intrinsic vs extrinsic identifier: toward more robustness?

[Simon Tournier]

Hi,

I would like to open a discussion about how we identify the source
origin (fixed output).  It is of vitally importance for being robust on
the long-term (say 3-5 years).  It matters in Reproducible Research
context, but not only.

# First thing first
===================

## What is an intrinsic identifier or an extrinsic one?
=======================================================

 - extrinsic: use a register to keep the correspondence between the
   identifier and the object; say label version as Git tag.

 - intrinsic: intimately bound to the designated object itself; say hash
   as Git blob or tree and at some extent commit.

The register must be a trusted authority and it resolves by mapping the
key identifier to the object.  Having the object at hand does not give
any clue about the key identifier.  And collisions are very frequent;
two key identifiers resolve to the same content – hopefully! we call
that mirrors. ;-)

Intrinsic identifier also relies on a (trusted) map but collisions are
avoided as much as possible.  Somehow it strongly reduces the power of
the authority and it is often more robust.

Please note that the identification and the integrity is not the same.
Since intrinsic identifier often uses cryptographic hash functions and
integrity too, it is often confusing.

Whatever the intrinsic identifier we consider – even ones based on very
weak cryptographic hash function as MD5, or based on non-crytographic
hash function as Pearson hashing, etc. – the integrity check is
currently done by SHA256.

## For example, consider this source origin,
==============

    (source (origin
              (method url-fetch)
              (uri (string-append "mirror://gnu/hello/hello-" version
                                  ".tar.gz"))
              (sha256
               (base32
                "086vqwk2wl8zfs47sq2xpjc9k066ilmb8z6dn0q6ymwjzlm196cd"))))

where ’mirror://gnu’ is resolved by Guix itself.  Or this one,

    (source
     (origin
       (method git-fetch)
       (uri (git-reference
             (url "https://github.com/FluxML/Zygote.jl";)
             (commit (string-append "v" version))))
       (file-name (git-file-name name version))
       (sha256
        (base32 "02bgj6m1j25sm3pa5sgmds706qpxk1qsbm0s2j3rjlrz9xn7glgk"))))

where Guix clones then checks out at the specification of the field
’commit’.

Here both are extrinsic identifiers.  For the first example, the register
is defined by ’%mirrors’.  For the second example, the register is the
folder ’.git/’.

Intrinsic identifier could be plain hash or hashed serialized data.
Using Guix b8f6ead:

--8<---------------cut here---------------start------------->8---
$ guix hash -S none -H sha256 -f nix-base32 -x $(guix build hello -S)
086vqwk2wl8zfs47sq2xpjc9k066ilmb8z6dn0q6ymwjzlm196cd

$ guix hash -S git -H sha256 -f nix-base32 -x $(guix build hello -S)
11kaw6m19rdj3d55y4cygk6k9zv6sn2iz4gpimx0j99ps87ij29l

$ guix hash -S nar -H sha256 -f nix-base32 -x 
/gnu/store/3dq55rw99wdc4g4wblz7xikc8a2jy7a3-hello-2.12.1.tar.gz
1lvqpbk2k1sb39z8jfxixf7p7v8sj4z6mmpa44nnmff3w1y6h8lh
--8<---------------cut here---------------end--------------->8---

Or some Git-like tree md5 of the decompressed data, e.g.,

--8<---------------cut here---------------start------------->8---
$ guix hash -S git -H md5 -f hex -x hello-2.12.1
3db60bcfecf17a5dd81e3fb5bfb1c191
--8<---------------cut here---------------end--------------->8---

Or some others.

--8<---------------cut here---------------start------------->8---
$ git clone https://github.com/FluxML/Zygote.jl
$ git -C Zygote.jl checkout v0.6.41

$ guix hash -S nar -H sha256 -f nix-base32 -x Zygote.jl
02bgj6m1j25sm3pa5sgmds706qpxk1qsbm0s2j3rjlrz9xn7glgk

$ guix hash -S git -H sha1 -f hex -x Zygote.jl
3cfdb31b517eec4173584fba2b1aa65daad46e09
--8<---------------cut here---------------end--------------->8---


# Second thing second
=====================

All that’s said, Guix uses extrinsic identifiers for almost all origins,
if not all.  Even for ’git-fetch’ method.

Consider that GitHub disappears and the default build farms ci.guix and
bordeaux.guix are unreachable for whatever reason.  Then Guix will
fallback to Software Heritage and will exploits its resolver.

