Fix index for mysql adapter

* similar to rails 5 it assures that index length is set for blob
  columns also in rails 4
* it also ignores multiple definitions of indexes for mysql, for some
  tables we define multiple indexes on the same set of columns, but with
  different parameters (opclasses, where), these are not supported by
  mysql adapter so the second definition of index is skipped

config/initializers/mysql_set_length_for_binary_indexes.rb

@@ -24,10 +24,25 @@ if defined?(ActiveRecord::ConnectionAdapters::Mysql2Adapter)
   ActiveRecord::ConnectionAdapters::Mysql2Adapter.send(:prepend, MysqlSetLengthForBinaryIndex)
 end
 
-if Gitlab.rails5?
-  module MysqlSetLengthForBinaryIndexAndIgnorePostgresOptionsForSchema
+module MysqlSetLengthForBinaryIndexAndIgnorePostgresOptionsForSchema
   # This method is used in Rails 5 schema loading as t.index
   def index(column_names, options = {})
+    # Ignore indexes that use opclasses,
+    # also see config/initializers/mysql_ignore_postgresql_options.rb
+    if options[:opclasses]
+      warn "WARNING: index on columns #{column_names} uses unsupported option, skipping."
+      return
+    end
+
+    # when running rails 4 with rails 5 schema, rails 4 doesn't support multiple
+    # indexes on the same set of columns. Mysql doesn't support partial indexes, so if
+    # an index already exists and we add another index, skip it if it's partial:
+    # see https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/21492#note_102821326
+    if !Gitlab.rails5? && indexes[column_names] && options[:where]
+      warn "WARNING: index on columns #{column_names} already exists and partial index is not supported, skipping."
+      return
+    end
+
     options[:length] ||= {}
     Array(column_names).each do |column_name|
       column = columns.find { |c| c.name == column_name }
@@ -37,15 +52,18 @@ if Gitlab.rails5?
       end
     end
 
-      # Ignore indexes that use opclasses,
-      # also see config/initializers/mysql_ignore_postgresql_options.rb
-      unless options[:opclasses]
     super(column_names, options)
   end
-    end
-  end
+end
 
+def mysql_adapter?
+  defined?(ActiveRecord::ConnectionAdapters::Mysql2Adapter) && ActiveRecord::Base.connection.is_a?(ActiveRecord::ConnectionAdapters::Mysql2Adapter)
+end
+
+if Gitlab.rails5?
   if defined?(ActiveRecord::ConnectionAdapters::MySQL::TableDefinition)
     ActiveRecord::ConnectionAdapters::MySQL::TableDefinition.send(:prepend, MysqlSetLengthForBinaryIndexAndIgnorePostgresOptionsForSchema)
   end
+elsif mysql_adapter? && defined?(ActiveRecord::ConnectionAdapters::TableDefinition)
+  ActiveRecord::ConnectionAdapters::TableDefinition.send(:prepend, MysqlSetLengthForBinaryIndexAndIgnorePostgresOptionsForSchema)
 end