>setContentsToForceRefresh nils the instvar because
> CodeHolder>>didCodeChangeElsewhere returns true.
> That method returns true because the Debugger's currentCompiledMethod
> instvar and the compiled method according to "aClass compiledMethodAt:
> aSelector" aren't the same object.
> And now I'm a bit stuck. I know the behaviour I'd expect, I know why the
> bug's happening, but how do I fix it? Does CodeHolder (or Debugger) need
> a better way of knowing whether the viewed method's changed? (For
> instance, comparing the two CompiledMethods' asCommaString shows
> identical contents.)

OK, I got a bit further. The Debugger's absolutely right in saying that 
the code has changed elsewhere. As soon as you delete the instvar, the 
entire class is recompiled and, in particular, the method referencing 
the deleted instvar changes.

In my tests my method #bar looks like this:

   self halt.
   baz := 1.

with bytecodes

17 <70> self
18 <D0> send: halt
19 <87> pop
20 <76> pushConstant: 1
21 <60> popIntoRcvr: 0
22 <78> returnSelf

After deleting the instvar baz, bar's bytecodes look like this:

21 <70> self
22 <D0> send: halt
23 <87> pop
24 <76> pushConstant: 1
25 <82 C1> popIntoLit: baz
27 <78> returnSelf

I can't argue with that. It does mean that the Debugger's job is harder. 
In this particular case, nilling out the Debugger's contents is the 
wrong thing to do, because it's surprising. You see the code, see the 
reference to the defunct instvar... and then your source is gone!

We fall into this hole because for most CodeHolders, when the code's 
changed, self hasUnacceptedEdits will return true, and you'll get that 
red border around your code. In this case, there aren't unaccepted edits 
because the user hasn't changed code. Instead, the Debugger has what I 
suppose one might call a stale copy of the method (in contextStack).

So what should the correct behaviour be? How about a message that says 
"Oops, this method has changed underneath your feet. Would you like to 
revert back to the context that called it?"


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