[PATCH v3 4/4] transport.c: introduce core.alternateRefsPrefixes

[Taylor Blau <me@ttaylorr.com>]

The recently-introduced "core.alternateRefsCommand" allows callers to
specify with high flexibility the tips that they wish to advertise from
alternates. This flexibility comes at the cost of some inconvenience
when the caller only wishes to limit the advertisement to one or more
prefixes.

For example, to advertise only tags, a caller using
'core.alternateRefsCommand' would have to do:

  $ git config core.alternateRefsCommand ' \
      git -C "$1" for-each-ref refs/tags --format="%(objectname)"'

The above is cumbersome to write, so let's introduce a
"core.alternateRefsPrefixes" to address this common case. Instead, the
caller can run:

  $ git config core.alternateRefsPrefixes 'refs/tags'

Which will behave identically to the longer example using
"core.alternateRefsCommand".

Since the value of "core.alternateRefsPrefixes" is appended to 'git
for-each-ref' and then executed, include a "--" before taking the
configured value to avoid misinterpreting arguments as flags to 'git
for-each-ref'.

In the case that the caller wishes to specify multiple prefixes, they
may separate them by whitespace. If "core.alternateRefsCommand" is set,
it will take precedence over "core.alternateRefsPrefixes".

Signed-off-by: Taylor Blau <me@ttaylorr.com>
---
 Documentation/config.txt | 7 +++++++
 t/t5410-receive-pack.sh  | 8 ++++++++
 transport.c              | 5 +++++
 3 files changed, 20 insertions(+)

diff --git a/Documentation/config.txt b/Documentation/config.txt
index afcb18331a..9ef792ef0d 100644
--- a/Documentation/config.txt
+++ b/Documentation/config.txt
@@ -627,6 +627,13 @@ alternate's references as ".have"'s. For example, to only advertise branch
 heads, configure `core.alternateRefsCommand` to the path of a script which runs
 `git --git-dir="$1" for-each-ref --format='%(objectname)' refs/heads`.
 
+core.alternateRefsPrefixes::
+	When listing references from an alternate, list only references that begin
+	with the given prefix. Prefixes match as if they were given as arguments to
+	linkgit:git-for-each-ref[1]. To list multiple prefixes, separate them with
+	whitespace. If `core.alternateRefsCommand` is set, setting
+	`core.alternateRefsPrefixes` has no effect.
+
 core.bare::
 	If true this repository is assumed to be 'bare' and has no
 	working directory associated with it.  If this is the case a
diff --git a/t/t5410-receive-pack.sh b/t/t5410-receive-pack.sh
index 503dde35a4..3449967cc7 100755
--- a/t/t5410-receive-pack.sh
+++ b/t/t5410-receive-pack.sh
@@ -46,4 +46,12 @@ test_expect_success 'with core.alternateRefsCommand' '
 	test_cmp expect actual.haves
 '
 
+test_expect_success 'with core.alternateRefsPrefixes' '
+	test_config -C fork core.alternateRefsPrefixes "refs/tags" &&
+	git rev-parse one three two >expect &&
+	printf "0000" | git receive-pack fork >actual &&
+	extract_haves <actual >actual.haves &&
+	test_cmp expect actual.haves
+'
+
 test_done
diff --git a/transport.c b/transport.c
index e271b66603..83474add28 100644
--- a/transport.c
+++ b/transport.c
@@ -1341,6 +1341,11 @@ static void fill_alternate_refs_command(struct child_process *cmd,
 		argv_array_pushf(&cmd->args, "--git-dir=%s", repo_path);
 		argv_array_push(&cmd->args, "for-each-ref");
 		argv_array_push(&cmd->args, "--format=%(objectname)");
+
+		if (!git_config_get_value("core.alternateRefsPrefixes", &value)) {
+			argv_array_push(&cmd->args, "--");
+			argv_array_split(&cmd->args, value);
+		}
 	}
 
 	cmd->env = local_repo_env;
-- 
2.19.0.221.g150f307af