support fetching into a shallow repository

A shallow commit is a commit which has parents, which in turn are
"grafted away", i.e. the commit appears as if it were a root.

Since these shallow commits should not be edited by the user, but
only by core git, they are recorded in the file $GIT_DIR/shallow.

A repository containing shallow commits is called shallow.

The advantage of a shallow repository is that even if the upstream
contains lots of history, your local (shallow) repository needs not
occupy much disk space.

The disadvantage is that you might miss a merge base when pulling
some remote branch.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <junkio@cox.net>

1 files changed, 4 insertions, 0 deletions

diff --git a/fetch-pack.c b/fetch-pack.c
index 0a169dce8..bc5e72505 100644
--- a/fetch-pack.c
+++ b/fetch-pack.c
@@ -180,6 +180,8 @@ static int find_common(int fd[2], unsigned char *result_sha1,
 			packet_write(fd[1], "want %s\n", sha1_to_hex(remote));
 		fetching++;
 	}
+	if (is_repository_shallow())
+		write_shallow_commits(fd[1], 1);
 	packet_flush(fd[1]);
 	if (!fetching)
 		return 1;
@@ -523,6 +525,8 @@ static int fetch_pack(int fd[2], int nr_match, char **match)
 	int status;
 
 	get_remote_heads(fd[0], &ref, 0, NULL, 0);
+	if (is_repository_shallow() && !server_supports("shallow"))
+		die("Server does not support shallow clients");
 	if (server_supports("multi_ack")) {
 		if (verbose)
 			fprintf(stderr, "Server supports multi_ack\n");