diff --git a/.gitignore b/.gitignore
index 8572c8c0b0..122336c504 100644
--- a/.gitignore
+++ b/.gitignore
@@ -171,6 +171,7 @@
 /test-date
 /test-delta
 /test-dump-cache-tree
+/test-scrap-cache-tree
 /test-genrandom
 /test-index-version
 /test-line-buffer
diff --git a/Makefile b/Makefile
index 3031be5ee6..6295422e10 100644
--- a/Makefile
+++ b/Makefile
@@ -435,6 +435,7 @@ TEST_PROGRAMS_NEED_X += test-ctype
 TEST_PROGRAMS_NEED_X += test-date
 TEST_PROGRAMS_NEED_X += test-delta
 TEST_PROGRAMS_NEED_X += test-dump-cache-tree
+TEST_PROGRAMS_NEED_X += test-scrap-cache-tree
 TEST_PROGRAMS_NEED_X += test-genrandom
 TEST_PROGRAMS_NEED_X += test-index-version
 TEST_PROGRAMS_NEED_X += test-line-buffer
diff --git a/builtin/commit.c b/builtin/commit.c
index d8d6dd5eaa..d749cb112a 100644
--- a/builtin/commit.c
+++ b/builtin/commit.c
@@ -394,6 +394,7 @@ static char *prepare_index(int argc, const char **argv, const char *prefix,
 		fd = hold_locked_index(&index_lock, 1);
 		add_files_to_cache(also ? prefix : NULL, pathspec, 0);
 		refresh_cache_or_die(refresh_flags);
+		update_main_cache_tree(1);
 		if (write_cache(fd, active_cache, active_nr) ||
 		    close_lock_file(&index_lock))
 			die(_("unable to write new_index file"));
@@ -414,6 +415,7 @@ static char *prepare_index(int argc, const char **argv, const char *prefix,
 		fd = hold_locked_index(&index_lock, 1);
 		refresh_cache_or_die(refresh_flags);
 		if (active_cache_changed) {
+			update_main_cache_tree(1);
 			if (write_cache(fd, active_cache, active_nr) ||
 			    commit_locked_index(&index_lock))
 				die(_("unable to write new_index file"));
@@ -862,10 +864,7 @@ static int prepare_to_commit(const char *index_file, const char *prefix,
 	 */
 	discard_cache();
 	read_cache_from(index_file);
-	if (!active_cache_tree)
-		active_cache_tree = cache_tree();
-	if (cache_tree_update(active_cache_tree,
-			      active_cache, active_nr, 0, 0) < 0) {
+	if (update_main_cache_tree(0)) {
 		error(_("Error building trees"));
 		return 0;
 	}
diff --git a/builtin/reset.c b/builtin/reset.c
index 811e8e252c..8c2c1d52a2 100644
--- a/builtin/reset.c
+++ b/builtin/reset.c
@@ -43,6 +43,7 @@ static int reset_index_file(const unsigned char *sha1, int reset_type, int quiet
 	int nr = 1;
 	int newfd;
 	struct tree_desc desc[2];
+	struct tree *tree;
 	struct unpack_trees_options opts;
 	struct lock_file *lock = xcalloc(1, sizeof(struct lock_file));
 
@@ -84,6 +85,12 @@ static int reset_index_file(const unsigned char *sha1, int reset_type, int quiet
 		return error(_("Failed to find tree of %s."), sha1_to_hex(sha1));
 	if (unpack_trees(nr, desc, &opts))
 		return -1;
+
+	if (reset_type == MIXED || reset_type == HARD) {
+		tree = parse_tree_indirect(sha1);
+		prime_cache_tree(&active_cache_tree, tree);
+	}
+
 	if (write_cache(newfd, active_cache, active_nr) ||
 	    commit_locked_index(lock))
 		return error(_("Could not write new index file."));
diff --git a/cache-tree.c b/cache-tree.c
index f755590da8..8de39590d5 100644
--- a/cache-tree.c
+++ b/cache-tree.c
@@ -150,7 +150,7 @@ void cache_tree_invalidate_path(struct cache_tree *it, const char *path)
 }
 
 static int verify_cache(struct cache_entry **cache,
-			int entries)
+			int entries, int silent)
 {
 	int i, funny;
 
