thodg/libgit2

diff --git a/CHANGELOG.md b/CHANGELOG.md
index eb7ae84..1340f78 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -69,6 +69,9 @@ support for HTTPS connections insead of OpenSSL.
   and `git_diff_buffers` now accept a new binary callback of type
   `git_diff_binary_cb` that includes the binary diff information.
 
+* The race condition mitigations described in `racy-git.txt` have been
+  implemented.
+
 ### API additions
 
 * The `git_merge_options` gained a `file_flags` member.
diff --git a/src/diff.c b/src/diff.c
index d7365ef..cc93f57 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/index.c b/src/index.c
index 1fb3c48..5ce5522 100644
--- a/src/index.c
+++ b/src/index.c
@@ -688,20 +688,59 @@ int git_index__changed_relative_to(
 	return !!git_oid_cmp(&index->checksum, checksum);
 }
 
+static bool is_racy_timestamp(git_time_t stamp, git_index_entry *entry)
+{
+	/* Git special-cases submodules in the check */
+	if (S_ISGITLINK(entry->mode))
+		return false;
+
+	/* If we never read the index, we can't have this race either */
+	if (stamp == 0)
+		return false;
+
+	/* If the timestamp is the same or newer than the index, it's racy */
+	return ((int32_t) stamp) <= entry->mtime.seconds;
+}
+
 /*
  * Force the next diff to take a look at those entries which have the
  * same timestamp as the current index.
  */
-static void truncate_racily_clean(git_index *index)
+static int truncate_racily_clean(git_index *index)
 {
 	size_t i;
+	int error;
 	git_index_entry *entry;
 	git_time_t ts = index->stamp.mtime;
+	git_diff_options diff_opts = GIT_DIFF_OPTIONS_INIT;
+	git_diff *diff;
 
+	/* Nothing to do if there's no repo to talk about */
+	if (!INDEX_OWNER(index))
+		return 0;
+
+	/* If there's no workdir, we can't know where to even check */
+	if (!git_repository_workdir(INDEX_OWNER(index)))
+		return 0;
+
+	diff_opts.flags |= GIT_DIFF_INCLUDE_TYPECHANGE | GIT_DIFF_IGNORE_SUBMODULES | GIT_DIFF_DISABLE_PATHSPEC_MATCH;
 	git_vector_foreach(&index->entries, i, entry) {
-		if (entry->mtime.seconds == ts || ts == 0)
+		if (!is_racy_timestamp(ts, entry))
+			continue;
+
+		diff_opts.pathspec.count = 1;
+		diff_opts.pathspec.strings = (char **) &entry->path;
+
+		if ((error = git_diff_index_to_workdir(&diff, INDEX_OWNER(index), index, &diff_opts)) < 0)
+			return error;
+
+		if (git_diff_num_deltas(diff) > 0)
 			entry->file_size = 0;
+
+		git_diff_free(diff);
 	}
+
+	return 0;
 }
 
