Re: [PATCH 2/4] Introduce optional "keywords" on tag objects

Johan Herland [off-list ref] writes:

For now, I couldn't find a good reason why the set of allowed characters
for keywords should be smaller than for the tag name.

The set of allowed tag names excludes shell metacharacters,
primarily to help scripting.  I think keywords can share the
same reasoning to exclude them.

It also excludes '^', '~' and ':', because tag names can be used
in revision range expressions (i.e. prefix '^' is the "exclude
from the resulting set" operation, postfix "~<number>" is the
"Nth generation ancestor" operation) and general SHA-1
expression (i.e. infix ':' is the "find in the tree-ish the
object at path" operation).  These reasons would not apply to 
keywords.

Having said all of that, I suspect it is premature to talk about
keywords, as it is unclear what their intended use is.  What
kind of operations are useful on them?  

It does not count that "git cat-file tag" would show "keywords
blah" on the header instead of in body.  It is not a compelling
enough reason to introduce a new header type. grep would work
just fine for such a use.

On the other hand, for example, if (the syntax is totally made
up) we make '::keywords=foo::' expand to set of all tags that
have the specified keyword 'foo', and it turns out to be useful
to be able to say "git show ::keywords=foo::" instead of listing
individual tags, that kind of use case may make it a good reason
to add such a new header type.