[PATCH 3/3] fetch-pack: Request fixed-off-by-one-depth when available

[Matthijs Kooijman <matthijs@stdin.nl>]

This server feature changes the meaning of the fetch depth, allowing
fetching only a single revision instead of at least two as before. To
make sure the behaviour only depends on the client version, the depth
value sent over the wire is corrected depending on wether the server has
the fix.

There is one corner case: A server without the fix cannot send less than
2 commmits, so when --depth=1 is specified a warning is shown and 2
commits are fetched instead of 1.

Signed-off-by: Matthijs Kooijman <matthijs@stdin.nl>
---
 fetch-pack.c | 26 ++++++++++++++++++++++++--
 1 file changed, 24 insertions(+), 2 deletions(-)

diff --git a/fetch-pack.c b/fetch-pack.c
index abe5ffb..799b2c1 100644
--- a/fetch-pack.c
+++ b/fetch-pack.c
@@ -39,6 +39,7 @@ static int marked;
 
 static struct commit_list *rev_list;
 static int non_common_revs, multi_ack, use_sideband, allow_tip_sha1_in_want;
+static int fixed_depth;
 
 static void rev_list_push(struct commit *commit, int mark)
 {
@@ -327,6 +328,7 @@ static int find_common(struct fetch_pack_args *args,
 			if (prefer_ofs_delta)   strbuf_addstr(&c, " ofs-delta");
 			if (agent_supported)    strbuf_addf(&c, " agent=%s",
 							    git_user_agent_sanitized());
+			if (fixed_depth)        strbuf_addstr(&c, " fixed-off-by-one-depth");
 			packet_buf_write(&req_buf, "want %s%s\n", remote_hex, c.buf);
 			strbuf_release(&c);
 		} else
@@ -342,8 +344,23 @@ static int find_common(struct fetch_pack_args *args,
 
 	if (is_repository_shallow())
 		write_shallow_commits(&req_buf, 1);
-	if (args->depth > 0)
-		packet_buf_write(&req_buf, "deepen %d", args->depth);
+	if (args->depth > 0) {
+		if (!fixed_depth && args->depth == 1)
+			warning("Server does not support depth=1, using depth=2 instead");
+		if (!fixed_depth && args->depth > 1) {
+			/* Old server that interprets "deepen 1" as
+			   "give me tip + 1 extra commit" */
+			packet_buf_write(&req_buf, "deepen %d", args->depth - 1);
+		} else if (!fixed_depth && args->depth == 1) {
+			/* Old servers cannot handle depth=1 (deepen=0
+			   means don't change depth / full depth). */
+			packet_buf_write(&req_buf, "deepen 1");
+		} else {
+			/* New server, send depth as-is */
+			packet_buf_write(&req_buf, "deepen %d", args->depth);
+		}
+	}
+
 	packet_buf_flush(&req_buf);
 	state_len = req_buf.len;
 
@@ -874,6 +891,11 @@ static struct ref *do_fetch_pack(struct fetch_pack_args *args,
 			fprintf(stderr, "Server supports ofs-delta\n");
 	} else
 		prefer_ofs_delta = 0;
+	if (server_supports("fixed-off-by-one-depth")) {
+		if (args->verbose)
+			fprintf(stderr, "Server has fixed meaning of depth value\n");
+		fixed_depth = 1;
+	}
 
 	if ((agent_feature = server_feature_value("agent", &agent_len))) {
 		agent_supported = 1;
-- 
1.8.3.rc1