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authorJeff King <peff@peff.net>2017-01-28 00:09:59 (GMT)
committerJunio C Hamano <gitster@pobox.com>2017-01-28 00:24:44 (GMT)
commit7dbabbbebe3ae047841690d035f302313a8fe51e (patch)
tree8c6d925f66d7a348935cc976c241eea0ae2c7e4d /builtin/pack-objects.c
parentad36dc8b4b165bf9eb3576b42a241164e312d48c (diff)
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pack-objects: enforce --depth limit in reused deltas
Since 898b14c (pack-objects: rework check_delta_limit usage, 2007-04-16), we check the delta depth limit only when figuring out whether we should make a new delta. We don't consider it at all when reusing deltas, which means that packing once with --depth=250, and then again with --depth=50, the second pack may still contain chains larger than 50. This is generally considered a feature, as the results of earlier high-depth repacks are carried forward, used for serving fetches, etc. However, since we started using cross-pack deltas in c9af708b1 (pack-objects: use mru list when iterating over packs, 2016-08-11), we are no longer bounded by the length of an existing delta chain in a single pack. Here's one particular pathological case: a sequence of N packs, each with 2 objects, the base of which is stored as a delta in a previous pack. If we chain all the deltas together, we have a cycle of length N. We break the cycle, but the tip delta is still at depth N-1. This is less unlikely than it might sound. See the included test for a reconstruction based on real-world actions. I ran into such a case in the wild, where a client was rapidly sending packs, and we had accumulated 10,000 before doing a server-side repack. The pack that "git repack" tried to generate had a very deep chain, which caused pack-objects to run out of stack space in the recursive write_one(). This patch bounds the length of delta chains in the output pack based on --depth, regardless of whether they are caused by cross-pack deltas or existed in the input packs. This fixes the problem, but does have two possible downsides: 1. High-depth aggressive repacks followed by "normal" repacks will throw away the high-depth chains. In the long run this is probably OK; investigation showed that high-depth repacks aren't actually beneficial, and we dropped the aggressive depth default to match the normal case in 07e7dbf0d (gc: default aggressive depth to 50, 2016-08-11). 2. If you really do want to store high-depth deltas on disk, they may be discarded and new delta computed when serving a fetch, unless you set pack.depth to match your high-depth size. The implementation uses the existing search for delta cycles. That lets us compute the depth of any node based on the depth of its base, because we know the base is DFS_DONE by the time we look at it (modulo any cycles in the graph, but we know there cannot be any because we break them as we see them). There is some subtlety worth mentioning, though. We record the depth of each object as we compute it. It might seem like we could save the per-object storage space by just keeping track of the depth of our traversal (i.e., have break_delta_chains() report how deep it went). But we may visit an object through multiple delta paths, and on subsequent paths we want to know its depth immediately, without having to walk back down to its final base (doing so would make our graph walk quadratic rather than linear). Likewise, one could try to record the depth not from the base, but from our starting point (i.e., start recursion_depth at 0, and pass "recursion_depth + 1" to each invocation of break_delta_chains()). And then when recursion_depth gets too big, we know that we must cut the delta chain. But that technique is wrong if we do not visit the nodes in topological order. In a chain A->B->C, it if we visit "C", then "B", then "A", we will never recurse deeper than 1 link (because we see at each node that we have already visited it). Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>
Diffstat (limited to 'builtin/pack-objects.c')
-rw-r--r--builtin/pack-objects.c18
1 files changed, 18 insertions, 0 deletions
diff --git a/builtin/pack-objects.c b/builtin/pack-objects.c
index 0fd52bd..5c42a1d 100644
--- a/builtin/pack-objects.c
+++ b/builtin/pack-objects.c
@@ -1541,6 +1541,8 @@ static int pack_offset_sort(const void *_a, const void *_b)
* 2. Updating our size/type to the non-delta representation. These were
* either not recorded initially (size) or overwritten with the delta type
* (type) when check_object() decided to reuse the delta.
+ *
+ * 3. Resetting our delta depth, as we are now a base object.
*/
static void drop_reused_delta(struct object_entry *entry)
{
@@ -1554,6 +1556,7 @@ static void drop_reused_delta(struct object_entry *entry)
p = &(*p)->delta_sibling;
}
entry->delta = NULL;
+ entry->depth = 0;
oi.sizep = &entry->size;
oi.typep = &entry->type;
@@ -1572,6 +1575,9 @@ static void drop_reused_delta(struct object_entry *entry)
* Follow the chain of deltas from this entry onward, throwing away any links
* that cause us to hit a cycle (as determined by the DFS state flags in
* the entries).
+ *
+ * We also detect too-long reused chains that would violate our --depth
+ * limit.
*/
static void break_delta_chains(struct object_entry *entry)
{
@@ -1589,6 +1595,18 @@ static void break_delta_chains(struct object_entry *entry)
*/
entry->dfs_state = DFS_ACTIVE;
break_delta_chains(entry->delta);
+
+ /*
+ * Once we've recursed, our base (if we still have one) knows
+ * its depth, so we can compute ours (and check it against
+ * the limit).
+ */
+ if (entry->delta) {
+ entry->depth = entry->delta->depth + 1;
+ if (entry->depth > depth)
+ drop_reused_delta(entry);
+ }
+
entry->dfs_state = DFS_DONE;
break;