Re: Conservative GC isn't safe

[Daniel Colascione <dancol@dancol.org>]

On 11/26/2016 01:01 AM, Eli Zaretskii wrote:
>> From: Daniel Colascione <dancol@dancol.org>
>> Date: Sat, 26 Nov 2016 00:33:13 -0800
>>
>>>>   2) INTERVAL is GCed, but it's not represented in the memory tree:
>>>> struct interval isn't a real lisp object and it's allocated as
>>>> MEM_TYPE_NON_LISP. Even a direct pointer to the start of an interval
>>>> won't protect it from GC. Shouldn't we treat intervals like conses?
>>>
>>> Does the code ever create an interval that is accessible only via locals
>>> when a GC occurs? If not, Emacs should be OK. (This should also be
>>> documented better.)
>>
>> Anywhere in the code? Forever? I wouldn't be confident saying so.
>
> A simple practical solution to such assumptions is to add an assertion
> in some strategic place(s).
>
> I don't think it's TRT to sprinkle our sources with code that is there
> "just in case", i.e. it will never actually run.

How would you assert dynamically that if an interval is reachable, its 
owning string or buffer must be too? It's not enough for the variable 
holding the reference to the string or buffer to be in scope: you have 
to be sure that the reference isn't dead.