[Bro-Dev] Feedback on configuration framework implementation

[Johanna Amann]

Hello everyone,

the branch topic/johanna/config contains an implementation of the
configuration framework as it was discussed in an earlier thread on this
list. GitHub link: https://github.com/bro/bro/compare/topic/johanna/config

The implementation is basically what we discussed in the earlier thread
with some additional components like a reader for configuration values and
a script-level framework.

It would be great if people could take a look at all of this and see if
this makes sense, or if they see any problems with the implementation as
it is at the moment.

In the rest of the mail I wil go into a bit more detail and describe the
different parts of this change. Note that the rest of this email will be
very similar to the git commit message which also describes this change :)

The configuration framework consists of three mostly distinct parts:

* option variables
* the config reader
* the script level framework

option variable
===============

The option keyword allows variables to be specified as run-tine options.
Such variables cannot be changed using normal assignments. Instead, they
can be changed using Option::set. It is possible to "subscribe" to
options and be notified when an option value changes.

Change handlers can also change values before they are applied; this
gives them the opportunity to reject changes. Priorities can be
specified if there are several handlers for one option.

Example script:

option testbool: bool = T;

function option_changed(ID: string, new_value: bool): bool
  {
  print fmt("Value of %s changed from %s to %s", ID, testbool, new_value);
  return new_value;
  }

event bro_init()
  {
  print "Old value", testbool;
  Option::set_change_handler("testbool", option_changed);
  Option::set("testbool", F);
  print "New value", testbool;
  }

config reader
=============

The config reader provides a way to read configuration files back into
Bro. Most importantly it automatically converts values to the correct
types. This is important because it is at least inconvenient (and
sometimes near impossible) to perform the necessary type conversions in
Bro scripts themselves. This is especially true for sets/vectors.

Configuration generally look like this:

[option name][tab/spaces][new variable value]

so, for example:

testaddr 2607:f8b0:4005:801::200e
testinterval 60
testtime 1507321987
test_set a	b	c	d	erdbeerschnitzel

The reader uses the option name to look up the type that variable has in
the Bro core and automatically converts the value to the correct type.

Example script use:

type Idx: record {
  option_name: string;
};

type Val: record {
  option_val: string;
};

global currconfig: table[string] of string = table();

event InputConfig::new_value(name: string, source: string, id: string, value: any)
  {
  print id, value;
  }

event bro_init()
  {
  Input::add_table([$reader=Input::READER_CONFIG, $source="../configfile", $name="configuration", $idx=Idx, $val=Val, $destination=currconfig, $want_record=F]);
  }

Script-level config framework
=============================

The script-level framework ties these two features together and makes
them a bit more convenient to use. Configuration files can simply be
specified by placing them into Config::config_files. The framework also
creates a config.log that shows all value changes that took place.

Usage example:

redef Config::config_files += {configfile};

export {
  option testbool : bool = F;
}

The file is now monitored for changes; when a change occurs the
respective option values are automatically updated and the value change
is written to config.log.

Other changes
=============

Internally, this commit also performs a range of changes to the Input
manager; it marks a lot of functions as const and introduces a new
ValueToVal method (which could in theory replace the already existing
one - it is a bit more powerful).

This also changes SerialTypes to have a subtype for Values, just as
Fields already have it; I think it was mostly an oversight that this was
not introduced from the beginning. This should not necessitate any code
changes for people already using SerialTypes.

Johanna