Use has_table_privilege for TRIGGER on PostgreSQL

This fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/38634.

changelogs/unreleased/fix-postgresql-table-grant.yml

+---
+title: Use has_table_privilege for TRIGGER on PostgreSQL
+merge_request:
+author:
+type: fixed

lib/gitlab/database/grant.rb

@@ -12,10 +12,22 @@ module Gitlab
       # Returns true if the current user can create and execute triggers on the
       # given table.
       def self.create_and_execute_trigger?(table)
-        priv =
         if Database.postgresql?
-            where(privilege_type: 'TRIGGER', table_name: table)
-              .where('grantee = user')
+          # We _must not_ use quote_table_name as this will produce double
+          # quotes on PostgreSQL and for "has_table_privilege" we need single
+          # quotes.
+          quoted_table = connection.quote(table)
+
+          begin
+            from(nil)
+              .pluck("has_table_privilege(#{quoted_table}, 'TRIGGER')")
+              .first
+          rescue ActiveRecord::StatementInvalid
+            # This error is raised when using a non-existing table name. In this
+            # case we just want to return false as a user technically can't
+            # create triggers for such a table.
+            false
+          end
         else
           queries = [
             Grant.select(1)
@@ -32,10 +44,8 @@ module Gitlab
 
           union = SQL::Union.new(queries).to_sql
 
-            Grant.from("(#{union}) privs")
+          Grant.from("(#{union}) privs").any?
         end
-
-        priv.any?
       end
     end
   end