Re: [Emacs-diffs] master d826037 3/3: Remove the need for temacs.in

[Paul Eggert <eggert@cs.ucla.edu>]

Daniel Colascione wrote:

>> what exactly is the failure scenario here? I'm not seeing any
>> failure scenarios for the current approach that can't also be failure
>> scenarios for the previous approach
> 
> It's likely that the linker to lay out objects in a section identically in
> two different builds when the only difference between the builds is the
> content of an array.

Sure, but it's even more likely for linkers to lay out objects in a section 
identically in two different builds when there is no difference whatsoever 
between the inputs to the builds. So I'm still not seeing the failure scenario 
for the current approach that wouldn't also be a failure scenario for the 
previous (temacs.in) approach.

> If we're worried about the array being folded into
> the code, we can make it volatile.

That wouldn't be enough; we'd need the volatile accesses to memory under the 
program's control being tricky enough (they aren't now) so that the compiler 
couldn't optimize them away or reorder the array or whatever. Admittedly this is 
getting a little theoretical (but then this particular point is pretty 
theoretical anyway :-).

> If someone changes a linker flag that
> changes object ordering, temacs.in will change.

Right, but that can also affect temacs in the previous approach; that is, 
temacs.in might be linked with different flags than temacs is. Or the linker 
might be upgraded between the time that temacs.in is built, and the time that 
temacs is built. So these failure scenarios apply to the previous approach too.

If we rely on a fingerprint we have to give up on the idea of an ironclad 
guarantee that if the fingerprint matches, Emacs is compatible. We have to 
settle for just a high-enough probability in practice.

We could document ways in which the low-probability events can occur (hash 
collision, linker change that breaks reproducible builds, etc.). Or we could 
change the pdumper so that it doesn't rely on a fingerprint: instead, Emacs 
could record a complete description of what it's assuming in the dump file, and 
check this description when it reads the dump back in. However, that'd be some 
work and is almost surely overkill.