[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>
---
 On Wed, Feb 2, 2011 at 2:09 AM, Ilari Liusvaara <ilari.liusvaara@elisanet.fi> wrote:
 > Yes, writing to index/working area. IIRC, having such entry in tree causes
 > a "ghost directory". I don't exactly recall what such thing broke, but I
 > remember that it broke something (merging?)...
 >
 > Those ghosts also had annoying tendency to persist between commits. Commits
 > didn't kill them. Rm didn't work. You had to create something on top/inside to
 > get rid of them.

 That's probably because of cache-tree. Empty trees can't exist in
 index so an operation "trees -> index -> trees" will remove empty
 trees.

 But read-tree can preserve the exact structure of original trees
 (including empty trees) so if that particular path is untouched,
 empty trees will remain.

 Perhaps a patch like this for pre-1.8.0?

 cache-tree.c |    2 ++
 1 files changed, 2 insertions(+), 0 deletions(-)

diff --git a/cache-tree.c b/cache-tree.c
index f755590..f717793 100644
--- a/cache-tree.c
+++ b/cache-tree.c
@@ -621,6 +621,8 @@ 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))
+				continue;
 			sub = cache_tree_sub(it, entry.path);
 			sub->cache_tree = cache_tree();
 			prime_cache_tree_rec(sub->cache_tree, subtree);
-- 
1.7.3.4.878.g439c7