[PATCH] cache-tree: populate cache-tree on successful merge

[David Turner <dturner@twopensource.com>]

When we unpack trees into an existing index, we discard the old index
and replace it with the new, merged index.  Ensure that this index has
its cache-tree populated.  This will make subsequent git status and
commit commands faster.

Signed-off-by: David Turner <dturner@twopensource.com>
Signed-off-by: Brian Degenhardt <bmd@bmdhacks.com>
---

This patch is by my colleague, Brian Degenhardt (as part of his work
on git at Twitter).  I'm sending it with his and Twitter's approval.

---
 t/t0090-cache-tree.sh | 24 ++++++++++++++++++++++++
 unpack-trees.c        |  7 +++++++
 2 files changed, 31 insertions(+)

diff --git a/t/t0090-cache-tree.sh b/t/t0090-cache-tree.sh
index 601d02d..055cc19 100755
--- a/t/t0090-cache-tree.sh
+++ b/t/t0090-cache-tree.sh
@@ -199,6 +199,30 @@ test_expect_success 'checkout -B gives cache-tree' '
 	test_cache_tree
 '
 
+test_expect_success 'merge --ff-only maintains cache-tree' '
+	git checkout current &&
+	git checkout -b changes &&
+	test_commit llamas &&
+	test_commit pachyderm &&
+	test_cache_tree &&
+	git checkout current &&
+	test_cache_tree &&
+	git merge --ff-only changes &&
+	test_cache_tree
+'
+
+test_expect_success 'merge maintains cache-tree' '
+	git checkout current &&
+	git checkout -b changes2 &&
+	test_commit alpacas &&
+	test_cache_tree &&
+	git checkout current &&
+	test_commit struthio &&
+	test_cache_tree &&
+	git merge changes2 &&
+	test_cache_tree
+'
+
 test_expect_success 'partial commit gives cache-tree' '
 	git checkout -b partial no-children &&
 	test_commit one &&
diff --git a/unpack-trees.c b/unpack-trees.c
index 2927660..befc247 100644
--- a/unpack-trees.c
+++ b/unpack-trees.c
@@ -1156,6 +1156,13 @@ int unpack_trees(unsigned len, struct tree_desc *t, struct unpack_trees_options
 	o->src_index = NULL;
 	ret = check_updates(o) ? (-2) : 0;
 	if (o->dst_index) {
+		if (!o->result.cache_tree)
+			o->result.cache_tree = cache_tree();
+
+		if (!cache_tree_fully_valid(o->result.cache_tree)) {
+			cache_tree_update(&o->result, WRITE_TREE_SILENT | WRITE_TREE_REPAIR);
+		}
+
 		discard_index(o->dst_index);
 		*o->dst_index = o->result;
 	} else {
-- 
2.0.4.315.gad8727a-twtrsrc