@@ -159,6 +159,8 @@ static int verify_cache(struct cache_entry **cache,
 	for (i = 0; i < entries; i++) {
 		struct cache_entry *ce = cache[i];
 		if (ce_stage(ce) || (ce->ce_flags & CE_INTENT_TO_ADD)) {
+			if (silent)
+				return -1;
 			if (10 < ++funny) {
 				fprintf(stderr, "...\n");
 				break;
@@ -370,10 +372,11 @@ int cache_tree_update(struct cache_tree *it,
 		      struct cache_entry **cache,
 		      int entries,
 		      int missing_ok,
-		      int dryrun)
+		      int dryrun,
+		      int silent)
 {
 	int i;
-	i = verify_cache(cache, entries);
+	i = verify_cache(cache, entries, silent);
 	if (i)
 		return i;
 	i = update_one(it, cache, entries, "", 0, missing_ok, dryrun);
@@ -573,7 +576,7 @@ int write_cache_as_tree(unsigned char *sha1, int flags, const char *prefix)
 
 		if (cache_tree_update(active_cache_tree,
 				      active_cache, active_nr,
-				      missing_ok, 0) < 0)
+				      missing_ok, 0, 0) < 0)
 			return WRITE_TREE_UNMERGED_INDEX;
 		if (0 <= newfd) {
 			if (!write_cache(newfd, active_cache, active_nr) &&
@@ -668,3 +671,11 @@ int cache_tree_matches_traversal(struct cache_tree *root,
 		return it->entry_count;
 	return 0;
 }
+
+int update_main_cache_tree (int silent)
+{
+	if (!the_index.cache_tree)
+		the_index.cache_tree = cache_tree();
+	return cache_tree_update(the_index.cache_tree,
+				 the_index.cache, the_index.cache_nr, 0, 0, silent);
+}
diff --git a/cache-tree.h b/cache-tree.h
index 3df641f593..0ec0b2a159 100644
--- a/cache-tree.h
+++ b/cache-tree.h
@@ -29,7 +29,9 @@ void cache_tree_write(struct strbuf *, struct cache_tree *root);
 struct cache_tree *cache_tree_read(const char *buffer, unsigned long size);
 
 int cache_tree_fully_valid(struct cache_tree *);
-int cache_tree_update(struct cache_tree *, struct cache_entry **, int, int, int);
+int cache_tree_update(struct cache_tree *, struct cache_entry **, int, int, int, int);
+
+int update_main_cache_tree(int);
 
 /* bitmasks to write_cache_as_tree flags */
 #define WRITE_TREE_MISSING_OK 1
diff --git a/merge-recursive.c b/merge-recursive.c
index cc664c39b6..e22a5195f6 100644
--- a/merge-recursive.c
+++ b/merge-recursive.c
@@ -265,7 +265,7 @@ struct tree *write_tree_from_memory(struct merge_options *o)
 
 	if (!cache_tree_fully_valid(active_cache_tree) &&
 	    cache_tree_update(active_cache_tree,
-			      active_cache, active_nr, 0, 0) < 0)
+			      active_cache, active_nr, 0, 0, 0) < 0)
 		die("error building trees");
 
