[PATCH 2/8] pack-objects: produce a stable pack when --skip is given

["Nguyễn Thái Ngọc Duy" <pclouds@gmail.com>]

Parallel delta search does not produce a stable pack so it's disabled
when --skip is used. zlib compression algorithm is stable. Ref
negotiation should be stable (at least on smart http). Ref changes
will be addressed separately.

So unless configuration files or git binary is changed, we should be
able to produce a stable pack. And if config or binaries are changed,
it's ok to abort and fetch a new (resumable) pack again.

Alternatively (and not implemented), pack-objects can be taught to
save the output pack when --skip=0 is passed in, then somehow find the
cached version and send the remaining when --skip=<non-zero> is
given. This saves processing power at the cost of disk and cache
management.

The key thing for caching though, is "find the cached version". We do
not provide any way for pack-objects to send a "key", like http
cookies, to the client, so that the client can pass it back on the
next fetch. Regardless, the input from client must be identical
between fetches, the server should be able to extract a signature from
that and use it to associate with a cached pack. So no need for
sending keys from the server side.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
---
 builtin/pack-objects.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/builtin/pack-objects.c b/builtin/pack-objects.c
index 417c830..c58a9cb 100644
--- a/builtin/pack-objects.c
+++ b/builtin/pack-objects.c
@@ -2709,6 +2709,11 @@ int cmd_pack_objects(int argc, const char **argv, const char *prefix)
 			if (get_oid_hex(skip_hash_hex, &skip_hash))
 				die(_("%s is not SHA-1"), skip_hash_hex);
 		}
+
+		/*
+		 * Parallel delta search can't produce stable packs.
+		 */
+		delta_search_threads = 1;
 	}
 
 	argv_array_push(&rp, "pack-objects");
-- 
2.7.0.377.g4cd97dd