upload-pack: avoid parsing tag destinations

When upload-pack advertises refs, it dereferences any tags
it sees, and shows the resulting sha1 to the client. It does
this by calling deref_tag. That function must load and parse
each tag object to find the sha1 of the tagged object.
However, it also ends up parsing the tagged object itself,
which is not strictly necessary for upload-pack's use.

Each tag produces two object loads (assuming it is not a
recursive tag), when it could get away with only a single
one. Dropping the second load halves the effort we spend.

The downside is that we are no longer verifying the
resulting object by loading it. In particular:

  1. We never cross-check the "type" field given in the tag
     object with the type of the pointed-to object.  If the
     tag says it points to a tag but doesn't, then we will
     keep peeling and realize the error.  If the tag says it
     points to a non-tag but actually points to a tag, we
     will stop peeling and just advertise the pointed-to
     tag.

  2. If we are missing the pointed-to object, we will not
     realize (because we never even look it up in the object
     db).

However, both of these are errors in the object database,
and both will be detected if a client actually requests the
broken objects in question. So we are simply pushing the
verification away from the advertising stage, and down to
the actual fetching stage.

On my test repo with 120K refs, this drops the time to
advertise the refs from ~3.2s to ~2.0s.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff --git a/tag.c b/tag.c
index 3aa186df628331e74e8a84d3cc2d313f4518a626..78d272b863f22285048cf54b9dcb03f80cb36f00 100644 (file)
--- a/tag.c
+++ b/tag.c
@@ -24,6 +24,18 @@ struct object *deref_tag(struct object *o, const char *warn, int warnlen)
        return o;
 }
 
+struct object *deref_tag_noverify(struct object *o)
+{
+       while (o && o->type == OBJ_TAG) {
+               o = parse_object(o->sha1);
+               if (o && o->type == OBJ_TAG && ((struct tag *)o)->tagged)
+                       o = ((struct tag *)o)->tagged;
+               else
+                       o = NULL;
+       }
+       return o;
+}
+
 struct tag *lookup_tag(const unsigned char *sha1)
 {
        struct object *obj = lookup_object(sha1);

diff --git a/tag.h b/tag.h
index 5ee88e6550cafa78b7e4acaa1285d5805974a037..bc8a1e40f04e87a6d502ab9d96022f734c57f4eb 100644 (file)
--- a/tag.h
+++ b/tag.h
@@ -16,6 +16,7 @@ extern struct tag *lookup_tag(const unsigned char *sha1);
 extern int parse_tag_buffer(struct tag *item, const void *data, unsigned long size);
 extern int parse_tag(struct tag *item);
 extern struct object *deref_tag(struct object *, const char *, int);
+extern struct object *deref_tag_noverify(struct object *);
 extern size_t parse_signature(const char *buf, unsigned long size);
 
 #endif /* TAG_H */

diff --git a/upload-pack.c b/upload-pack.c
index 65cb0ff0fbe6356eb62fb0936a14d46670a62ed4..c01e161a9d713b64951210416d44e96835d12a64 100644 (file)
--- a/upload-pack.c
+++ b/upload-pack.c
@@ -741,8 +741,7 @@ static int send_ref(const char *refname, const unsigned char *sha1, int flag, vo
                nr_our_refs++;
        }
        if (o->type == OBJ_TAG) {
-               o = parse_object(o->sha1);
-               o = deref_tag(o, refname, 0);
+               o = deref_tag_noverify(o);
                if (o)
                        packet_write(1, "%s %s^{}\n", sha1_to_hex(o->sha1), refname_nons);
        }