As reported in
https://lore.kernel.org/git/a812a664-67ea-c0ba-599f-cb79e2d96694@gmail.com/ (local)
changing the default "tgz" output method of from "gzip(1)" to our
internal "git archive gzip" (using zlib ) broke things for users in
the wild that assume that the "git archive" output is stable, most
notably GitHub: https://github.com/orgs/community/discussions/45830

Leaving aside the larger question of whether we're going to promise
output stability for "git archive" in general, the motivation for that
change was to have a working compression method on systems that lacked
a gzip(1).

As the disruption of changing the default isn't worth it, let's use
gzip(1) again by default, and only fall back on the new "git archive
gzip" if it isn't available.

The later parts of this series then document and test for the output
stability of the command.

We're not promising anything new there, except that we now promise
that we're going to use "gzip" as the default compressor, but that
it's up to that command to be stable, should the user desire output
stability.

The documentation discusses the various caveats involved, suggests
alternatives to checksumming compressed archives, but in the end notes
what's been the policy so far: We're not promising that the "tar"
output is going to be stable.

The early parts of this series (1-2/9) are clean-up for existing
config drift, as later in the series we'll otherwise need to change
the divergent config documentation in two places.

CI & branch for this at:
https://github.com/avar/git/tree/avar/archive-internal-gzip-not-the-default

Ævar Arnfjörð Bjarmason (9):
  archive & tar config docs: de-duplicate configuration section
  git config docs: document "tar.<format>.{command,remote}"
  archiver API: make the "flags" in "struct archiver" an enum
  archive: omit the shell for built-in "command" filters
  archive-tar.c: move internal gzip implementation to a function
  archive: use "gzip -cn" for stability, not "git archive gzip"
  test-lib.sh: add a lazy GZIP prerequisite
  archive tests: test for "gzip -cn" and "git archive gzip" stability
  git archive docs: document output non-stability

 Documentation/config/tar.txt           | 29 +++++++-
 Documentation/git-archive.txt          | 96 +++++++++++++++++++-------
 archive-tar.c                          | 78 ++++++++++++++-------
 archive.h                              | 11 +--
 t/t5000-tar-tree.sh                    |  2 -
 t/t5005-archive-stability.sh           | 70 +++++++++++++++++++
 t/t5562-http-backend-content-length.sh |  2 -
 t/test-lib.sh                          |  4 ++
 8 files changed, 231 insertions(+), 61 deletions(-)
 create mode 100755 t/t5005-archive-stability.sh

-- 
2.39.1.1392.g63e6d408230