[Bug 1521172] Re: [FFe][Ubuntu 16.04] Use glibc-2.23 in Ubuntu 16.04

[Steve Langasek]

As mentioned by Adam, there is some contract pressure to ship a newer
version of glibc than 2.21, which is now a year old.  It is also better
for the LTS in general if we can have this newer upstream version of
glibc.  However, as always, that needs to be balanced against the risks.

The risks are mitigated here by the fact that we have extensive
autopkgtest coverage for the reverse-dependencies of libc (aka, "the
archive"), and that proposed-migration gates on autopkgtest regressions
for 5 of 7 release architectures.  The remaining two architectures are
powerpc, which is a best-effort community port; and arm64, which
Canonical accepts responsibility for finding and fixing regressions on
before release.

I understand that we will also have an archive rebuild test between now
and release.  It's unfortunate that this glibc update has not been
included in the most recent rebuild test, but as long as we are still
able to get one before 16.04 release that should suffice.

Along with some basic boot smoketesting prior to release from xenial-
proposed, I believe this provides adequate coverage for us regarding
risk of regressions.

So +1 from me for the FFe.

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https://bugs.launchpad.net/bugs/1521172

Title:
  [FFe][Ubuntu 16.04] Use glibc-2.23 in Ubuntu 16.04

Status in glibc package in Ubuntu:
  Triaged

Bug description:
  Repurposing this as a Feature Freeze request bug for the glibc update.
  The justification for POWER is laid out below but, obviously,
  2.21->2.23 brings in a lot of random minor improvements and bugfixes
  for all architectures, and has been requested by several different
  contracts.

  The plan is to get it into the archive Friday evening/night at the
  latest, watch all the automated testing, iterate through bugfixes as
  necessary, and see it ship in the final beta next week.

  As for the GCC feature mentioned in the original bug report, that was
  fixed by wschmidt and doko a few hours ago to actually work in xenial.

  [Original Report]
  glibc-2.23 will introduce an important ABI change for Power. It will be a prerequisite for __builtin_cpu_is() and __builtin_cpu_supports() in gcc.

  The __builtin_cpu_is () / __builtin_cpu_supports()  are required to
  bring Power on-par with Intel regarding CPU and capability
  identification during runtime, allowing our customers to optimize
  their code path to the Power processor they are running on.

  These gcc builtins will be disabled when the binary is built against a
  glibc older than 2.23, making glibc-2.23 a requirement for this
  feature to work.

  This version of glibc will be released in early February/2016.

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