grep: fix race conditions on userdiff calls

git-grep uses an internal grep_read_mutex to protect object reading
operations. Similarly, there's a grep_attr_mutex to protect calls to the
gitattributes machinery. However, two of the three functions protected
by the last mutex may also perform object reading, as seen below:

- userdiff_get_textconv() > notes_cache_init() >
  notes_cache_match_validity() > lookup_commit_reference_gently() >
  parse_object() > repo_has_object_file() >
  repo_has_object_file_with_flags() > oid_object_info_extended()

- userdiff_find_by_path() > git_check_attr() > collect_some_attrs() >
  prepare_attr_stack() > read_attr() > read_attr_from_index() >
  read_blob_data_from_index() > read_object_file()

As these calls are not protected by grep_read_mutex, there might be race
conditions with other threads performing object reading (e.g. threads
calling fill_textconv() at grep.c:fill_textconv_grep()). To prevent
that, let's make sure to acquire the lock before both of these calls.

Note: this patch might slow down the threaded grep in worktree, for the
sake of thread-safeness. However, in the following patches, we should
regain performance by replacing grep_read_mutex for an internal object
reading lock and allowing parallel inflation during object reading.

Signed-off-by: Matheus Tavares <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: matheustavares

grep.c

@@ -1816,7 +1816,9 @@ static int grep_source_1(struct grep_opt *opt, struct grep_source *gs, int colle
 		 * is not thread-safe.
 		 */
 		grep_attr_lock();
+		grep_read_lock();
 		textconv = userdiff_get_textconv(opt->repo, gs->driver);
+		grep_read_unlock();
 		grep_attr_unlock();
 	}
 
@@ -2184,8 +2186,11 @@ void grep_source_load_driver(struct grep_source *gs,
 		return;
 
 	grep_attr_lock();
-	if (gs->path)
+	if (gs->path) {
+		grep_read_lock();
 		gs->driver = userdiff_find_by_path(istate, gs->path);
+		grep_read_unlock();
+	}
 	if (!gs->driver)
 		gs->driver = userdiff_find_by_name("default");
 	grep_attr_unlock();