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

From: Paul Eggert
Subject: Re: [Emacs-diffs] master d826037 3/3: Remove the need for temacs.in
Date: Sun, 14 Apr 2019 17:19:33 -0700
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.6.1

Daniel Colascione wrote:
If the temacs.in scheme *were* to
break, it would have to be in such a way that 1) some change in Emacs or
the environment resulted in temacs.in_1 (before the change) and
temacs.in_2 (after the change) having the same hash, but temacs_1 (before
the change) and temacs_2 (after the change) being different in a way that
breaks pdumper. Can you think of such a scenario?


   cd src
   make LDFLAGS="something"
   rm temacs
   make LDFLAGS="something else" temacs

Here, temacs.in_1 and temacs.in_2 are identical (because temacs.in is built just once) but temacs_1 and temacs_2 can differ in a way that breaks the portable dumper.

None of the approaches proposed in this thread work reliably in the presence of all plausible linkers, even ignoring the problem of hash collisions. All the approaches have false alarms (where the Emacs loader incorrectly reports an incompatibility) and false negatives (where the Emacs loader incorrectly fails to report an incompatibility that hurts later execution), and these flaws are pretty much inherent to the current fingerprint strategy. That's OK: the point of the fingerprint is not to be perfect (it can't be); it's only to be good enough. However, nobody knows even which of the discussed approaches will be best in practice, because nobody has practical experience with any of these approaches in a realistic deployment (and also, because we probably don't agree 100% on what constitutes "best".)

If you want to address the build time issue, just rewrite the fingerprint
in place.

Although I don't agree that this approach works reliably (in the sense described above), it's better than the old temacs.in approach and it should be good enough in practice, so I installed the attached patch to do that.

Attachment: 0001-Replace-executable-s-fingerprint-in-place.patch
Description: Text Data

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