 	result = lookup_tree(active_cache_tree->sha1);
diff --git a/t/t0090-cache-tree.sh b/t/t0090-cache-tree.sh
new file mode 100755
index 0000000000..6c33e28ee8
--- /dev/null
+++ b/t/t0090-cache-tree.sh
@@ -0,0 +1,93 @@
+#!/bin/sh
+
+test_description="Test whether cache-tree is properly updated
+
+Tests whether various commands properly update and/or rewrite the
+cache-tree extension.
+"
+ . ./test-lib.sh
+
+cmp_cache_tree () {
+	test-dump-cache-tree >actual &&
+	sed "s/$_x40/SHA/" <actual >filtered &&
+	test_cmp "$1" filtered
+}
+
+# We don't bother with actually checking the SHA1:
+# test-dump-cache-tree already verifies that all existing data is
+# correct.
+test_shallow_cache_tree () {
+	printf "SHA  (%d entries, 0 subtrees)\n" $(git ls-files|wc -l) >expect &&
+	cmp_cache_tree expect
+}
+
+test_invalid_cache_tree () {
+	echo "invalid                                   (0 subtrees)" >expect &&
+	printf "SHA #(ref)  (%d entries, 0 subtrees)\n" $(git ls-files|wc -l) >>expect &&
+	cmp_cache_tree expect
+}
+
+test_no_cache_tree () {
+	: >expect &&
+	cmp_cache_tree expect
+}
+
+test_expect_failure 'initial commit has cache-tree' '
+	test_commit foo &&
+	test_shallow_cache_tree
+'
+
+test_expect_success 'read-tree HEAD establishes cache-tree' '
+	git read-tree HEAD &&
+	test_shallow_cache_tree
+'
+
+test_expect_success 'git-add invalidates cache-tree' '
+	test_when_finished "git reset --hard; git read-tree HEAD" &&
+	echo "I changed this file" > foo &&
+	git add foo &&
+	test_invalid_cache_tree
+'
+
+test_expect_success 'update-index invalidates cache-tree' '
+	test_when_finished "git reset --hard; git read-tree HEAD" &&
+	echo "I changed this file" > foo &&
+	git update-index --add foo &&
+	test_invalid_cache_tree
+'
+
+test_expect_success 'write-tree establishes cache-tree' '
+	test-scrap-cache-tree &&
+	git write-tree &&
+	test_shallow_cache_tree
+'
+
+test_expect_success 'test-scrap-cache-tree works' '
+	git read-tree HEAD &&
+	test-scrap-cache-tree &&
+	test_no_cache_tree
+'
+
+test_expect_success 'second commit has cache-tree' '
+	test_commit bar &&
+	test_shallow_cache_tree
+'
+
+test_expect_success 'reset --hard gives cache-tree' '
+	test-scrap-cache-tree &&
+	git reset --hard &&
+	test_shallow_cache_tree
+'
+
+test_expect_success 'reset --hard without index gives cache-tree' '
+	rm -f .git/index &&
+	git reset --hard &&
+	test_shallow_cache_tree
+'
+
+test_expect_failure 'checkout gives cache-tree' '
+	git checkout HEAD^ &&
+	test_shallow_cache_tree
+'
+
+test_done
diff --git a/test-dump-cache-tree.c b/test-dump-cache-tree.c
index 1f73f1ea7d..e6c292385f 100644
--- a/test-dump-cache-tree.c
+++ b/test-dump-cache-tree.c
@@ -59,6 +59,6 @@ int main(int ac, char **av)
 	struct cache_tree *another = cache_tree();
 	if (read_cache() < 0)
 		die("unable to read index file");
-	cache_tree_update(another, active_cache, active_nr, 0, 1);
+	cache_tree_update(another, active_cache, active_nr, 0, 1, 0);
 	return dump_cache_tree(active_cache_tree, another, "");
 }
diff --git a/test-scrap-cache-tree.c b/test-scrap-cache-tree.c
new file mode 100644
index 0000000000..4728013910
--- /dev/null
+++ b/test-scrap-cache-tree.c
@@ -0,0 +1,17 @@
+#include "cache.h"
+#include "tree.h"
+#include "cache-tree.h"
+
+static struct lock_file index_lock;
+
+int main(int ac, char **av)
+{
+	int fd = hold_locked_index(&index_lock, 1);
+	if (read_cache() < 0)
+		die("unable to read index file");
+	active_cache_tree = NULL;
+	if (write_cache(fd, active_cache, active_nr)
+	    || commit_lock_file(&index_lock))
+		die("unable to write index file");
+	return 0;
+}