Implicit (default) state file lines hard for external apps to know
Right now we have stanzas in relay state files like:
BWHistoryReadEnds 2010-07-08 17:52:41 BWHistoryReadValues 438064128,395179008,...
but we leave "BWHistoryReadInterval 900" implicit because it's the default value from config.c.
That's fine -- if we ever change the default, we'll have to teach Tor to look at the TorVersion line in the state file, and derive the correct default when loading the state file.
But now we have apps like arm looking at our state file. Those apps need to know that a missing value means 900, and if we ever change it, they need to know what Tor versions have what defaults. Having that logic separately in every controller app is the wrong place for it.
Does that mean we want a "getstate" controller option (along with a getinfo state/names), and it will write out the lines it would write to the state file, but include the defaults?
Or should we just write the defaults to the state file, and call that close enough? It seems we shouldn't do that, since I can imagine two classes of defaults (first is "I don't care what the value is, please choose the right number for me Tor", and the second is "I am relying on this number, and it happens to be the default number") and we don't distinguish between them. On the third hand, I don't actually see any of the former category in _state_vars in config.c, so maybe I'm generalizing too much.