dag-json: cleanup and fully specify reserved space

[rvagg]

I've been working ok figuring out how to codify the rules that we discussed sometime last meeting .. during some meeting. I can't find the particular meeting by looking at the minutes in https://github.com/ipld/team-mgmt so I'm doing this from memory. But the main points were:

• Strictly only allow certain forms in the {"/":...} namespace so that tokenizing parsers didn't have to buffer and unroll too much data but can quickly reject form without getting too deep.
• Remove the multibase prefix from the encoded bytes.

After working through some of this, I've flipped on my opinion of the second point and would rather it stay as proper multibase, even if it's still strictly base64. It adds clarity and there's scope for using this for further validation if we want to. It's a single byte and I think it increases the clarity and reduces the number of edge cases. So I haven't included that change here, but discussion is welcome if you feel strongly!

The bit to pay most attention to below is "The Reserved Namespace" where I've tried to enumerate the rules. It turns out to be quite hard to explain it with crystal clarity, but that could be me so suggestions welcome!

Along with this, as exploration, I've been working on a version of JS DAG-JSON that does tokenized decoding and uses the same base encoder/decoder as the new JS DAG-CBOR codec, just by swapping out the backend. There's some consistency benefits here in having the same code paths and same affordances for applying data model strictness. Still uncertain whether I'll actually propose that we merge this, although I did uncover some bugs in the current implementation in the process, but the code is here: https://github.com/ipld/js-dag-json/compare/rvagg/cborg (see the innards of DagJsonTokenizer for the token parsing rules that arise from the reserved space rules).

[warpfork]

I went digging for our discussion notes (and merged some stale PRs) and found them here:

https://github.com/ipld/team-mgmt/blob/master/meeting-notes/2020/2020-11-05--ipld-specs.md#dag-json-bytes-and-links-discuss-parsing-rules-edge-cases

[rvagg]

I've addressed the reviews so far and I think this is ready for landing. The reserved namespace rules are hard to be clear about but I've done the shuffle recommended by @vmx and separated things out a little more to hopefully make it easier to digest.

I went with the .0 rule for floats with no fractional component - although this is no help to JavaScript but Go can implement it.

I also have a working JavaScript version that I'm pretty much ready to release. It uses the same backend as the DAG-CBOR codec but with JSON semantics. We've lost some speed in the process but we get all the reserved namespace rules and all the strictness.

@warpfork care to review the changes?

[rvagg]

As per a brief discussion today:

• go-ipld-prime just had "bytes" implemented @ https://github.com/ipld/go-ipld-prime/pull/166 but this was done without the multibase prefix as per the current version of the spec
• We have previously discussed removal of the multibase prefix but this spec change didn't include that -- see the original post for a discussion on that and why it was removed from scope.
• The current Rust implementation has a multibase prefix and Forest has some test fixtures using dag-json, and the current JavaScript dag-json has a multibase prefix too.

So either we should fix go-ipld-prime to match spec or we just bite the bullet and remove it and get it done everywhere—I can do this in JavaScript pretty quickly after merging this.

@warpfork @vmx @willscott (and anyone else) 👍 for just remove multibase prefix and 👎 for leave the spec alone darn it!.

[vmx]

My reasons for having the multibase prefix:

• consistency: the CIDs are also Multibases. The rule would be: all base encoded things are Multibases (how could would it be if that would be true across all our specs?)
• we do self-describing things: to me having a multibase isn't necessarily a signal that any arbitrary Multibase is allowed, it's for being self-described.
• not performance: if you care about performance, don't use DAG-JSON. An even if you care about allocations, for base encodings you need to over-allocate slightly anyway, so just allocate one byte more for the prefix
• why change now: generally we are hesitate to change things in specs that are already out there, unless we really have to. We don't know who else has implemented that spec already. So why changing the status quo if it doesn't really make a practical difference (except for one of our own libraries we control that didn't follow the spec).

