[RFC/PATCH 11/11] WIP fetch: add a --fetch-prune option and fetch.pruneTags config

["Ævar Arnfjörð Bjarmason" <avarab@gmail.com>]

[WIP: This doesn't (yet) work as advertised, see further WIP note
below]

Add a --fetch-prune option to git-fetch along with fetch.pruneTags
config option. This allows for doing:

    git fetch origin -p -P

Or simply:

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

Instead of the much more verbose:

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

A use-case of git which is a pain to support now is frequent pulling
from a repo which is having both its branches *and* tags deleted
regularly. 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 we want.

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 details.

[WIP: This doesn't actually work, since the implementation is to just
pretend that 'refs/tags/*:refs/tags/*' was supplied on the
command-line, we'll consequently stop fetching any branches, since we
don't use the default refspec found in .git/config anymore

I'm not familiar enough with this code and the refspec machinery to
know if this argument injection is crazy, and how to tell it to *also*
do whatever it's doing by default]

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
---
 Documentation/config.txt        | 15 +++++++++++++++
 Documentation/fetch-options.txt | 15 ++++++++++++++-
 builtin/fetch.c                 | 37 ++++++++++++++++++++++++++++++++++---
 remote.c                        |  2 ++
 remote.h                        |  1 +
 t/t5510-fetch.sh                |  1 +
 6 files changed, 67 insertions(+), 4 deletions(-)

diff --git a/Documentation/config.txt b/Documentation/config.txt
index 0f27af5760..476175ef8d 100644
--- a/Documentation/config.txt
+++ b/Documentation/config.txt
@@ -1401,6 +1401,14 @@ fetch.prune::
 	option was given on the command line.  See also `remote.<name>.prune`
 	and the PRUNING section of linkgit:git-fetch[1].
 
+fetch.pruneTags::
+	If true, fetch will automatically behave as if the
+	`refs/tags/*:refs/tags/*` refspec was provided when pruning,
+	if not set already. This allows for setting both this option
+	and `fetch.prune` to maintain a 1=1 mapping to upstrem
+	refs. See also `remote.<name>.pruneTags` and the PRUNING
+	section of linkgit:git-fetch[1].
+
 fetch.output::
 	Control how ref update status is printed. Valid values are
 	`full` and `compact`. Default value is `full`. See section
@@ -2945,6 +2953,13 @@ remote.<name>.prune::
 	remove any remote-tracking references that no longer exist on the
 	remote (as if the `--prune` option was given on the command line).
 	Overrides `fetch.prune` settings, if any.
+
+remote.<name>.pruneTags::
+	When set to true, fetching from this remote by default will also
+	remove any local tags that no longer exist on the remote (as
+	if the `--prune` option was given on the command line in
+	conjunction with the `refs/tags/*:refs/tags/*` refspec).
+	Overrides `fetch.prune` settings, if any.
 +
 See also `remote.<name>.prune` and the PRUNING section of
 linkgit:git-fetch[1].
diff --git a/Documentation/fetch-options.txt b/Documentation/fetch-options.txt
index 9f5c85ad96..dc13bed741 100644
--- a/Documentation/fetch-options.txt
+++ b/Documentation/fetch-options.txt
@@ -73,7 +73,20 @@ ifndef::git-pull[]
 	are fetched due to an explicit refspec (either on the command
 	line or in the remote configuration, for example if the remote
 	was cloned with the --mirror option), then they are also
-	subject to pruning.
+	subject to pruning. Supplying `--prune-tags` is a shorthand for
+	providing the tag refspec.
++
+See the PRUNING section below for more details.
+
+-P::
+--prune-tags::
+	Before fetching, remove any local tags that no longer exist on
+	the remote if `--prune` is enabled. This option should be used
+	more carefully, unlike `--prune` it will remove any local
+	references (local tags) that have been created. This option is
+	merely a shorthand for providing the explicit tag refspec
+	along with `--prune`, see the discussion about that in its
+	documentation.
 +
 See the PRUNING section below for more details.
 
diff --git a/builtin/fetch.c b/builtin/fetch.c
index b34665db9e..57669a000c 100644
--- a/builtin/fetch.c
+++ b/builtin/fetch.c
@@ -38,6 +38,10 @@ static int fetch_prune_config = -1; /* unspecified */
 static int prune = -1; /* unspecified */
 #define PRUNE_BY_DEFAULT 0 /* do we prune by default? */
 
+static int fetch_prune_tags_config = -1; /* unspecified */
+static int prune_tags = -1; /* unspecified */
+#define PRUNE_TAGS_BY_DEFAULT 0 /* do we prune tags by default? */
+
 static int all, append, dry_run, force, keep, multiple, update_head_ok, verbosity, deepen_relative;
 static int progress = -1;
 static int tags = TAGS_DEFAULT, unshallow, update_shallow, deepen;
@@ -64,6 +68,11 @@ static int git_fetch_config(const char *k, const char *v, void *cb)
 		return 0;
 	}
 
