add 'ls-remote' option to limit output records

add 'ls-remote' option to limit output records

[David Carson]

Given that 'ls-remote' can be sorted, it would be useful to be able to ask for a subset of the total number of result records.

For example, if I want to retrieve only the tag with the highest version, I would do so by adding this new option (-n1 in my example):

	$ git ls-remote -n1 --tags --sort=v:refname origin "v*"

That would save from having to receive a large list of tags from the remote, only one of which I care about.

~David

Re: add 'ls-remote' option to limit output records

[Bryan Turner]

On Fri, Apr 26, 2019 at 11:57 AM David Carson
<DCarson@extremenetworks.com> wrote:
>
> Given that 'ls-remote' can be sorted, it would be useful to be able to ask for a subset of the total number of result records.
>
> For example, if I want to retrieve only the tag with the highest version, I would do so by adding this new option (-n1 in my example):
>
>         $ git ls-remote -n1 --tags --sort=v:refname origin "v*"
>
> That would save from having to receive a large list of tags from the remote, only one of which I care about.

That sort of filtering would have to be applied client side, at least
partially, so it wouldn't necessarily save as much as you'd hope.

With the v0/v1 wire protocol, the server speaks first and sends the
entire ref advertisement, so the filtering would have to be 100%
client side.

With the v2 wire protocol, the client could provide a prefix (like
`refs/tags/`, or potentially `refs/tags/v`) to reduce what the server
included in the ref advertisement, but even the v2 protocol doesn't
have anything for telling the server "Version parse these names and
then return the first N". That means the parsing, sorting and trimming
for the advertised tags would still all have to happen locally. (I'm
sure someone can correct me if I've misstated what protocol v2 can do
for filtering, but I don't see any "ls-refs" options that look like
they enable anything other than prefix matching.)

Not trying to imply any judgments on the feature request itself; I'm
just noting that if it was added it may not be quite the win you were
hoping for. In fact, with the existing wire protocol versions (all of
them), I suspect this would end up with identical bandwidth usage to:

$ git ls-remote --tags --sort=v:refname origin "v*" | head -1

It would take modifications to the wire protocol itself for this to
happen server-side, not just the `ls-remote` command.

Bryan

Re: add 'ls-remote' option to limit output records

[Jeff King]

On Fri, Apr 26, 2019 at 12:32:51PM -0700, Bryan Turner wrote:

> >         $ git ls-remote -n1 --tags --sort=v:refname origin "v*"
> [...]
> With the v2 wire protocol, the client could provide a prefix (like
> `refs/tags/`, or potentially `refs/tags/v`) to reduce what the server
> included in the ref advertisement, but even the v2 protocol doesn't
> have anything for telling the server "Version parse these names and
> then return the first N". That means the parsing, sorting and trimming
> for the advertised tags would still all have to happen locally. (I'm
> sure someone can correct me if I've misstated what protocol v2 can do
> for filtering, but I don't see any "ls-refs" options that look like
> they enable anything other than prefix matching.)

No, that sounds right. The "--tags" in the original command should cause
v2 to ask only for refs/tags/. But because of the way ls-remote matching
works, "v*" is not a pure prefix match, and would find
"refs/tags/foo/v1.2.3". So we can't use protocol-level matching to ask
for "refs/tags/v*".

There are no provisions in the protocol for sorting, nor for limiting
the output.

I didn't check, but I suspect some hosting-platform APIs might be able
to answer this with a single query.

-Peff