diff --git a/swh/storage/postgresql/storage.py b/swh/storage/postgresql/storage.py
--- a/swh/storage/postgresql/storage.py
+++ b/swh/storage/postgresql/storage.py
@@ -219,24 +219,27 @@
 
         with self.db() as db:
             with db.transaction() as cur:
-                missing = list(
-                    self.content_missing(
-                        map(Content.to_dict, contents),
-                        key_hash="sha1_git",
-                        db=db,
-                        cur=cur,
-                    )
-                )
-                contents = [c for c in contents if c.sha1_git in missing]
-
-                self.journal_writer.content_add(contents)
-                self._content_add_metadata(db, cur, contents)
+                nb_added = self._content_add(contents, db, cur)
 
         return {
-            "content:add": len(contents),
+            "content:add": nb_added,
             "content:add:bytes": objstorage_summary["content:add:bytes"],
         }
 
+    def _content_add(self, contents: List[Content], db, cur) -> int:
+        """Called by `content_add`, and also used by `test_content_add_race`."""
+        missing = list(
+            self.content_missing(
+                map(Content.to_dict, contents), key_hash="sha1_git", db=db, cur=cur,
+            )
+        )
+        contents = [c for c in contents if c.sha1_git in missing]
+
+        self.journal_writer.content_add(contents)
+        self._content_add_metadata(db, cur, contents)
+
+        return len(contents)
+
     @timed
     @db_transaction()
     def content_update(
diff --git a/swh/storage/tests/test_postgresql.py b/swh/storage/tests/test_postgresql.py
--- a/swh/storage/tests/test_postgresql.py
+++ b/swh/storage/tests/test_postgresql.py
@@ -48,14 +48,14 @@
 class TestStorageRaceConditions:
     @pytest.mark.xfail
     def test_content_add_race(self, swh_storage, sample_data):
-        content = sample_data.content
+        content = attr.evolve(sample_data.content, ctime=now())
 
         results = queue.Queue()
 
         def thread():
             try:
                 with db_transaction(swh_storage) as (db, cur):
-                    ret = swh_storage.content_add([content], db=db, cur=cur)
+                    ret = swh_storage._content_add([content], db=db, cur=cur)
                 results.put((threading.get_ident(), "data", ret))
             except Exception as e:
                 results.put((threading.get_ident(), "exc", e))