diff: check files with the same or newer timestamps

When a file on the workdir has the same or a newer timestamp than the
index, we need to perform a full check of the contents, as the update of
the file may have happened just after we wrote the index.

The iterator changes are such that we can reach inside the workdir
iterator from the diff, though it may be better to have an accessor
instead of moving these structs into the header.

src/diff.c

@@ -816,11 +816,11 @@ static int maybe_modified(
 	} else if (git_oid_iszero(&nitem->id) && new_is_workdir) {
 		bool use_ctime = ((diff->diffcaps & GIT_DIFFCAPS_TRUST_CTIME) != 0);
 		bool use_nanos = ((diff->diffcaps & GIT_DIFFCAPS_TRUST_NANOSECS) != 0);
+		git_index *index;
+		git_iterator_index(&index, info->new_iter);
 
 		status = GIT_DELTA_UNMODIFIED;
 
-		/* TODO: add check against index file st_mtime to avoid racy-git */
-
 		if (S_ISGITLINK(nmode)) {
 			if ((error = maybe_modified_submodule(&status, &noid, diff, info)) < 0)
 				return error;
@@ -839,7 +839,8 @@ static int maybe_modified(
 			 !diff_time_eq(&oitem->ctime, &nitem->ctime, use_nanos)) ||
 			oitem->ino != nitem->ino ||
 			oitem->uid != nitem->uid ||
-			oitem->gid != nitem->gid)
+			oitem->gid != nitem->gid ||
+			(index && nitem->mtime.seconds >= index->stamp.mtime))
 		{
 			status = GIT_DELTA_MODIFIED;
 			modified_uncertain = true;

src/iterator.c

@@ -1762,6 +1762,18 @@ int git_iterator_current_workdir_path(git_buf **path, git_iterator *iter)
 	return 0;
 }
 
+int git_iterator_index(git_index **out, git_iterator *iter)
+{
+	workdir_iterator *wi = (workdir_iterator *)iter;
+
+	if (iter->type != GIT_ITERATOR_TYPE_WORKDIR)
+		*out = NULL;
+
+	*out = wi->index;
+
+	return 0;
+}
+
 int git_iterator_advance_over_with_status(
 	const git_index_entry **entryptr,
 	git_iterator_status_t *status,

src/iterator.h

@@ -11,6 +11,7 @@
 #include "git2/index.h"
 #include "vector.h"
 #include "buffer.h"
+#include "ignore.h"
 
 typedef struct git_iterator git_iterator;
 
@@ -286,4 +287,11 @@ typedef enum {
 extern int git_iterator_advance_over_with_status(
 	const git_index_entry **entry, git_iterator_status_t *status, git_iterator *iter);
 
+/**
+ * Retrieve the index stored in the iterator.
+ *
+ * Only implemented for the workdir iterator
+ */
+extern int git_iterator_index(git_index **out, git_iterator *iter);
+
 #endif

tests/diff/racy.c

 #include "clar_libgit2.h"
+#include "../checkout/checkout_helpers.h"
 
 #include "buffer.h"
 
@@ -37,3 +38,35 @@ void test_diff_racy__diff(void)
 	cl_git_pass(git_diff_index_to_workdir(&diff, g_repo, index, NULL));
 	cl_assert_equal_i(1, git_diff_num_deltas(diff));
 }
+
+void test_diff_racy__write_index_just_after_file(void)
+{
+	git_index *index;
+	git_diff *diff;
+	git_buf path = GIT_BUF_INIT;
+
+	/* Make sure we do have a timestamp */
+	cl_git_pass(git_repository_index(&index, g_repo));
+	cl_git_pass(git_index_write(index));
+	/* The timestamp will be one second before we change the file */
+	sleep(1);
+
+	cl_git_pass(git_buf_joinpath(&path, git_repository_workdir(g_repo), "A"));
+	cl_git_mkfile(path.ptr, "A");
+
+	/*
+	 * Put 'A' into the index, the size field will be filled,
+	 * because the index' on-disk timestamp does not match the
+	 * file's timestamp. Both timestamps will however match after
+	 * writing out the index.
+	 */
+	cl_git_pass(git_repository_index(&index, g_repo));
+	cl_git_pass(git_index_add_bypath(index, "A"));
+	cl_git_pass(git_index_write(index));
+
+	/* Change its contents quickly, so we get the same timestamp */
+	cl_git_mkfile(path.ptr, "B");
+
+	cl_git_pass(git_diff_index_to_workdir(&diff, g_repo, index, NULL));
+	cl_assert_equal_i(1, git_diff_num_deltas(diff));
+}

tests/diff/workdir.c

@@ -1623,6 +1623,8 @@ void test_diff_workdir__can_update_index(void)
 
 	/* now if we do it again, we should see fewer OID calculations */
 
+	/* tick again as the index updating from the previous diff might have reset the timestamp */
+	tick_index(index);
 	basic_diff_status(&diff, &opts);
 
 	cl_git_pass(git_diff_get_perfdata(&perf, diff));

tests/status/worktree.c

@@ -985,6 +985,8 @@ void test_status_worktree__update_stat_cache_0(void)
 
 	opts.flags &= ~GIT_STATUS_OPT_UPDATE_INDEX;
 
+	/* tick again as the index updating from the previous diff might have reset the timestamp */
+	tick_index(index);
 	cl_git_pass(git_status_list_new(&status, repo, &opts));
 	check_status0(status);
 	cl_git_pass(git_status_list_get_perfdata(&perf, status));