path: root/pack-objects.h
diff options
authorJeff King <>2016-08-11 09:26:36 (GMT)
committerJunio C Hamano <>2016-08-11 17:44:13 (GMT)
commit4cf2143e02993440d6661ddccbd896bee7756c02 (patch)
tree3320029fcb1d4d211a6dc883e59eafda221d7453 /pack-objects.h
parentca79c985727653656443da8d92c9f5c2b4da93ed (diff)
pack-objects: break delta cycles before delta-search phase
We do not allow cycles in the delta graph of a pack (i.e., A is a delta of B which is a delta of A) for the obvious reason that you cannot actually access any of the objects in such a case. There's a last-ditch attempt to notice cycles during the write phase, during which we issue a warning to the user and write one of the objects out in full. However, this is "last-ditch" for two reasons: 1. By this time, it's too late to find another delta for the object, so the resulting pack is larger than it otherwise could be. 2. The warning is there because this is something that _shouldn't_ ever happen. If it does, then either: a. a pack we are reusing deltas from had its own cycle b. we are reusing deltas from multiple packs, and we found a cycle among them (i.e., A is a delta of B in one pack, but B is a delta of A in another, and we choose to use both deltas). c. there is a bug in the delta-search code So this code serves as a final check that none of these things has happened, warns the user, and prevents us from writing a bogus pack. Right now, (2b) should never happen because of the static ordering of packs in want_object_in_pack(). If two objects have a delta relationship, then they must be in the same pack, and therefore we will find them from that same pack. However, a future patch would like to change that static ordering, which will make (2b) a common occurrence. In preparation, we should be able to handle those kinds of cycles better. This patch does by introducing a cycle-breaking step during the get_object_details() phase, when we are deciding which deltas can be reused. That gives us the chance to feed the objects into the delta search as if the cycle did not exist. We'll leave the detection and warning in the write_object() phase in place, as it still serves as a check for case (2c). This does mean we will stop warning for (2a). That case is caused by bogus input packs, and we ideally would warn the user about it. However, since those cycles show up after picking reusable deltas, they look the same as (2b) to us; our new code will break the cycles early and the last-ditch check will never see them. We could do analysis on any cycles that we find to distinguish the two cases (i.e., it is a bogus pack if and only if every delta in the cycle is in the same pack), but we don't need to. If there is a cycle inside a pack, we'll run into problems not only reusing the delta, but accessing the object data at all. So when we try to dig up the actual size of the object, we'll hit that same cycle and kick in our usual complain-and-try-another-source code. Signed-off-by: Jeff King <> Signed-off-by: Junio C Hamano <>
Diffstat (limited to 'pack-objects.h')
1 files changed, 9 insertions, 0 deletions
diff --git a/pack-objects.h b/pack-objects.h
index d1b98b3..cc9b9a9 100644
--- a/pack-objects.h
+++ b/pack-objects.h
@@ -27,6 +27,15 @@ struct object_entry {
unsigned no_try_delta:1;
unsigned tagged:1; /* near the very tip of refs */
unsigned filled:1; /* assigned write-order */
+ /*
+ * State flags for depth-first search used for analyzing delta cycles.
+ */
+ enum {
+ DFS_NONE = 0,
+ } dfs_state;
struct packing_data {