Re: [PATCH] show-ref: make --head always show the HEAD ref

On Jul 11, 2013, at 10:41 AM, Junio C Hamano [off-list ref] wrote:

Doug Bell [off-list ref] writes:

quoted

The docs seem to say that doing

	git show-ref --head --tags

would show both the HEAD ref and all the tag refs. However, doing
both --head and either of --tags or --heads would filter out the HEAD
ref.

Signed-off-by: Doug Bell <redacted>
---

I think this patch fell through the cracks, and looking at it, I am
somewhat torn.

The command help for "--head" says "show the HEAD reference", which
may mean:

(1) in addition to everything else the command would do if there
    weren't this option specified, show HEAD;

(2) show the HEAD and nothing else; or

(3) add HEAD to the candidates to be shown, but apply the usual
    filtering rule based on --heads, --tags and/or pattern
    arguments.

While the last interpretation is what we have used since the
beginning of the command at 358ddb62 (Add "git show-ref" builtin
command, 2006-09-15), I tend to agree with you that the first
interpretation may make more sense, at least from the end user's
point of view.

But at a deeper level, it makes the command somewhat inconsistent.

What happens in the command is

- We iterate over "candidates to be shown", which is usually
  "everything under refs/", but with "--head", HEAD is added to
  this set.  For each of these candidates:

  - if one or more <pattern> parameters are given, reject the
    candidate ref if it does not tail-match with any of the
    patterns;

  - if either "--heads" or "--tags" is given, among the ones that
    pass <pattern> filter, check if they:

    - begin with "refs/heads/" (if "--heads" is given); or
    - begin with "refs/tags/" (if "--tags" is given).

    and reject those that don't.

  - show it if it is still surviving after these two tests.

And taht is why "git show-ref --tags master v1.3.0" shows only the
v1.3.0 tag without showing the master branch, and giving "--heads"
instead of "--tags" yields only the master branch without the tag.

The semantics your patch wants, by changing the definition of
"--head" from (3) to (1), is:

- If "--head" is given, show HEAD no matter what.

- Iterate over everything under refs/, and for each of them, do the
  same filter-and-show as we currently do (see above).

While I think the new semantics is also understandable as the
current one, and personally I think it is a better behaviour than
the current one, it will require an update to the document to
highlight that "--head" is special-cased in a big way, to bypass all
the filtering that is applied to normal refs.

A few additional observations (these are not complaints to this
patch and please do not take them as such):

- The command help says "(can be combined with heads)" for "--tags"
  and vice versa, but does not mention their interaction with
  "--head".  This is because we take interpretation (3) above and
  do not treat "--head" as a mechanism to add to <pattern>
  parameter like these two.

- The command help for "--heads" and "--tags" says "only show
  heads/tags", which technically does not contradict with "can be
  combined with" above, but a logical consequence of combining
  ought to be showing nothing, as a ref cannot be a head (an old
  nomenclature for a "branch") and a tag at the same time.  

I think we should find a word better than "only" to use here, but I
am not sure what would be a good phrase to use.

The reason I had initially wanted both --tags and --head was I wanted to compare HEAD against all the tags to see which one(s) I was on (if any). I was eventually pointed to `git describe`, but I ended up just using show-ref without any options and filtering the result using Perl (the entire application is in Perl, so this wasn't a big deal). Then, yeah, I figured it was confusing enough to either patch the code or the docs.

For the doc changes, I think if it's explained that by default it show-ref shows refs/{tags,heads,remotes}, it becomes easier to explain what the options will end up doing. I'll put together a second patch.