fetch: add a --prune-tags option and fetch.pruneTags config

Add a --prune-tags option to git-fetch, along with fetch.pruneTags
config option and a -P shorthand (-p is --prune). This allows for
doing any of:

    git fetch -p -P
    git fetch --prune --prune-tags
    git fetch -p -P origin
    git fetch --prune --prune-tags origin

Or simply:

    git config fetch.prune true &&
    git config fetch.pruneTags true &&
    git fetch

Instead of the much more verbose:

    git fetch --prune origin 'refs/tags/*:refs/tags/*' '+refs/heads/*:refs/remotes/origin/*'

Before this feature it was painful to support the use-case of pulling
from a repo which is having both its branches *and* tags deleted
regularly, and have our local references to reflect upstream.

At work we create deployment tags in the repo for each rollout, and
there's *lots* of those, so they're archived within weeks for
performance reasons.

Without this change it's hard to centrally configure such repos in
/etc/gitconfig (on servers that are only used for working with
them). You need to set fetch.prune=true globally, and then for each
repo:

    git -C {} config --replace-all remote.origin.fetch "refs/tags/*:refs/tags/*" "^\+*refs/tags/\*:refs/tags/\*$"

Now I can simply set fetch.pruneTags=true in /etc/gitconfig as well,
and users running "git pull" will automatically get the pruning
semantics I want.

Even though "git remote" has corresponding "prune" and "update
--prune" subcommands I'm intentionally not adding a corresponding
prune-tags or "update --prune --prune-tags" mode to that command.

It's advertised (as noted in my recent "git remote doc: correct
dangerous lies about what prune does") as only modifying remote
tracking references, whereas any --prune-tags option is always going
to modify what from the user's perspective is a local copy of the tag,
since there's no such thing as a remote tracking tag.

Ideally add_prune_tags_to_fetch_refspec() would be something that
would use ALLOC_GROW() to grow the 'fetch` member of the 'remote'
struct. Instead I'm realloc-ing remote->fetch and adding the
tag_refspec to the end.

The reason is that parse_{fetch,push}_refspec which allocate the
refspec (ultimately remote->fetch) struct are called many places that
don't have access to a 'remote' struct. It would be hard to change all
their callsites to be amenable to carry around the bookkeeping
variables required for dynamic allocation.

All the other callers of the API first incrementally construct the
string version of the refspec in remote->fetch_refspec via
add_fetch_refspec(), before finally calling parse_fetch_refspec() via
some variation of remote_get().

It's less of a pain to deal with the one special case that needs to
modify already constructed refspecs than to chase down and change all
the other callsites. The API I'm adding is intentionally not
generalized because if we add more of these we'd probably want to
re-visit how this is done.

See my "Re: [BUG] git remote prune removes local tags, depending on
fetch config" (87po6ahx87.fsf@evledraar.gmail.com;
https://public-inbox.org/git/87po6ahx87.fsf@evledraar.gmail.com/) for
more background info.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 47 insertions, 0 deletions

diff --git a/Documentation/git-fetch.txt b/Documentation/git-fetch.txt
index e94bcfb..af12310 100644
--- a/Documentation/git-fetch.txt
+++ b/Documentation/git-fetch.txt
@@ -148,6 +148,53 @@ So be careful when using this with a refspec like
 `refs/tags/*:refs/tags/*`, or any other refspec which might map
 references from multiple remotes to the same local namespace.
 
+Since keeping up-to-date with both branches and tags on the remote is
+a common use-case the `--prune-tags` option can be supplied along with
+`--prune` to prune local tags that don't exist on the remote, and
+force-update those tags that differ. Tag pruning can also be enabled
+with `fetch.pruneTags` or `remote.<name>.pruneTags` in the config. See
+linkgit:git-config[1].
+
+The `--prune-tags` option is equivalent to having
+`refs/tags/*:refs/tags/*` declared in the refspecs of the remote. This
+can lead to some seemingly strange interactions:
+
+------------------------------------------------
+# These both fetch tags
+$ git fetch --no-tags origin 'refs/tags/*:refs/tags/*'
+$ git fetch --no-tags --prune-tags origin
+------------------------------------------------
+
+The reason it doesn't error out when provided without `--prune` or its
+config versions is for flexibility of the configured versions, and to
+maintain a 1=1 mapping between what the command line flags do, and
+what the configuration versions do.
+
+It's reasonable to e.g. configure `fetch.pruneTags=true` in
+`~/.gitconfig` to have tags pruned whenever `git fetch --prune` is
+run, without making every invocation of `git fetch` without `--prune`
+an error.
+
+Another special case of `--prune-tags` is that
+`refs/tags/*:refs/tags/*` will not be implicitly provided if an URL is
+being fetched. I.e.:
+
+------------------------------------------------
+$ git fetch <url> --prune --prune-tags
+------------------------------------------------
+
+Will prune no tags, as opposed to:
+
+------------------------------------------------
+$ git fetch origin --prune --prune-tags
+------------------------------------------------
+
+To prune tags given a URL supply the refspec explicitly:
+
+------------------------------------------------
+$ git fetch <url> --prune 'refs/tags/*:refs/tags/*'
+------------------------------------------------
+
 OUTPUT
 ------