Updating LLVM on Debian/Ubuntu

[Renato Golin]

Hey,

Regarding the GCC ABI 5 issue, I was wondering what's the policy
behind updating packages on stable updates for both Debian and Ubuntu.

Our time frame is a bit constrained, and we definitely will have to
take some hard decisions in the next six months, so I'd like to
understand everything that is at stake before I have my own opinion.

LLVM has a 6 month major cycle, releasing around February / August.
Major releases are allowed to break the ABI. Major breakages need one
release warning period.

Ubuntu has a 6 month release cycle, around April / October. IIUC,
major releases are allowed to have new versions of packages, but
updates for the next few years have to keep within the same major
release.

Debian has a -1 years release cycle (heh), and has the same major /
minor policy, which makes it a lot harder to update major versions.
However, I believe unstable is still not closed, nor will be in August
this year, so updating to LLVM 3.9 will not be a problem, but it will
mean users will have to wait a bit more to get a working LLVM.

The time frame is then:

3.8.0 released March (without the fix)
Ubuntu X released April
3.9.0 releases August (hopefully with a fix)
Ubuntu X+1 released October

Debian freezes ??
LLVM 3.8.1 ??

If we don't back-port GCC ABI 5 into 3.8.1, Ubuntu users will not have
the fix ever, unless you *can* update to 3.9.0 in August.

Ubuntu X+1 will be fine using 3.9, as will Debian after August, unless
you guys freeze before that.

I believe both Debian and Ubuntu have a trunk-based LLVM package for
experimental use only, and it would be bad, but not completely broken,
to recommend users to use that meanwhile.

If Debian freezes *before* 3.9.0 is out, or if Ubuntu can't update to
3.9.0 on April's release, then we'll have a strong reason to back-port
the change to 3.8.x. If not, even though it will be uncomfortable for
users until August, the argument is not that strong and will be hard
to get it through.

Any comments? Ideas? Does any of that make sense?

cheers,
--renato

[Matthias Klose]

Post by Renato Golin
Hey,
Regarding the GCC ABI 5 issue, I was wondering what's the policy
behind updating packages on stable updates for both Debian and Ubuntu.
Our time frame is a bit constrained, and we definitely will have to
take some hard decisions in the next six months, so I'd like to
understand everything that is at stake before I have my own opinion.
LLVM has a 6 month major cycle, releasing around February / August.
Major releases are allowed to break the ABI. Major breakages need one
release warning period.
Ubuntu has a 6 month release cycle, around April / October. IIUC,
major releases are allowed to have new versions of packages, but
updates for the next few years have to keep within the same major
release.

except for the LTS releases, which have a two year release cycle. 16.04 LTS will
be the next LTS.

Post by Renato Golin
Debian has a -1 years release cycle (heh), and has the same major /
minor policy, which makes it a lot harder to update major versions.

This is more like a 18-24 months cycle.

Post by Renato Golin
However, I believe unstable is still not closed, nor will be in August
this year, so updating to LLVM 3.9 will not be a problem, but it will
mean users will have to wait a bit more to get a working LLVM.

As announced on debian-devel-announce, the freeze will be in Feb 2017.

Post by Renato Golin
3.8.0 released March (without the fix)
Ubuntu X released April
3.9.0 releases August (hopefully with a fix)
Ubuntu X+1 released October
Debian freezes ??
LLVM 3.8.1 ??
If we don't back-port GCC ABI 5 into 3.8.1, Ubuntu users will not have
the fix ever, unless you *can* update to 3.9.0 in August.

Ubuntu LTS users won't see any 3.9 updates in August. So yes, a backport of
these changes to 3.8.1 would be appreciated.

Post by Renato Golin
Ubuntu X+1 will be fine using 3.9, as will Debian after August, unless
you guys freeze before that.

While I'm still listed as an uploader for some llvm packages in Debian; I'm only
contributing some build fixes to the Debian packages if I see these, however the
roadmap for Debian maybe should be to go with 3.9 as the default for the
release, and maybe keep some legacy versions (I assume at least haskell needs 3.5).

Ubuntu usually adds new llvm versions after release as part of the regular
xserver/mesa updates, using a llvm-x.y version in a newer Ubuntu release. So
you might get 3.9 into 16.04 LTS either in late 2016 (after the 16.10 release),
or in mid 2017 (after the 17.04 release).

Post by Renato Golin
I believe both Debian and Ubuntu have a trunk-based LLVM package for
experimental use only, and it would be bad, but not completely broken,
to recommend users to use that meanwhile.

Debian keeps these in experimental, which is not recommended for "users"; Ubuntu
keeps these in the -proposed pocket only, these package never reach the release
pocket, even during development of a new release. So you can't safely point
users to this version.

Post by Renato Golin
If Debian freezes *before* 3.9.0 is out, or if Ubuntu can't update to
3.9.0 on April's release, then we'll have a strong reason to back-port
the change to 3.8.x. If not, even though it will be uncomfortable for
users until August, the argument is not that strong and will be hard
to get it through.
Any comments? Ideas? Does any of that make sense?

So from my point of view, I'd like to see these patches on the 3.8 branch, or on
a separate branch, if upstream thinks that these are not suitable for the 3.8
branch.

Matthias

[Renato Golin]

Post by Matthias Klose
except for the LTS releases, which have a two year release cycle. 16.04 LTS
will be the next LTS.

Damn it.

Post by Matthias Klose
As announced on debian-devel-announce, the freeze will be in Feb 2017.

Ok, less problematic in Debian's case.

Post by Matthias Klose
Ubuntu LTS users won't see any 3.9 updates in August. So yes, a backport of
these changes to 3.8.1 would be appreciated.

Damn it. :)

Post by Matthias Klose
While I'm still listed as an uploader for some llvm packages in Debian; I'm
only contributing some build fixes to the Debian packages if I see these,
however the roadmap for Debian maybe should be to go with 3.9 as the default
for the release, and maybe keep some legacy versions (I assume at least
haskell needs 3.5).

That'd be ok, since Haskell will probably not be affected by the ABI bug.

But you should make sure you mark that version as deprecated and as
not fulfilling the "llvm" requirement, so that packages don't accept
llvm-3.5 as a dependency.

Post by Matthias Klose
Ubuntu usually adds new llvm versions after release as part of the regular
xserver/mesa updates, using a llvm-x.y version in a newer Ubuntu release.
So you might get 3.9 into 16.04 LTS either in late 2016 (after the 16.10
release), or in mid 2017 (after the 17.04 release).

If this happens, could it become the default, replacing 3.8?

Post by Matthias Klose
Debian keeps these in experimental, which is not recommended for "users";
Ubuntu keeps these in the -proposed pocket only, these package never reach
the release pocket, even during development of a new release. So you can't
safely point users to this version.

Ok.

Post by Matthias Klose
So from my point of view, I'd like to see these patches on the 3.8 branch,
or on a separate branch, if upstream thinks that these are not suitable for
the 3.8 branch.

It's either 3.8 or nothing. We don't do multiple release branches, so
once 3.9 is out, there will be no more 3.8.x (or 3.7.x, foobar, etc).

Once the ABI is fixed in trunk, we shall start the discussion about
the back-port. I think we have enough information to take it to the
community and make sure we take the right decision for all affected
parties.

Thanks!
--renato