thodg/libgit2

diff --git a/include/git2/diff.h b/include/git2/diff.h
index 273f471..afdb2c9 100644
--- a/include/git2/diff.h
+++ b/include/git2/diff.h
@@ -145,6 +145,13 @@ typedef enum {
 	 */
 	GIT_DIFF_ENABLE_FAST_UNTRACKED_DIRS = (1u << 14),
 
+	/** When diff finds a file in the working directory with stat
+	 * information different from the index, but the OID ends up being the
+	 * same, write the correct stat information into the index.  Note:
+	 * without this flag, diff will always leave the index untouched.
+	 */
+	GIT_DIFF_UPDATE_INDEX = (1u << 15),
+
 	/*
 	 * Options controlling how output will be generated
 	 */
diff --git a/src/checkout.c b/src/checkout.c
index d94cb0c..7279116 100644
--- a/src/checkout.c
+++ b/src/checkout.c
@@ -184,7 +184,7 @@ static bool checkout_is_workdir_modified(
 	if (baseitem->size && wditem->file_size != baseitem->size)
 		return true;
 
-	if (git_diff__oid_for_entry(&oid, data->diff, wditem) < 0)
+	if (git_diff__oid_for_entry(&oid, data->diff, wditem, NULL) < 0)
 		return false;
 
 	return (git_oid__cmp(&baseitem->id, &oid) != 0);
diff --git a/src/diff.c b/src/diff.c
index eae4543..caed8bf 100644
--- a/src/diff.c
+++ b/src/diff.c
@@ -442,6 +442,14 @@ static int diff_list_apply_options(
 		diff->new_src = tmp_src;
 	}
 
+	/* Unset UPDATE_INDEX unless diffing workdir and index */
+	if (DIFF_FLAG_IS_SET(diff, GIT_DIFF_UPDATE_INDEX) &&
+		(!(diff->old_src == GIT_ITERATOR_TYPE_WORKDIR ||
+		   diff->new_src == GIT_ITERATOR_TYPE_WORKDIR) ||
+		 !(diff->old_src == GIT_ITERATOR_TYPE_INDEX ||
+		   diff->new_src == GIT_ITERATOR_TYPE_INDEX)))
+		diff->opts.flags &= ~GIT_DIFF_UPDATE_INDEX;
+
 	/* if ignore_submodules not explicitly set, check diff config */
 	if (diff->opts.ignore_submodules <= 0) {
 		const git_config_entry *entry;
@@ -523,11 +531,14 @@ int git_diff__oid_for_file(
 	entry.file_size = size;
 	entry.path = (char *)path;
 
-	return git_diff__oid_for_entry(out, diff, &entry);
+	return git_diff__oid_for_entry(out, diff, &entry, NULL);
 }
 
 int git_diff__oid_for_entry(
-	git_oid *out, git_diff *diff, const git_index_entry *src)
+	git_oid *out,
+	git_diff *diff,
+	const git_index_entry *src,
+	const git_oid *update_match)
 {
 	int error = 0;
 	git_buf full_path = GIT_BUF_INIT;
@@ -595,7 +606,16 @@ int git_diff__oid_for_entry(
 		git_filter_list_free(fl);
 	}
 
-	/* TODO: update index for entry if requested */
+	/* update index for entry if requested */
+	if (!error && update_match && git_oid_equal(out, update_match)) {
+		git_index *idx;
+
+		if (!(error = git_repository_index(&idx, diff->repo))) {
+			memcpy(&entry.id, out, sizeof(entry.id));
+			error = git_index_add(idx, &entry);
+			git_index_free(idx);
+		}
+ 	}
 
 	git_buf_free(&full_path);
 	return error;
@@ -776,7 +796,12 @@ static int maybe_modified(
 	 */
 	if (modified_uncertain && git_oid_iszero(&nitem->id)) {
 		if (git_oid_iszero(&noid)) {
-			if ((error = git_diff__oid_for_entry(&noid, diff, nitem)) < 0)
+			const git_oid *update_check =
+				DIFF_FLAG_IS_SET(diff, GIT_DIFF_UPDATE_INDEX) ?
+				&oitem->id : NULL;
+
+			if ((error = git_diff__oid_for_entry(
+					&noid, diff, nitem, update_check)) < 0)
 				return error;
 		}
 
@@ -1208,6 +1233,9 @@ int git_diff_index_to_workdir(
 			&b, repo, GIT_ITERATOR_DONT_AUTOEXPAND, pfx, pfx)
 	);
 
