fetch: fetch objects by their exact SHA-1 object names

Teach "git fetch" to accept an exact SHA-1 object name the user may
obtain out of band on the LHS of a pathspec, and send it on a "want"
message when the server side advertises the allow-tip-sha1-in-want
capability.
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

fetch-pack.c

@@ -36,7 +36,7 @@ static int marked;
 #define MAX_IN_VAIN 256
 
 static struct commit_list *rev_list;
-static int non_common_revs, multi_ack, use_sideband;
+static int non_common_revs, multi_ack, use_sideband, allow_tip_sha1_in_want;
 
 static void rev_list_push(struct commit *commit, int mark)
 {
@@ -563,6 +563,21 @@ static void filter_refs(struct fetch_pack_args *args,
 		}
 	}
 
+	/* Append unmatched requests to the list */
+	if (allow_tip_sha1_in_want) {
+		for (i = 0; i < nr_sought; i++) {
+			ref = sought[i];
+			if (ref->matched)
+				continue;
+			if (get_sha1_hex(ref->name, ref->old_sha1))
+				continue;
+
+			ref->matched = 1;
+			*newtail = ref;
+			ref->next = NULL;
+			newtail = &ref->next;
+		}
+	}
 	*refs = newlist;
 }
 
@@ -803,6 +818,11 @@ static struct ref *do_fetch_pack(struct fetch_pack_args *args,
 			fprintf(stderr, "Server supports side-band\n");
 		use_sideband = 1;
 	}
+	if (server_supports("allow-tip-sha1-in-want")) {
+		if (args->verbose)
+			fprintf(stderr, "Server supports allow-tip-sha1-in-want\n");
+		allow_tip_sha1_in_want = 1;
+	}
 	if (!server_supports("thin-pack"))
 		args->use_thin_pack = 0;
 	if (!server_supports("no-progress"))

remote.c

@@ -15,6 +15,7 @@ static struct refspec s_tag_refspec = {
 	0,
 	1,
 	0,
+	0,
 	"refs/tags/*",
 	"refs/tags/*"
 };
@@ -565,9 +566,13 @@ static struct refspec *parse_refspec_internal(int nr_refspec, const char **refsp
 		flags = REFNAME_ALLOW_ONELEVEL | (is_glob ? REFNAME_REFSPEC_PATTERN : 0);
 
 		if (fetch) {
+			unsigned char unused[40];
+
 			/* LHS */
 			if (!*rs[i].src)
 				; /* empty is ok; it means "HEAD" */
+			else if (llen == 40 && !get_sha1_hex(rs[i].src, unused))
+				rs[i].exact_sha1 = 1; /* ok */
 			else if (!check_refname_format(rs[i].src, flags))
 				; /* valid looking ref is ok */
 			else
@@ -1495,7 +1500,12 @@ int get_fetch_map(const struct ref *remote_refs,
 	} else {
 		const char *name = refspec->src[0] ? refspec->src : "HEAD";
 
-		ref_map = get_remote_ref(remote_refs, name);
+		if (refspec->exact_sha1) {
+			ref_map = alloc_ref(name);
+			get_sha1_hex(name, ref_map->old_sha1);
+		} else {
+			ref_map = get_remote_ref(remote_refs, name);
+		}
 		if (!missing_ok && !ref_map)
 			die("Couldn't find remote ref %s", name);
 		if (ref_map) {

remote.h

@@ -62,6 +62,7 @@ struct refspec {
 	unsigned force : 1;
 	unsigned pattern : 1;
 	unsigned matching : 1;
+	unsigned exact_sha1 : 1;
 
 	char *src;
 	char *dst;

t/t5516-fetch-push.sh

@@ -1064,4 +1064,38 @@ do
 	'
 done
 
+test_expect_success 'fetch exact SHA1' '
+	mk_test heads/master hidden/one &&
+	git push testrepo master:refs/hidden/one &&
+	(
+		cd testrepo &&
+		git config transfer.hiderefs refs/hidden
+	) &&
+	check_push_result $the_commit hidden/one &&
+
+	mk_child child &&
+	(
+		cd child &&
+
+		# make sure $the_commit does not exist here
+		git repack -a -d &&
+		git prune &&
+		test_must_fail git cat-file -t $the_commit &&
+
+		# fetching the hidden object should fail by default
+		test_must_fail git fetch -v ../testrepo $the_commit:refs/heads/copy &&
+		test_must_fail git rev-parse --verify refs/heads/copy &&
+
+		# the server side can allow it to succeed
+		(
+			cd ../testrepo &&
+			git config uploadpack.allowtipsha1inwant true
+		) &&
+
+		git fetch -v ../testrepo $the_commit:refs/heads/copy &&
+		result=$(git rev-parse --verify refs/heads/copy) &&
+		test "$the_commit" = "$result"
+	)
+'
+
 test_done