Fix TRIGGER checks for MySQL

This ensures we can check if the user has TRIGGER permissions without
querying restricted tables. Thanks to Steve Norman
(https://gitlab.com/stevenorman) for helping out with this merge
request.

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/38372

changelogs/unreleased/fix-mysql-grant-check.yml

+---
+title: Fix TRIGGER checks for MySQL
+merge_request:
+author:
+type: fixed

lib/gitlab/database/grant.rb

@@ -6,28 +6,36 @@ module Gitlab
         if Database.postgresql?
           'information_schema.role_table_grants'
         else
-          'mysql.user'
+          'information_schema.schema_privileges'
         end
 
-      def self.scope_to_current_user
-        if Database.postgresql?
-          where('grantee = user')
-        else
-          where("CONCAT(User, '@', Host) = current_user()")
-        end
-      end
-
       # 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')
           else
-            where(Trigger_priv: 'Y')
+            queries = [
+              Grant.select(1)
+                .from('information_schema.user_privileges')
+                .where("PRIVILEGE_TYPE = 'SUPER'")
+                .where("GRANTEE = CONCAT('\\'', REPLACE(CURRENT_USER(), '@', '\\'@\\''), '\\'')"),
+
+              Grant.select(1)
+                .from('information_schema.schema_privileges')
+                .where("PRIVILEGE_TYPE = 'TRIGGER'")
+                .where('TABLE_SCHEMA = ?', Gitlab::Database.database_name)
+                .where("GRANTEE = CONCAT('\\'', REPLACE(CURRENT_USER(), '@', '\\'@\\''), '\\'')")
+            ]
+
+            union = SQL::Union.new(queries).to_sql
+
+            Grant.from("(#{union}) privs")
           end
 
-        priv.scope_to_current_user.any?
+        priv.any?
       end
     end
   end

spec/lib/gitlab/database/grant_spec.rb

 require 'spec_helper'
 
 describe Gitlab::Database::Grant do
-  describe '.scope_to_current_user' do
-    it 'scopes the relation to the current user' do
-      user = Gitlab::Database.username
-      column = Gitlab::Database.postgresql? ? :grantee : :User
-      names = described_class.scope_to_current_user.pluck(column).uniq
-
-      expect(names).to eq([user])
-    end
-  end
-
   describe '.create_and_execute_trigger' do
     it 'returns true when the user can create and execute a trigger' do
       # We assume the DB/user is set up correctly so that triggers can be
@@ -18,13 +8,11 @@ describe Gitlab::Database::Grant do
       expect(described_class.create_and_execute_trigger?('users')).to eq(true)
     end
 
-    it 'returns false when the user can not create and/or execute a trigger' do
-      allow(described_class).to receive(:scope_to_current_user)
-        .and_return(described_class.none)
-
-      result = described_class.create_and_execute_trigger?('kittens')
-
-      expect(result).to eq(false)
+    it 'returns false when the user can not create and/or execute a trigger', :postgresql do
+      # In case of MySQL the user may have SUPER permissions, making it
+      # impossible to have `false` returned when running tests; hence we only
+      # run these tests on PostgreSQL.
+      expect(described_class.create_and_execute_trigger?('foo')).to eq(false)
     end
   end
 end