--8<---------------cut here---------------start------------->8---
Initialized empty Git repository in 
/gnu/store/ns1f3b4wm5n470bczd2k5li6xpgbqkz7-julia-zygote-0.6.41-checkout/.git/
fatal: unable to access 'https://github.com/FluxML/Zygote.jl/': Could not 
resolve host: github.com
Failed to do a shallow fetch; retrying a full fetch...
fatal: unable to access 'https://github.com/FluxML/Zygote.jl/': Could not 
resolve host: github.com
git-fetch: 
'/gnu/store/55ba5ragbd5sd4r45n0q24vrxx9rigrm-git-minimal-2.39.1/bin/git fetch 
origin' failed with exit code 128
Trying content-addressed mirror at berlin.guix.gnu.org...
Trying content-addressed mirror at berlin.guix.gnu.org...
Trying to download from Software Heritage...
SWH: found revision 4777767737b4c95d2cea842933c5b2edae2771b2 with directory at 
'https://archive.softwareheritage.org/api/1/directory/3cfdb31b517eec4173584fba2b1aa65daad46e09/'
swh:1:dir:3cfdb31b517eec4173584fba2b1aa65daad46e09/
--8<---------------cut here---------------end--------------->8---

That’s SWH which finds the revision
4777767737b4c95d2cea842933c5b2edae2771b2 from the contextual information
URL + label version and from this revision SWH associates the content
having the intrinsic identifier
swh:1:dir:3cfdb31b517eec4173584fba2b1aa65daad46e09.


## First, please note that the SWHID is just Git,
========

--8<---------------cut here---------------start------------->8---
guix hash -S git -H sha1 -f hex \
     /gnu/store/ns1f3b4wm5n470bczd2k5li6xpgbqkz7-julia-zygote-0.6.41-checkout
3cfdb31b517eec4173584fba2b1aa65daad46e09
--8<---------------cut here---------------end--------------->8---

Other said, SWH information is somehow the same information as the one
of Git objects.  Specifically, from the Git checkout,

--8<---------------cut here---------------start------------->8---
$ git cat-file -p v0.6.41
object 4777767737b4c95d2cea842933c5b2edae2771b2
type commit
tag v0.6.41

$ git cat-file -p 4777767737b4c95d2cea842933c5b2edae2771b2
tree 3cfdb31b517eec4173584fba2b1aa65daad46e09
--8<---------------cut here---------------end--------------->8---


## Second, SWH acts as a resolver here, i.e.,
=========

     (find (lambda (branch)
             (or
              ;; Git specific.
              (string=? (string-append "refs/tags/" tag)
                        (branch-name branch))
              ;; Hg specific.
              (string=? tag
                        (branch-name branch))))
           (snapshot-branches snapshot))

and this is not robust.  For one, it fails for Git lightweight tag as
exposed with the package ’open-zwave’ tag 1.6.

--8<---------------cut here---------------start------------->8---
$ for t in $(git tag); do printf "$t "; git cat-file -t $t ;done
Rel-1.0 commit
V1.5 tag
v1.2 commit
v1.3 tag
v1.4 tag
v1.6 commit
--8<---------------cut here---------------end--------------->8---

It means that the code above would be able to find V1.5 or v1.4 but not
v1.6 or v1.2.  Well, we can consider that as a bug and improve the
snapshot machinery for also collecting more ’refs’.  But, for two…

…the current code (guix swh) does not deal with several snapshots and
only consider the latest one.  Therefore, it fails for some in-place
replacements – upstream tags a specific revision then later removes it
and upstream re-use the same tag label for another revision booo!, if
SWH ingests after the first tag, SWH creates one snapshot, then if SWH
ingests again after the second re-tag, SWH creates another snapshot.


## Third, Disarchive is helping.
========

Aside adding a layer to maintain does not help when speaking about
long-term (3-5 years), well, the reduction of layers is often better for
long-term.  That’s said, there is a work in progress to have Disarchive
features directly from SWH.

What does Disarchive do?  It maps various intrinsic identifiers.

Remember ’hello’ from above?

