Introduce the ls-refs server command.  In protocol v2, the ls-refs
command is used to request the ref advertisement from the server.  Since
it is a command which can be requested (as opposed to mandatory in v1),
a client can sent a number of parameters in its request to limit the ref
advertisement based on provided ref-patterns.

Signed-off-by: Brandon Williams <bmw...@google.com>
---
 Documentation/technical/protocol-v2.txt |  32 +++++++++
 Makefile                                |   1 +
 ls-refs.c                               |  96 ++++++++++++++++++++++++++
 ls-refs.h                               |   9 +++
 serve.c                                 |   8 +++
 t/t5701-git-serve.sh                    | 115 ++++++++++++++++++++++++++++++++
 6 files changed, 261 insertions(+)
 create mode 100644 ls-refs.c
 create mode 100644 ls-refs.h

diff --git a/Documentation/technical/protocol-v2.txt 
b/Documentation/technical/protocol-v2.txt
index f87372f9b..ef81df868 100644
--- a/Documentation/technical/protocol-v2.txt
+++ b/Documentation/technical/protocol-v2.txt
@@ -112,3 +112,35 @@ printable ASCII characters except space (i.e., the byte 
range 32 < x <
 "git/1.8.3.1"). The agent strings are purely informative for statistics
 and debugging purposes, and MUST NOT be used to programmatically assume
 the presence or absence of particular features.
+
+ ls-refs
+---------
+
+`ls-refs` is the command used to request a reference advertisement in v2.
+Unlike the current reference advertisement, ls-refs takes in parameters
+which can be used to limit the refs sent from the server.
+
+Additional features not supported in the base command will be advertised
+as the value of the command in the capability advertisement in the form
+of a space separated list of features, e.g.  "<command>=<feature 1>
+<feature 2>".
+
+ls-refs takes in the following parameters wrapped in packet-lines:
+
+    symrefs
+       In addition to the object pointed by it, show the underlying ref
+       pointed by it when showing a symbolic ref.
+    peel
+       Show peeled tags.
+    ref-pattern <pattern>
+       When specified, only references matching the one of the provided
+       patterns are displayed.
+
+The output of ls-refs is as follows:
+
+    output = *ref
+            flush-pkt
+    ref = PKT-LINE(obj-id SP refname *(SP ref-attribute) LF)
+    ref-attribute = (symref | peeled)
+    symref = "symref-target:" symref-target
+    peeled = "peeled:" obj-id
diff --git a/Makefile b/Makefile
index 18c255428..e50927cfb 100644
--- a/Makefile
+++ b/Makefile
@@ -825,6 +825,7 @@ LIB_OBJS += list-objects-filter-options.o
 LIB_OBJS += ll-merge.o
 LIB_OBJS += lockfile.o
 LIB_OBJS += log-tree.o
+LIB_OBJS += ls-refs.o
 LIB_OBJS += mailinfo.o
 LIB_OBJS += mailmap.o
 LIB_OBJS += match-trees.o
diff --git a/ls-refs.c b/ls-refs.c
new file mode 100644
index 000000000..70682b4f7
--- /dev/null
+++ b/ls-refs.c
@@ -0,0 +1,96 @@
+#include "cache.h"
+#include "repository.h"
+#include "refs.h"
+#include "remote.h"
+#include "argv-array.h"
+#include "ls-refs.h"
+#include "pkt-line.h"
+
+struct ls_refs_data {
+       unsigned peel;
+       unsigned symrefs;
+       struct argv_array patterns;
+};
+
+/*
+ * Check if one of the patterns matches the tail part of the ref.
