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.

6 files changed, 61 insertions, 3 deletions

diff --git a/src/diff.c b/src/diff.c
index d7365ef77..cc93f57cd 100644
--- a/src/diff.c
+++ b/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;
diff --git a/src/iterator.c b/src/iterator.c
index 7807a1636..d5f7eec34 100644
--- a/src/iterator.c
+++ b/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,
diff --git a/src/iterator.h b/src/iterator.h
index db1f325a7..57f82416a 100644
--- a/src/iterator.h
+++ b/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
diff --git a/tests/diff/racy.c b/tests/diff/racy.c
index a109f8c3b..66630cc4d 100644
--- a/tests/diff/racy.c
+++ b/tests/diff/racy.c
@@ -1,4 +1,5 @@
 #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));
+}
diff --git a/tests/diff/workdir.c b/tests/diff/workdir.c
index 6b72f3286..3ca82e733 100644
--- a/tests/diff/workdir.c
+++ b/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));
diff --git a/tests/status/worktree.c b/tests/status/worktree.c
index 56f98a882..75c7b71b0 100644
--- a/tests/status/worktree.c
+++ b/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));