--8<---------------cut here---------------start------------->8---
$ guix shell disarchive guile-lzma guile
$ disarchive disassemble hello-2.12.1
(disarchive
  (version 0)
  (directory-ref
    (version 0)
    (name "hello-2.12.1")
    (addresses
      (swhid "swh:1:dir:ad5fc7c3062e8426b7936588e7a27d51ace0e508"))
    (digest
      (sha256
        "cc7d5c45cfa1f5fba96c8b32d933734b24377a3c1ac776650044e497469affd4"))))

$ guix hash -S git -H sha1 -f hex hello-2.12.1
ad5fc7c3062e8426b7936588e7a27d51ace0e508
$ guix hash -S git -H sha256 -f hex hello-2.12.1
cc7d5c45cfa1f5fba96c8b32d933734b24377a3c1ac776650044e497469affd4
--8<---------------cut here---------------end--------------->8---

Well, the fixed-outputs is a compressed tarball, it reads,

--8<---------------cut here---------------start------------->8---
$ disarchive disassemble $(guix build -S hello)
(disarchive
  (gzip-member
    (name "3dq55rw99wdc4g4wblz7xikc8a2jy7a3-hello-2.12.1.tar.gz")
    (digest
      (sha256
        "8d99142afd92576f30b0cd7cb42a8dc6809998bc5d607d88761f512e26c7db20"))
    (header (mtime 0) (extra-flags 2) (os 3))
    (footer (crc 2707092614) (isize 4945920))
    (compressor gnu-best-rsync)
    (input (tarball
             (name "3dq55rw99wdc4g4wblz7xikc8a2jy7a3-hello-2.12.1.tar")
             (digest
               (sha256
                 
"a2c33fd13c555015433956bcf06609293a34ce5c5e6a2070990bfb86070dc554"))
[...]

    (input (directory-ref
             (version 0)
             (name "3dq55rw99wdc4g4wblz7xikc8a2jy7a3-hello-2.12.1")
             (addresses
               (swhid "swh:1:dir:9c1eecffa866f7cb9ffdd56c32ad0cecb11fcf2a"))
             (digest
               (sha256
                 
"1cb6effd40736b441a2a6dd49e56b3dfd4f6550e8ae1a8ac34ed4b1674097bc0"))))))))
--8<---------------cut here---------------end--------------->8---

where the values are just (considering that ’guix hash -S none -H sha256
-f hex’ is equivalent to ’sha256sum’)

--8<---------------cut here---------------start------------->8---
$ guix hash -S none -H sha256 -f hex $(guix build hello -S)
8d99142afd92576f30b0cd7cb42a8dc6809998bc5d607d88761f512e26c7db20
$ gzip -d $(guix build -S hello) -c | sha256sum
a2c33fd13c555015433956bcf06609293a34ce5c5e6a2070990bfb86070dc554  -
--8<---------------cut here---------------end--------------->8---

However the fields ’swhid’ and the other SHA256 ’digest’ are different
from above.  That’s because the dots [...] part.  It probably comes from
the normalization process. Well, I am not sure to deeply understand why
it is different but that’s another story. :-)


## Fourth, it misses a bridge using NAR normalization (serialization).
=========

Disarchive can (or could) provides a bridge (map) between SWHID+SHA1 and
NAR+SHA256.  But it could be nice if it was implemented in SWH
directly.  It would ease previous drawbacks.

For the interested reader, discussion there
<https://gitlab.softwareheritage.org/swh/meta/-/issues/4538>.  Moreover,
<https://gitlab.softwareheritage.org/swh/meta/-/issues/4538#note_121067>
provides simple examples about NAR and how to implement it using Python.


# Discussion asking for comments and feedback
=============================================

Still there?  If yes, thanks for reading. :-)

As shown in,

1: <https://lists.gnu.org/archive/html/guix-devel/2023-02/msg00398.html>
2: <https://lists.gnu.org/archive/html/guix-devel/2023-03/msg00007.html>

we have holes and we are not currently robust for long-term (3-5 years)
if our lovely build-farms are down for whatever reasons.

For sure, we have to fix the holes and bugs. :-)  However, I am asking
what we could add for having more robustness on the long term.

It is not affordable, neither wanted, to switch from the current
extrinsic identification to a complete intrinsic one.  Although it would
fix many issues. ;-)

Guix and ’guix time-machine’ provides all the machinery for being able
to redeploy later but as I have tried to point in the two links above
[1,2], we are lacking tools for retrieving contents; well having the
machinery does not mean that such machinery works well or is robust. :-)

The discussion could also fit how to distribute using ERIS.

At some point, I was thinking to have something like “guix freeze -m
manifest.scm” returning a map of all the sources from the deep bootstrap
to the leaf packages described in manifest.scm.  However, maybe
something is poor in the metadata we collect at package time.

For instance, the substitutions work more or less using intrinsic
identifier so it helps, I guess. :-)

Well, we could imagine the addition of another option field, say under
’properties’, that could store the intrinsic identifier of the
fixed-outputs such as SWHID or Git tree / commit hash or else.  It would
add robustness for later.

Or maybe an optional field of the ’origin’ record for the same purpose.

WDYT?

Cheers,
simon