+ * If no patterns were provided, all refs match.
+ */
+static int ref_match(const struct argv_array *patterns, const char *refname)
+{
+       char *pathbuf;
+       int i;
+
+       if (!patterns->argc)
+               return 1; /* no restriction */
+
+       pathbuf = xstrfmt("/%s", refname);
+       for (i = 0; i < patterns->argc; i++) {
+               if (!wildmatch(patterns->argv[i], pathbuf, 0)) {
+                       free(pathbuf);
+                       return 1;
+               }
+       }
+       free(pathbuf);
+       return 0;
+}
+
+static int send_ref(const char *refname, const struct object_id *oid,
+                   int flag, void *cb_data)
+{
+       struct ls_refs_data *data = cb_data;
+       const char *refname_nons = strip_namespace(refname);
+       struct strbuf refline = STRBUF_INIT;
+
+       if (!ref_match(&data->patterns, refname))
+               return 0;
+
+       strbuf_addf(&refline, "%s %s", oid_to_hex(oid), refname_nons);
+       if (data->symrefs && flag & REF_ISSYMREF) {
+               struct object_id unused;
+               const char *symref_target = resolve_ref_unsafe(refname, 0,
+                                                              &unused,
+                                                              &flag);
+
+               if (!symref_target)
+                       die("'%s' is a symref but it is not?", refname);
+
+               strbuf_addf(&refline, " symref-target:%s", symref_target);
+       }
+
+       if (data->peel) {
+               struct object_id peeled;
+               if (!peel_ref(refname, &peeled))
+                       strbuf_addf(&refline, " peeled:%s", 
oid_to_hex(&peeled));
+       }
+
+       strbuf_addch(&refline, '\n');
+       packet_write(1, refline.buf, refline.len);
+
+       strbuf_release(&refline);
+       return 0;
+}
+
+int ls_refs(struct repository *r, struct argv_array *keys, struct argv_array 
*args)
+{
+       int i;
+       struct ls_refs_data data = { 0, 0, ARGV_ARRAY_INIT };
+
+       for (i = 0; i < args->argc; i++) {
+               const char *arg = args->argv[i];
+               const char *out;
+
+               if (!strcmp("peel", arg))
+                       data.peel = 1;
+               else if (!strcmp("symrefs", arg))
+                       data.symrefs = 1;
+               else if (skip_prefix(arg, "ref-pattern ", &out))
+                       argv_array_pushf(&data.patterns, "*/%s", out);
+       }
+
+       head_ref_namespaced(send_ref, &data);
+       for_each_namespaced_ref(send_ref, &data);
+       packet_flush(1);
+       argv_array_clear(&data.patterns);
+       return 0;
+}
diff --git a/ls-refs.h b/ls-refs.h
new file mode 100644
index 000000000..9e4c57bfe
--- /dev/null
+++ b/ls-refs.h
@@ -0,0 +1,9 @@
+#ifndef LS_REFS_H
+#define LS_REFS_H
+
+struct repository;
+struct argv_array;
+extern int ls_refs(struct repository *r, struct argv_array *keys,
+                  struct argv_array *args);
+
+#endif /* LS_REFS_H */
diff --git a/serve.c b/serve.c
index cf23179b9..c7925c5c7 100644
--- a/serve.c
+++ b/serve.c
@@ -4,8 +4,15 @@
 #include "pkt-line.h"
 #include "version.h"
 #include "argv-array.h"
+#include "ls-refs.h"
 #include "serve.h"
 
