"breaking" command output message parsing (was: [RFC PATCH] vreportf: ensure sensible ordering of normal and error output)

On Tue, Nov 30 2021, Eric Sunshine wrote:

On Tue, Nov 30, 2021 at 9:05 AM Eric Sunshine [off-list ref] wrote:

quoted

(2) With git-worktree being four or five years old, for
backward-compatibility concerns, I worry that "that ship has sailed",
where 'that' is the freedom to relocate those status-like messages
from stdout to stderr. I don't want to break tooling which exists
around git-worktree.

I'd be happy to be wrong on the second point -- indeed, git-worktree
is still marked "experimental" in the man-page, but that may not mean
anything this late in the game -- and submit a patch which places
git-worktree's status-like messages on stderr instead of stdout.
Thoughts?

If that ship has indeed sailed, then perhaps the best and safest thing
to do is admit that git-worktree is an outlier in terms of sending
status-like messages to stdout, and just sprinkle the necessary
fflush(stdout) around in builtin/worktree.c and live with that
localized ugliness. Thoughts?

I really don't think that ship has sailed at all. We're at full liberty
to change these error messages, and have even done so for some plumbing
in the past (being sensitive to what sort of messages, sometimes they
are important).

See e.g. my 9144ba4cf52 (remote: add meaningful exit code on
missing/existing, 2020-10-27), that was a case where we knew about a
in-the-wild parser of the output, and git-remote is in the (I suppose
"pseudo-plumbing?") "Ancillary Commands" category.

I think the one grey area here is as in 9144ba4cf52 where we'd expect
people to reasonably script around these commands, and parsing the
output was the only way to accomplish something reasonable. E.g. in that
case automation around adding remotes & the handling of failure
scenarios. The case of [1] for "git pull" is another potential recent
one. I.e. someone grepping "Everything up-to-date".

From some brief skimming of the worktree.c code that doesn't seem to
apply, i.e. it's just chattyness.

I doubt anyone cares if it's blathering about "preparing a worktree" or
whatever, it just matters if "git worktree add" and the like fail with
non-zero, but perhaps there's cases of conflated states, as in that case
of "git remote" and "git pull".

1. https://lore.kernel.org/git/211130.86a6hleo84.gmgdl@evledraar.gmail.com/ (local)