Test the current state of the cache-tree optimization

The cache-tree optimization originally helped speed up write-tree
operation.  However, many commands no longer properly maintain -- or
use an opportunity to cheaply generate -- the cache-tree data.  In
particular, this affects commit, checkout and reset.  The notable
examples that *do* write cache-tree data are read-tree and write-tree.

This sadly means most people no longer benefit from the optimization,
as they would not normally use the plumbing commands.

Document the current state of affairs in a test file, in preparation
for improvements in the area.

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t0090-cache-tree.sh

@@ -0,0 +1,95 @@
+#!/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 "$1" filtered
+}
+
+# We don't bother with actually checking the SHA1:
+# test-dump-cache-tree already verifies that all existing data is
+# correct.
+test_shallow_cache_tree () {
+	echo "SHA " \
+	    "($(git ls-files|wc -l) entries, 0 subtrees)" >expect &&
+	cmp_cache_tree expect
+}
+
+test_invalid_cache_tree () {
+	echo "invalid                                   (0 subtrees)" >expect &&
+	echo "SHA #(ref) " \
+	    "($(git ls-files|wc -l) entries, 0 subtrees)" >>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_failure 'second commit has cache-tree' '
+	test_commit bar &&
+	test_shallow_cache_tree
+'
+
+test_expect_failure 'reset --hard gives cache-tree' '
+	test-scrap-cache-tree &&
+	git reset --hard &&
+	test_shallow_cache_tree
+'
+
+test_expect_failure '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