[RFC PATCH v3 1/8] Prevent diff machinery from examining assume-unchanged entries on worktree

@@ -162,7 +162,8 @@ int run_diff_files(struct rev_info *revs, unsigned int option)
 		if (ce_uptodate(ce))
 			continue;
 
-		changed = check_removed(ce, &st);
+		/* If CE_VALID is set, don't look at workdir for file removal */
+		changed = (ce->ce_flags & CE_VALID) ? 0 : check_removed(ce, &st);
 		if (changed) {
 			if (changed < 0) {
 				perror(ce->name);

@@ -337,6 +338,8 @@ static void do_oneway_diff(struct unpack_trees_options *o,
 	struct rev_info *revs = o->unpack_data;
 	int match_missing, cached;
 
+	/* if the entry is not checked out, don't examine work tree */
+	cached = o->index_only || (idx && (idx->ce_flags & CE_VALID));
 	/*
 	 * Backward compatibility wart - "diff-index -m" does
 	 * not mean "do not ignore merges", but "match_missing".

@@ -344,7 +347,6 @@ static void do_oneway_diff(struct unpack_trees_options *o,
 	 * But with the revision flag parsing, that's found in
 	 * "!revs->ignore_merges".
 	 */
-	cached = o->index_only;
 	match_missing = !revs->ignore_merges;
 
 	if (cached && idx && ce_stage(idx)) {

@@ -0,0 +1,31 @@
+#!/bin/sh
+
+test_description='diff with assume-unchanged entries'
+
+. ./test-lib.sh
+
+# external diff has been tested in t4020-diff-external.sh
+
+test_expect_success 'setup' '
+	echo zero > zero &&
+	git add zero &&
+	git commit -m zero &&
+	echo one > one &&
+	echo two > two &&
+	git add one two &&
+	git commit -m onetwo &&
+	git update-index --assume-unchanged one &&
+	echo borked >> one &&
+	test "$(git ls-files -v one)" = "h one"
+'
+
+test_expect_success 'diff-index does not examine assume-unchanged entries' '
+	git diff-index HEAD^ -- one | grep -q 5626abf0f72e58d7a153368ba57db4c673c0e171
+'
+
+test_expect_success 'diff-files does not examine assume-unchanged entries' '
+	rm one &&
+	test -z "$(git diff-files -- one)"
+'
+
+test_done