Re: Ambiguous reference weirdness

On Wed, Feb 22, 2012 at 09:48:23AM -0500, Phil Hord wrote:

quoted

What is 1147? Is it supposed to be a partial sha1, or is it a ref you
have?

1147 was a typo.  It was a Gerritt changeset ID I forgot to expand.
When I type "git cherry-pick 1147<TAB>", autocompletion expands this
for me to "git cherry-pick origin/changes/47/1147/1".  Except in this
case I misfired the TAB and got the weird "BUG:" report.  But I didn't
see the same problem with other invalid refs, so I went searching for
the variants and to see what caused it.

Ah. It's a little annoying that Gerrit names refs that look kind of
like partial sha1s. But I guess most of the time it's not that big a
deal (it is only because you were using the partial Gerrit ref with tab
completion). And I don't think we're about to change how Gerrit names
things. :)

quoted

Have you looked at the object that it resolves to? I suspect it is the
partial sha1 of a non-commit object. E.g.:

All of these examples were run in current git.git, so you can try them
yourself if needed.  But I did figure out that 1147 resolves to a
blob, and that's apparently the difference between these three:

Yeah. I figured that after reading more, but didn't go back and revise
the first part of my message. I was able to easily get the same results
as you (actually, in my git.git, 1147 is ambiguous because I happen to
have some extra refs, but it was easy to replicate with a fresh clone).

$ git cherry-pick 1147
fatal: BUG: expected exactly one commit from walk

$ git cherry-pick 1146
error: short SHA1 1146 is ambiguous.
error: short SHA1 1146 is ambiguous.
fatal: ambiguous argument '1146': unknown revision or path not in the
working tree.
Use '--' to separate paths from revisions

$ git cherry-pick 114333
fatal: ambiguous argument '114333': unknown revision or path not in
the working tree.
Use '--' to separate paths from revisions

I consider the first two responses to be UI bugs.   The second one is
minor (the twice-reported error message), and the first one is pretty
rude.   I would expect all three to report the same conclusion,
"fatal: ambiguous argument 'XXXXX': unknown revision or path not in
the working tree."  But the first one doesn't.

Right. Because cherry-pick's logic is:

  if the arguments do not resolve to any objects at all
      complain of unknown revision
  if the resolved object is not a single commit
      die of BUG

And I think you just want to collapse those two conditions into a single
conditional, which seems reasonable (the error message does say "unknown
revision", not "unknown object". It's a little more complicated than
that, because you are crossing a boundary between reusable library code
and cherry-pick specific code.

As for the doubled "ambiguous" warning, I'm not sure what the cause is
for that. I suspect it is because cherry-pick tries to parse one way (as
a revision walk specifier, like "a..b"), and then parses again (as a
single commit) when that fails, due to historical reasons. Obviously
it would be nice to see that improved, but I suspect it will be annoying
to do so; the error message is emitted by a low-level, and cherry-pick
has no idea that it has happened (it only sees "this didn't result in
parsing anything").

Thanks.  I'm not familiar with the {tree} syntax -- in fact I'd like
to find a dictionary for all the reference spelling variants -- but
this is elucidating.

See "git help rev-parse", section "Specifying Revisions".

quoted

In the cherry-pick case, the code is checking the right thing, but the
message is horrible. It is not a bug, but merely unexpected input, and
it should provide a usage message.

Bug is too strong a word in one sense, but from the user perspective I
consider this "horrible message" a bug.

Sorry, I meant git is wrong to use the word "BUG". For us, die("BUG:
...") is the equivalent of an assert. It means something totally
unexpected happened that should never happen, and therefore there is a
bug in git. But this is not a bug in git (well, it is, but not that
kind). It is a totally reasonable and expected malformed input that git
should diagnose and report correctly.

So it is a bug that git says "BUG". It should say "you gave me objects,
but they are not revisions" or something similar.

quoted

I think checkout has the same "is this a path or a revision" ambiguity
to resolve. But rather than be explicit that you might have meant "114"
as a tree, the error message assumes you meant a path. That might be
worth improving, similar to the above example.

Again, you can disambiguate with:

 $ git checkout -- 1147
 error: pathspec '1147' did not match any file(s) known to git.

 $ git checkout 1147 --
 fatal: reference is not a tree: 1147

quoted

$ git checkout 1147
fatal: reference is not a tree: 1147

Yes, I understand this.  This was a typo and it was ambiguous.  But
shouldn't we tell the user the same thing when encountering the same
failure?

No, because there are really three cases here:

  1. The user told us 1147 is a path.

  2. The user told us 1147 is a tree.

  3. The user did not tell us which, and we must guess.

We guess (1) if the name does not resolve at all, and (2) otherwise. And
the error message only indicates the particular guess we made.

So while I don't think they should all have the same error message,
there are two improvements I can see:

  a. If we are guessing, and 1147 resolves but is _not_ a tree, we
     could guess path instead.

  b. When our guess results in an error, it would be better to be
     explicit about the fact that we guessed (i.e., say "this is neither
     a tree nor a path that git knows about", similar to git-log).

quoted

I think the outcomes are all working as intended, but the error messages
could stand to be improved.

Yes, I agree.  I only meant to complain about the error messages, not
the results.  Thanks for the discussion.  I'll try to look for where
these come from and see if they can be improved within reason.

Great. I look forward to it.

-Peff