All that said, I don't have a strong opinion and can live with either way.

[rvagg]

• https://github.com/ipld/go-ipld-prime/pull/168 has a multibase prefix
• https://github.com/ipld/js-dag-json/pull/36 has the same test case ipld-prime has

at first I couldn't get them making the same bytes and thought there must be a multibase incompatibility (the horror!) but it's []byte('deadbeef') being deceptive cause that's actually a string, not a hex value in spite of the nice hex characters..

also, sorting, ipld-prime don't sort and that engages hostilities in @warpfork and is not fun for compatibility fixtures :facepalm: and I kind of don't want to have that argument over and over so will skip it for now.

But basically, it's not a big deal to do it in go-ipld-prime. It's also not a big deal to strip it in @ipld/dag-json, but I'm also a soft -1 to changing the spec and leaving the prefix in there for reasons stated in this thread and restated by @vmx. But, it's not a big enough deal if someone else has particularly strong feelings and justification.

[warpfork]

Arguments against inserting multibase in this bytes field, just to have them written:

• it will become "another one of those bytes" that anyone wanting compactness will question and question and question again -- just like the extra "zero" byte in CIDs in DAG-CBOR which confuses, surprises, and annoys every single person who ever has cause to notice it.
  • I've stopped saving references to every time this has happened about that zero in DAG-CBOR, but it's... many times. People (me included) repeatedly implement DAG-CBOR without it, and then go "what?!" when a fixture points out that it's needed; or people think it's a bug, and try to remove it again later on in development cycles! And this regularly comes from people working "down the hall" from us, not just distant strangers. Even if we nevermind the excess bytes on the wire, the number of human hours this has wasted over time does not sit well with me. I'd like to not do this again.
• having a multibase indicator would suggest it's acceptable to use other multibase indicators here... which it is not; in turn, it is not desirable to all multiple multibases to be used here because it would provide negative value if DAG-JSON were to be more expressive than the IPLD Data Model in this position.
• holistically: I think multibases are to be used when you've got a string and it's fairly user-facing, and there are no other disambiguators or indicators in the area that could help you know what that value is. This is not the scenario we have in the middle of dag-json: we already have 1) the multicodec indicator, from the outside, 2) all the information/confidence provided by the success of parsing as dag-json throughout the block, 3) the information/confidence provided by a single-key map with a "/" key, and 4) the information/confidence provided by a single-key map with a "bytes" key, and then 5) the confidence provided by the success of parsing the value therein as base64. This is a huge amount of confidence and a ton of non-accidental structure, and it seems to me heaping on another byte just to "be sure" it's clear what base is expected here would be wild overkill.
  • The (admittedly rather fine) distinction I'm trying to make here is that the "natural form" of this data is bytes -- it's not a string of some multibase, it's bytes -- and dag-json is in the business of specifying how those bytes are encoded in the serial format. I think that we have a goal of using indicators like multibase in contexts where there are not yet indicators; but in the the middle of a codec's serial forms is not such a context.
  • Can you imagine if we put a "zero" prefix on every bytes scalar in DAG-CBOR, too, just to make it really clear that they're bytes?
• We argued about this before in https://github.com/ipld/team-mgmt/blob/master/meeting-notes/2020/2020-11-05--ipld-specs.md#votes.
  • I distinctly remember trying to hold a vote and do an actual formal count -- and now do regret not forcing that! -- but as the notes say, and we reviewed and merged, in video, we had entirely assenting voices for removing multibase. There were more than a half dozen people there, fully engaged, and having just collaboratively reviewed the entire subject.
  • That meeting had more people and more of their attention dedicated to it than this re-review today has got, so I'd prefer to stick with it.

[rvagg]

fine, you appear to have a stronger opinion than either of us (although I'm not sure I'm convinced it's all that strong)

have pushed a new version that removes it and will do likewise in the JS impl

[rvagg]

(PTAL & merge if :thumbsup:)