[PATCH v2 01/11] grep: fix race conditions on userdiff calls

[Matheus Tavares]

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 bellow:

- 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 <matheus.bernard...@usp.br>
---
 grep.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/grep.c b/grep.c
index cd952ef5d3..b29946def2 100644
--- a/grep.c
+++ b/grep.c
@@ -1809,7 +1809,9 @@ 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();
        }
 
@@ -2177,8 +2179,11 @@ 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();
-- 
2.23.0