[PATCH 0/9] git archive: use gzip again by default, document output stabilty

As reported in https://lore.kernel.org/git/a812a664-67ea-c0ba-599f-cb79e2d96694@gmail.com/ 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