 int git_index_write(git_index *index)
diff --git a/src/iterator.c b/src/iterator.c
index 7807a16..d5f7eec 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 db1f325..57f8241 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
deleted file mode 100644
index a109f8c..0000000
--- a/tests/diff/racy.c
+++ /dev/null
@@ -1,39 +0,0 @@
-#include "clar_libgit2.h"
-
-#include "buffer.h"
-
-static git_repository *g_repo;
-
-void test_diff_racy__initialize(void)
-{
-	cl_git_pass(git_repository_init(&g_repo, "diff_racy", false));
-}
-
-void test_diff_racy__cleanup(void)
-{
-	cl_git_sandbox_cleanup();
-}
-
-void test_diff_racy__diff(void)
-{
-	git_index *index;
-	git_diff *diff;
-	git_buf path = GIT_BUF_INIT;
-
-	cl_git_pass(git_buf_joinpath(&path, git_repository_workdir(g_repo), "A"));
-	cl_git_mkfile(path.ptr, "A");
-
-	/* Put 'A' into 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));
-
-	cl_git_pass(git_diff_index_to_workdir(&diff, g_repo, index, NULL));
-	cl_assert_equal_i(0, git_diff_num_deltas(diff));
-
-	/* 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 ecc556c..13de6a9 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/index/racy.c b/tests/index/racy.c
new file mode 100644
index 0000000..3a4bc43
--- /dev/null
+++ b/tests/index/racy.c
@@ -0,0 +1,143 @@
+#include "clar_libgit2.h"
+#include "../checkout/checkout_helpers.h"
+
+#include "buffer.h"
+#include "index.h"
+
+static git_repository *g_repo;
+
+void test_index_racy__initialize(void)
+{
+	cl_git_pass(git_repository_init(&g_repo, "diff_racy", false));
+}
+
+void test_index_racy__cleanup(void)
+{
+	git_repository_free(g_repo);
+	g_repo = NULL;
+
+	cl_fixture_cleanup("diff_racy");
+}
+
+void test_index_racy__diff(void)
+{
+	git_index *index;
+	git_diff *diff;
+	git_buf path = GIT_BUF_INIT;
+
+	cl_git_pass(git_buf_joinpath(&path, git_repository_workdir(g_repo), "A"));
+	cl_git_mkfile(path.ptr, "A");
+
+	/* Put 'A' into 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));
+
+	cl_git_pass(git_diff_index_to_workdir(&diff, g_repo, index, NULL));
+	cl_assert_equal_i(0, git_diff_num_deltas(diff));
+	git_diff_free(diff);
+
+	/* 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));
+
+	git_index_free(index);
+	git_diff_free(diff);
+	git_buf_free(&path);
+}
+
+void test_index_racy__write_index_just_after_file(void)
+{
+	git_index *index;
+	git_diff *diff;
+	git_buf path = GIT_BUF_INIT;
+	struct timeval times[2];
+
+	/* Make sure we do have a timestamp */
+	cl_git_pass(git_repository_index(&index, g_repo));
+	cl_git_pass(git_index_write(index));
+
+	cl_git_pass(git_buf_joinpath(&path, git_repository_workdir(g_repo), "A"));
+	cl_git_mkfile(path.ptr, "A");
+	/* Force the file's timestamp to be a second after we wrote the index */
+	times[0].tv_sec = index->stamp.mtime + 1;
+	times[0].tv_usec = 0;
+	times[1].tv_sec = index->stamp.mtime + 1;
+	times[1].tv_usec = 0;
+	cl_git_pass(p_utimes(path.ptr, times));
+
+	/*
+	 * Put 'A' into the index, the size field will be filled,
+	 * because the index' on-disk timestamp does not match the
+	 * file's timestamp.
+	 */
+	cl_git_pass(git_index_add_bypath(index, "A"));
+	cl_git_pass(git_index_write(index));
+
+	cl_git_mkfile(path.ptr, "B");
+	/*
+	 * Pretend this index' modification happend a second after the
+	 * file update, and rewrite the file in that same second.
+	 */
+	times[0].tv_sec = index->stamp.mtime + 2;
+	times[0].tv_usec = 0;
+	times[1].tv_sec = index->stamp.mtime + 2;
+	times[0].tv_usec = 0;
+
+	cl_git_pass(p_utimes(git_index_path(index), times));
+	cl_git_pass(p_utimes(path.ptr, times));
+
+	cl_git_pass(git_index_read(index, true));
+
+	cl_git_pass(git_diff_index_to_workdir(&diff, g_repo, index, NULL));
+	cl_assert_equal_i(1, git_diff_num_deltas(diff));
+
+	git_buf_free(&path);
+	git_diff_free(diff);
+	git_index_free(index);
+}
+
+void test_index_racy__empty_file_after_smudge(void)
+{
+	git_index *index;
+	git_diff *diff;
+	git_buf path = GIT_BUF_INIT;
+	int i, found_race = 0;
+	const git_index_entry *entry;
+
+	/* Make sure we do have a timestamp */
+	cl_git_pass(git_repository_index(&index, g_repo));
+	cl_git_pass(git_index_write(index));
+
+	cl_git_pass(git_buf_joinpath(&path, git_repository_workdir(g_repo), "A"));
+
+	/* Make sure writing the file, adding and rewriting happen in the same second */
+	for (i = 0; i < 10; i++) {
+		struct stat st;
+		cl_git_mkfile(path.ptr, "A");
+
+		cl_git_pass(git_index_add_bypath(index, "A"));
+		cl_git_mkfile(path.ptr, "B");
+		cl_git_pass(git_index_write(index));
+
+		cl_git_mkfile(path.ptr, "");
+
+		cl_git_pass(p_stat(path.ptr, &st));
+		cl_assert(entry = git_index_get_bypath(index, "A", 0));
+		if (entry->mtime.seconds == (int32_t) st.st_mtime) {
+			found_race = 1;
+			break;
+		}
+
+	}
+
+	if (!found_race)
+		cl_fail("failed to find race after 10 attempts");
+
+	cl_assert_equal_i(0, entry->file_size);
+
+	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/merge/workdir/dirty.c b/tests/merge/workdir/dirty.c
index 30c404b..4bf984c 100644
--- a/tests/merge/workdir/dirty.c
+++ b/tests/merge/workdir/dirty.c
@@ -133,12 +133,25 @@ static void hack_index(char *files[])
 	struct stat statbuf;
 	git_buf path = GIT_BUF_INIT;
 	git_index_entry *entry;
+	struct timeval times[2];
+	time_t now;
 	size_t i;
 
 	/* Update the index to suggest that checkout placed these files on
 	 * disk, keeping the object id but updating the cache, which will
 	 * emulate a Git implementation's different filter.
+	 *
+	 * We set the file's timestamp to before now to pretend that
+	 * it was an old checkout so we don't trigger the racy
+	 * protections would would check the content.
 	 */
+
+	now = time(NULL);
+	times[0].tv_sec  = now - 5;
+	times[0].tv_usec = 0;
+	times[1].tv_sec  = now - 5;
+	times[1].tv_usec = 0;
+
 	for (i = 0, filename = files[i]; filename; filename = files[++i]) {
 		git_buf_clear(&path);
 
@@ -146,6 +159,7 @@ static void hack_index(char *files[])
 			git_index_get_bypath(repo_index, filename, 0));
 
 		cl_git_pass(git_buf_printf(&path, "%s/%s", TEST_REPO_PATH, filename));
+		cl_git_pass(p_utimes(path.ptr, times));
 		cl_git_pass(p_stat(path.ptr, &statbuf));
 
 		entry->ctime.seconds = (git_time_t)statbuf.st_ctime;
@@ -245,7 +259,6 @@ static int merge_differently_filtered_files(char *files[])
 	write_files(files);
 	hack_index(files);
 
-	repo_index->stamp.mtime = time(NULL) + 1;
 	cl_git_pass(git_index_write(repo_index));
 
 	error = merge_branch();
diff --git a/tests/status/worktree.c b/tests/status/worktree.c
index 56f98a8..75c7b71 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));