+	if (!error && DIFF_FLAG_IS_SET(*diff, GIT_DIFF_UPDATE_INDEX))
+		error = git_index_write(index);
+
 	return error;
 }
 
diff --git a/src/diff.h b/src/diff.h
index 8fa3c9b..b2b7dba 100644
--- a/src/diff.h
+++ b/src/diff.h
@@ -97,7 +97,7 @@ extern int git_diff_delta__format_file_header(
 extern int git_diff__oid_for_file(
 	git_oid *out, git_diff *, const char *, uint16_t, git_off_t);
 extern int git_diff__oid_for_entry(
-	git_oid *out, git_diff *, const git_index_entry *entry);
+	git_oid *out, git_diff *, const git_index_entry *, const git_oid *update);
 
 extern int git_diff__from_iterators(
 	git_diff **diff_ptr,
diff --git a/tests/diff/workdir.c b/tests/diff/workdir.c
index 84c8866..9e4608e 100644
--- a/tests/diff/workdir.c
+++ b/tests/diff/workdir.c
@@ -1502,3 +1502,90 @@ void test_diff_workdir__with_stale_index(void)
 
 	git_index_free(idx);
 }
+
+static int touch_file(void *payload, git_buf *path)
+{
+	int fd;
+	char b;
+
+	GIT_UNUSED(payload);
+	if (git_path_isdir(path->ptr))
+		return 0;
+
+	cl_assert((fd = p_open(path->ptr, O_RDWR)) >= 0);
+	cl_assert_equal_i(1, p_read(fd, &b, 1));
+	cl_must_pass(p_lseek(fd, 0, SEEK_SET));
+	cl_must_pass(p_write(fd, &b, 1));
+	cl_must_pass(p_close(fd));
+
+	return 0;
+}
+
+static void basic_diff_status(git_diff **out, const git_diff_options *opts)
+{
+	diff_expects exp;
+
+	cl_git_pass(git_diff_index_to_workdir(out, g_repo, NULL, opts));
+
+	memset(&exp, 0, sizeof(exp));
+
+	cl_git_pass(git_diff_foreach(
+		*out, diff_file_cb, diff_hunk_cb, diff_line_cb, &exp));
+
+	cl_assert_equal_i(13, exp.files);
+	cl_assert_equal_i(0, exp.file_status[GIT_DELTA_ADDED]);
+	cl_assert_equal_i(4, exp.file_status[GIT_DELTA_DELETED]);
+	cl_assert_equal_i(4, exp.file_status[GIT_DELTA_MODIFIED]);
+	cl_assert_equal_i(1, exp.file_status[GIT_DELTA_IGNORED]);
+	cl_assert_equal_i(4, exp.file_status[GIT_DELTA_UNTRACKED]);
+}
+
+void test_diff_workdir__can_update_index(void)
+{
+	git_diff_options opts = GIT_DIFF_OPTIONS_INIT;
+	git_diff *diff = NULL;
+
+	g_repo = cl_git_sandbox_init("status");
+
+	/* touch all the files so stat times are different */
+	{
+		git_buf path = GIT_BUF_INIT;
+		cl_git_pass(git_buf_sets(&path, "status"));
+		cl_git_pass(git_path_direach(&path, 0, touch_file, NULL));
+		git_buf_free(&path);
+	}
+
+	opts.flags |= GIT_DIFF_INCLUDE_IGNORED | GIT_DIFF_INCLUDE_UNTRACKED;
+
+	basic_diff_status(&diff, &opts);
+#ifdef GIT_PERF
+	cl_assert_equal_sz(diff->stat_calls, 13 + 3);
+	cl_assert_equal_sz(diff->oid_calculations, 5);
+	cl_assert_equal_sz(diff->submodule_lookups, 1);
+#endif
+
+	git_diff_free(diff);
+
+	/* now allow diff to update stat cache */
+	opts.flags |= GIT_DIFF_UPDATE_INDEX;
+
+	basic_diff_status(&diff, &opts);
+#ifdef GIT_PERF
+	cl_assert_equal_sz(diff->stat_calls, 13 + 3);
+	cl_assert_equal_sz(diff->oid_calculations, 5);
+	cl_assert_equal_sz(diff->submodule_lookups, 1);
+#endif
+
+	git_diff_free(diff);
+
+	/* now if we do it again, we should see fewer OID calculations */
+
+	basic_diff_status(&diff, &opts);
+#ifdef GIT_PERF
+	cl_assert_equal_sz(diff->stat_calls, 13 + 3);
+	cl_assert_equal_sz(diff->oid_calculations, 0); /* Yay */
+	cl_assert_equal_sz(diff->submodule_lookups, 1);
+#endif
+
+	git_diff_free(diff);
+}