[PATCH 2/3] fast-import: add a check for tree delta base sha1

[Dmitry Ivankov <divanorama@gmail.com>]

fast-import is able to write imported tree objects in delta format.
It holds a tree structure in memory where each tree entry may have
a delta base sha1 assigned. When delta base data is needed it is
reconstructed from this in-memory structure. Though sometimes the
delta base data doesn't match the delta base sha1 so wrong or even
corrupt pack is produced.

To create a small easily reproducible test, add an excessive check
for delta base sha1. It's not likely that computing sha1 for each
tree delta base costs us much.

Signed-off-by: Dmitry Ivankov <divanorama@gmail.com>
---
 fast-import.c          |   20 +++++++++++++++-----
 t/t9300-fast-import.sh |   38 ++++++++++++++++++++++++++++++++++++++
 2 files changed, 53 insertions(+), 5 deletions(-)

diff --git a/fast-import.c b/fast-import.c
index d0f8580..8196d1b 100644
--- a/fast-import.c
+++ b/fast-import.c
@@ -1455,12 +1455,22 @@ static void store_tree(struct tree_entry *root)
 			store_tree(t->entries[i]);
 	}
 
-	le = find_object(root->versions[0].sha1);
-	if (S_ISDIR(root->versions[0].mode) && le && le->pack_id == pack_id) {
+	if (!is_null_sha1(root->versions[0].sha1)
+					&& S_ISDIR(root->versions[0].mode)) {
+		unsigned char old_tree_sha1[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, old_tree_sha1);
+
+		if (hashcmp(old_tree_sha1, root->versions[0].sha1))
+			die("internal tree delta base sha1 mismatch");
+
+		le = find_object(root->versions[0].sha1);
+		if (le && le->pack_id == pack_id) {
+			lo.data = old_tree;
+			lo.offset = le->idx.offset;
+			lo.depth = t->delta_depth;
+		}
 	}
 
 	mktree(t, 1, &new_tree);
diff --git a/t/t9300-fast-import.sh b/t/t9300-fast-import.sh
index f256475..c70e489 100755
--- a/t/t9300-fast-import.sh
+++ b/t/t9300-fast-import.sh
@@ -734,6 +734,44 @@ test_expect_success \
 	 git diff-tree --abbrev --raw L^ L >output &&
 	 test_cmp expect output'
 
+cat >input <<INPUT_END
+blob
+mark :1
+data <<EOF
+the data
+EOF
+
+commit refs/heads/L2
+committer $GIT_COMMITTER_NAME <$GIT_COMMITTER_EMAIL> $GIT_COMMITTER_DATE
+data <<COMMIT
+init L2
+COMMIT
+
+M 644 :1 a/b/c
+M 644 :1 a/b/d
+M 644 :1 a/e/f
+INPUT_END
+
+cat >input2 <<INPUT_END
+commit refs/heads/L2
+committer $GIT_COMMITTER_NAME <$GIT_COMMITTER_EMAIL> $GIT_COMMITTER_DATE
+data <<COMMIT
+update L2
+COMMIT
+from refs/heads/L2^0
+M 040000 @A g
+M 040000 @E g/b
+M 040000 @E g/b/h
+INPUT_END
+
+test_expect_failure \
+    'L: verify internal tree delta base' \
+	'git fast-import <input &&
+	A=$(git ls-tree L2 a | tr " " "\t" | cut -f 3) &&
+	E=$(git ls-tree L2 a/e | tr " " "\t" | cut -f 3) &&
+	cat input2 | sed -e "s/@A/$A/" -e "s/@E/$E/" >input &&
+	git fast-import <input'
+
 ###
 ### series M
 ###
-- 
1.7.3.4