[tor-dev] An ANTLR 4 grammar for Tor bridge network statuses

Thu Nov 5 13:21:30 UTC 2015

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On 04/11/15 17:43, Nick Mathewson wrote:
> On Wed, Nov 4, 2015 at 4:06 AM, Karsten Loesing
> <karsten at torproject.org> wrote:
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>> 
>> Hello developers,
>> 
>> in the past few days I have been working on a grammar to parse
>> Tor bridge network statuses and hopefully other Tor descriptors
>> in the future.  It's working, for some definition of working, but
>> some issues remain and I need some help.
>> 
>> I just uploaded my sources, consisting only of the grammar with a
>> fair amount of documentation:
>> 
>> https://people.torproject.org/~karsten/volatile/BridgeNetworkStatus.g4
>
>> 
> 
> Nice work, Karsten!  I'm hoping we move towards some kind of 
> machine-readable grammar/schema for all our data formats, and that
> we have our actual parsing/encoding code generated from it.
> 
> (When I did a survey of where all our crash/assertion bugs for the 
> last few years were, they seemed to have a higher-than-usual 
> concentration in our parsing code.)

Great, added as goal #4 in the document.

> One thing about this grammar in particular, though: It is
> over-strict. It matches only the formats we use today, and not the
> formats we are allowed to use in the future.  For one example, a
> flag on an 's' line can be any non-space string - but this grammar
> will fail to parse unrecognized flags.

Right, that one would be easy to fix by just accepting any string and
moving the checks whether a string is a valid flag to the parser.

> On the other hand, while we specify the order of r, s, w, p, a,
> lines in a generated consensus, clients are required to parse the
> s, w, p, and a lines in any order, but not to allow two s lines in
> a single 'r' entry.
> 
> I think that because of the free-ordering and
> multiplicity-restriction rules for our data formats, a context-free
> grammar simply isn't going to match our spec very well.

Indeed, this grammar accepts any number of s, w, and p lines per r
entry and leaves it to the parser to make sure there's at most one.
It would be good to look for alternatives here.  I added this as
another item for the "open issues and questions" section.

>> Quoting from that file to facilitate discussion here:
>> 
>> There are multiple goals of having a grammar for Tor descriptors 
>> available on CollecTor:
>> 
>> 1. Translate descriptors to JSON for statistical analysis: Some
>> tools and databases require Tor descriptors in a standard format
>> like JSON. This grammar and a parser generated from it can help
>> making that translation as easy as possible, also to keep future
>> maintenance as low as possible.
>> 
>> 2. Provide a basis for descriptor-parsing libraries: As of late
>> 2015, there are three libraries for parsing Tor descriptors:
>> metrics-lib for Java, Stem for Python, and Zoossh for Go.  It
>> would be beneficial to place as much knowledge about the
>> descriptor format into a grammar shared by all those libraries
>> and then generate parsers for different languages from that
>> grammar.
>> 
>> 3. Serve as documentation for the Tor directory protocol 
>> specification: Tor descriptors are already documented using a 
>> hand-written grammar, but that may contain slight inaccuracies
>> because it's not verified.  This grammar could fix that by either
>> detecting inaccuracies while trying to rewrite it to an
>> executable grammar form or by replacing the grammar in the
>> specification documentation with this executable grammar.
>> 
>> Open issues and questions:
>> 
>> - Was it smart to explicitly include all those SP tokens in the 
>> rules, or should those be discarded right away by the lexer?  The
>> main reason for keeping them was to stay as close to the
>> specification as possible, but maybe that has downsides on the
>> other goals.
> 
> IMO, once we have a grammar that is truly correct, that grammar
> should _be_ the spec, and we should revise the main spec to
> reference the grammar.
> 
>> - If a bridge uses a nickname (or other token that's supposed to
>> be a STRING) that is also a keyword like "r" or "published",
>> things get confusing.  Try editing the input bridge network
>> status and observe the result.  But those are perfectly valid
>> nicknames, so what can we do?
> 
> Change the lexing rules so that keywords are only recognized as
> such at position 0 on the line, outside of a base64 block?

That would work, but I have no clue how to convince ANTLR 4 to
generate such a lexer.  I added this suggestion to the file in the
hope that somebody else who knows more about this stuff can pick this up.

Thanks for the great suggestions!

New version available here, if somebody else wants to look:

https://people.torproject.org/~karsten/volatile/BridgeNetworkStatus.g4

All the best,
Karsten

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