+	if (!strcmp(k, "fetch.prunetags")) {
+		fetch_prune_tags_config = git_config_bool(k, v);
+		return 0;
+	}
+
 	if (!strcmp(k, "submodule.recurse")) {
 		int r = git_config_bool(k, v) ?
 			RECURSE_SUBMODULES_ON : RECURSE_SUBMODULES_OFF;
@@ -126,6 +135,8 @@ static struct option builtin_fetch_options[] = {
 		    N_("number of submodules fetched in parallel")),
 	OPT_BOOL('p', "prune", &prune,
 		 N_("prune remote-tracking branches no longer on remote")),
+	OPT_BOOL('P', "prune-tags", &prune_tags,
+		 N_("prune local tags not found on remote")),
 	{ OPTION_CALLBACK, 0, "recurse-submodules", &recurse_submodules, N_("on-demand"),
 		    N_("control recursive fetching of submodules"),
 		    PARSE_OPT_OPTARG, option_fetch_parse_recurse_submodules },
@@ -1212,6 +1223,8 @@ static void add_options_to_argv(struct argv_array *argv)
 		argv_array_push(argv, "--dry-run");
 	if (prune != -1)
 		argv_array_push(argv, prune ? "--prune" : "--no-prune");
+	if (prune_tags != -1)
+		argv_array_push(argv, prune_tags ? "--prune-tags" : "--no-prune-tags");
 	if (update_head_ok)
 		argv_array_push(argv, "--update-head-ok");
 	if (force)
@@ -1270,6 +1283,7 @@ static int fetch_one(struct remote *remote, int argc, const char **argv)
 	static const char **refs = NULL;
 	struct refspec *refspec;
 	int ref_nr = 0;
+	int j = 0;
 	int exit_code;
 
 	if (!remote)
@@ -1288,10 +1302,27 @@ static int fetch_one(struct remote *remote, int argc, const char **argv)
 			prune = PRUNE_BY_DEFAULT;
 	}
 
+	if (prune_tags < 0) {
+		/* no command line request */
+		if (0 <= gtransport->remote->prune_tags)
+			prune_tags = gtransport->remote->prune_tags;
+		else if (0 <= fetch_prune_tags_config)
+			prune_tags = fetch_prune_tags_config;
+		else
+			prune_tags = PRUNE_TAGS_BY_DEFAULT;
+	}
+
+	if (argc > 0 || prune_tags) {
+		size_t nr_alloc = st_add3(argc, prune_tags, 1);
+		refs = xcalloc(nr_alloc, sizeof(const char *));
+		if (prune_tags) {
+			refs[j++] = xstrfmt("refs/tags/*:refs/tags/*");
+			ref_nr++;
+		}
+	}
+
 	if (argc > 0) {
-		int j = 0;
 		int i;
-		refs = xcalloc(st_add(argc, 1), sizeof(const char *));
 		for (i = 0; i < argc; i++) {
 			if (!strcmp(argv[i], "tag")) {
 				i++;
@@ -1301,8 +1332,8 @@ static int fetch_one(struct remote *remote, int argc, const char **argv)
 						    argv[i], argv[i]);
 			} else
 				refs[j++] = argv[i];
+			ref_nr++;
 		}
-		ref_nr = j;
 	}
 
 	sigchain_push_common(unlock_pack_on_signal);
diff --git a/remote.c b/remote.c
index 4e93753e19..9eb6d24009 100644
--- a/remote.c
+++ b/remote.c
@@ -391,6 +391,8 @@ static int handle_config(const char *key, const char *value, void *cb)
 		remote->skip_default_update = git_config_bool(key, value);
 	else if (!strcmp(subkey, "prune"))
 		remote->prune = git_config_bool(key, value);
+	else if (!strcmp(subkey, "prunetags"))
+		remote->prune_tags = git_config_bool(key, value);
 	else if (!strcmp(subkey, "url")) {
 		const char *v;
 		if (git_config_string(&v, key, value))
diff --git a/remote.h b/remote.h
index 1f6611be21..04d7994cbf 100644
--- a/remote.h
+++ b/remote.h
@@ -47,6 +47,7 @@ struct remote {
 	int skip_default_update;
 	int mirror;
 	int prune;
+	int prune_tags;
 
 	const char *receivepack;
 	const char *uploadpack;
diff --git a/t/t5510-fetch.sh b/t/t5510-fetch.sh
index 18280df4fc..1a822861e0 100755
--- a/t/t5510-fetch.sh
+++ b/t/t5510-fetch.sh
@@ -617,6 +617,7 @@ test_configured_prune () {
 test_configured_prune unset unset unset unset kept   kept   ""
 test_configured_prune unset unset unset unset kept   kept   "--no-prune"
 test_configured_prune unset unset unset unset pruned kept   "--prune"
+test_configured_prune unset unset unset unset pruned pruned "--prune --prune-tags" # WIP: broken, see commit msg
 test_configured_prune unset unset unset unset kept   pruned \
 	"--prune origin 'refs/tags/*:refs/tags/*'"
 test_configured_prune unset unset unset unset pruned pruned \
-- 
2.15.1.424.g9478a66081