[PATCH] cache-tree: do not cache empty trees

["Nguyễn Thái Ngọc Duy" <pclouds@gmail.com>]

Current index does not support empty trees. But users can construct
empty trees directly using plumbing. When empty trees are checked out,
things become inconsistent:

 - If cache-tree somehow is invalidated, when a tree is read to index,
   empty trees disappear. When we write trees back, empty trees will
   be gone.

 - If cache-tree is generated by read-tree and remains valid by the
   time trees are written back, empty trees remain.

Let's do it in a consistent way, always disregard empty trees in
index. If users choose to create empty trees their own way, they
should not use index at all.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
---
 Scratch the first version. This one actually works.

 cache-tree.c               |    9 +++++++++
 t/t1013-read-tree-empty.sh |   38 ++++++++++++++++++++++++++++++++++++++
 2 files changed, 47 insertions(+), 0 deletions(-)
 create mode 100755 t/t1013-read-tree-empty.sh

diff --git a/cache-tree.c b/cache-tree.c
index f755590..03732ad 100644
--- a/cache-tree.c
+++ b/cache-tree.c
@@ -621,9 +621,18 @@ static void prime_cache_tree_rec(struct cache_tree *it, struct tree *tree)
 			struct tree *subtree = lookup_tree(entry.sha1);
 			if (!subtree->object.parsed)
 				parse_tree(subtree);
+			if (!hashcmp(entry.sha1, (unsigned char *)EMPTY_TREE_SHA1_BIN)) {
+				warning("empty tree detected! Will be removed from new commits");
+				cnt = -1;
+				break;
+			}
 			sub = cache_tree_sub(it, entry.path);
 			sub->cache_tree = cache_tree();
 			prime_cache_tree_rec(sub->cache_tree, subtree);
+			if (sub->cache_tree->entry_count == -1) {
+				cnt = -1;
+				break;
+			}
 			cnt += sub->cache_tree->entry_count;
 		}
 	}
diff --git a/t/t1013-read-tree-empty.sh b/t/t1013-read-tree-empty.sh
new file mode 100755
index 0000000..a9279f0
--- /dev/null
+++ b/t/t1013-read-tree-empty.sh
@@ -0,0 +1,38 @@
+#!/bin/sh
+
+test_description='read-tree with empty trees'
+
+. ./test-lib.sh
+
+T1=f4ec99e8174c01eab488469b4c2680500bbb18da
+T2=4b825dc642cb6eb9a060e54bf8d69288fbee4904
+
+test_expect_success 'setup' '
+	printf "40000 empty\0\x4b\x82\x5d\xc6\x42\xcb\x6e\xb9\xa0\x60\xe5\x4b\xf8\xd6\x92\x88\xfb\xee\x49\x04" >newtree &&
+	git hash-object -w -t tree newtree >actual &&
+	echo $T1 >expected
+	test_cmp expected actual
+'
+
+test_expect_success 'ls-tree T1 (with empty tree)' '
+	git ls-tree $T1 >actual &&
+	cat <<EOF >expected &&
+040000 tree 4b825dc642cb6eb9a060e54bf8d69288fbee4904	empty
+EOF
+	test_cmp expected actual
+'
+
+test_expect_success 'write-tree removes empty tree' '
+	git read-tree "$T1" &&
+	git write-tree >actual
+	echo $T2 >expected
+	test_cmp expected actual
+'
+
+test_expect_success 'ls-tree T2 (without empty tree)' '
+	git ls-tree $T2 >actual &&
+	: >expected &&
+	test_cmp expected actual
+'
+
+test_done
-- 
1.7.3.4.878.g439c7