+static int always_advertise(struct repository *r,
+                           struct strbuf *value)
+{
+       return 1;
+}
+
 static int agent_advertise(struct repository *r,
                           struct strbuf *value)
 {
@@ -44,6 +51,7 @@ struct protocol_capability {
 
 static struct protocol_capability capabilities[] = {
        { "agent", agent_advertise, NULL },
+       { "ls-refs", always_advertise, ls_refs },
 };
 
 static void advertise_capabilities(void)
diff --git a/t/t5701-git-serve.sh b/t/t5701-git-serve.sh
index affbad097..33536254e 100755
--- a/t/t5701-git-serve.sh
+++ b/t/t5701-git-serve.sh
@@ -8,6 +8,7 @@ test_expect_success 'test capability advertisement' '
        cat >expect <<-EOF &&
        version 2
        agent=git/$(git version | cut -d" " -f3)
+       ls-refs
        0000
        EOF
 
@@ -57,4 +58,118 @@ test_expect_success 'request invalid command' '
        test_i18ngrep "invalid command" err
 '
 
+# Test the basics of ls-refs
+#
+test_expect_success 'setup some refs and tags' '
+       test_commit one &&
+       git branch dev master &&
+       test_commit two &&
+       git symbolic-ref refs/heads/release refs/heads/master &&
+       git tag -a -m "annotated tag" annotated-tag
+'
+
+test_expect_success 'basics of ls-refs' '
+       test-pkt-line pack >in <<-EOF &&
+       command=ls-refs
+       0000
+       EOF
+
+       cat >expect <<-EOF &&
+       $(git rev-parse HEAD) HEAD
+       $(git rev-parse refs/heads/dev) refs/heads/dev
+       $(git rev-parse refs/heads/master) refs/heads/master
+       $(git rev-parse refs/heads/release) refs/heads/release
+       $(git rev-parse refs/tags/annotated-tag) refs/tags/annotated-tag
+       $(git rev-parse refs/tags/one) refs/tags/one
+       $(git rev-parse refs/tags/two) refs/tags/two
+       0000
+       EOF
+
+       git serve --stateless-rpc <in >out &&
+       test-pkt-line unpack <out >actual &&
+       test_cmp actual expect
+'
+
+test_expect_success 'basic ref-patterns' '
+       test-pkt-line pack >in <<-EOF &&
+       command=ls-refs
+       0001
+       ref-pattern master
+       ref-pattern one
+       0000
+       EOF
+
+       cat >expect <<-EOF &&
+       $(git rev-parse refs/heads/master) refs/heads/master
+       $(git rev-parse refs/tags/one) refs/tags/one
+       0000
+       EOF
+
+       git serve --stateless-rpc <in >out &&
+       test-pkt-line unpack <out >actual &&
+       test_cmp actual expect
+'
+
+test_expect_success 'wildcard ref-patterns' '
+       test-pkt-line pack >in <<-EOF &&
+       command=ls-refs
+       0001
+       ref-pattern refs/heads/*
+       0000
+       EOF
+
+       cat >expect <<-EOF &&
+       $(git rev-parse refs/heads/dev) refs/heads/dev
+       $(git rev-parse refs/heads/master) refs/heads/master
+       $(git rev-parse refs/heads/release) refs/heads/release
+       0000
+       EOF
+
+       git serve --stateless-rpc <in >out &&
+       test-pkt-line unpack <out >actual &&
+       test_cmp actual expect
+'
+
+test_expect_success 'peel parameter' '
+       test-pkt-line pack >in <<-EOF &&
+       command=ls-refs
+       0001
+       peel
+       ref-pattern refs/tags/*
+       0000
+       EOF
+
+       cat >expect <<-EOF &&
+       $(git rev-parse refs/tags/annotated-tag) refs/tags/annotated-tag 
peeled:$(git rev-parse refs/tags/annotated-tag^{})
+       $(git rev-parse refs/tags/one) refs/tags/one
+       $(git rev-parse refs/tags/two) refs/tags/two
+       0000
+       EOF
+
+       git serve --stateless-rpc <in >out &&
+       test-pkt-line unpack <out >actual &&
+       test_cmp actual expect
+'
+
+test_expect_success 'symrefs parameter' '
+       test-pkt-line pack >in <<-EOF &&
+       command=ls-refs
+       0001
+       symrefs
+       ref-pattern refs/heads/*
+       0000
+       EOF
+
+       cat >expect <<-EOF &&
+       $(git rev-parse refs/heads/dev) refs/heads/dev
+       $(git rev-parse refs/heads/master) refs/heads/master
+       $(git rev-parse refs/heads/release) refs/heads/release 
symref-target:refs/heads/master
+       0000
+       EOF
+
+       git serve --stateless-rpc <in >out &&
+       test-pkt-line unpack <out >actual &&
+       test_cmp actual expect
+'
+
 test_done
-- 
2.16.0.rc1.238.g530d649a79-goog

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