[PATCH/WIP 6/7] fast-import: workaround data corruption

[Dmitry Ivankov <divanorama@gmail.com>]

fast-import keeps track of some delta-base for tree objects. When it is
time to compute the delta, base object is constructed from in-memory
tree representation using children's delta bases sha1. But these can be
unrelated due to several bugs, and it leads to object with wrong sha1
being delta-written to the packfile.

We have the base sha1 and what we think it's data is. Verify sha1 and if
it doesn't match, report it to stderr and don't use delta for this tree.

We could also die() here when bugs are fixed. Or we can see if the data
we've got is from our pack file and so still try to use it as a base.

Signed-off-by: Dmitry Ivankov <divanorama@gmail.com>
---
 fast-import.c |   14 +++++++++++++-
 1 files changed, 13 insertions(+), 1 deletions(-)

diff --git a/fast-import.c b/fast-import.c
index 9f0d2fe..14a2a63 100644
--- a/fast-import.c
+++ b/fast-import.c
@@ -1469,7 +1469,8 @@ static void drop_old(struct tree_entry *root)
 static void store_tree(struct tree_entry *root)
 {
 	struct tree_content *t = root->tree;
-	struct last_object lo = { STRBUF_INIT, 0, 0, /* no_swap */ 1 };
+	struct strbuf empty = STRBUF_INIT;
+	struct last_object lo = { empty, 0, 0, /* no_swap */ 1 };
 	struct object_entry *le;
 	unsigned int i;
 
@@ -1486,10 +1487,21 @@ static void store_tree(struct tree_entry *root)
 
 	le = find_object(root->versions[0].sha1);
 	if (S_ISDIR(root->versions[0].mode) && le && le->pack_id == pack_id) {
+		unsigned char sh[20];
 		mktree(t, 0, &old_tree);
 		lo.data = old_tree;
 		lo.offset = le->idx.offset;
 		lo.depth = t->delta_depth;
+
+		prepare_object_hash(OBJ_TREE, &old_tree, NULL, NULL, sh);
+		if (hashcmp(sh, root->versions[0].sha1)) {
+			fprintf(stderr, "internal sha1 delta base mismatch,"
+					" won't use delta for that tree\n");
+			lo.data = empty;
+			lo.offset = 0;
+			lo.depth = 0;
+		}
+
 	}
 
 	mktree(t, 1, &new_tree);
-- 
1.7.3.4