Token limit fix CVE-2023-49559

[uvzz]

Hi,

I added a default token limit value of 15000 and a token count inside next() function in parser.go

the limit should be configured by callers of the parser (i.e. gqlgen, genqlient, etc.). - Can you assist with that?

[coveralls]

coverage: 88.477% (-0.1%) from 88.575%
when pulling f1ffdaf on uvzz:token-limit-fix-CVE-2023-49559
into 591c98b on vektah:master.

[benjaminjkraft]

This doesn't seem to let callers set the token limit? All the structs in question are private. We will need to figure out how to wire it in, which is a bit painful since there is no existing options API.

One kind of strange way is to make it a field of Source. It's a bit weird at first blush but it actually kinda makes sense that the parsing limit might vary from source to source. That wouldn't make it easy to set a default, and doesn't work for the main LoadQuery API -- but that one could have a default and callers who care could use ParseQuery + Validate, I think?

BTW, my opinion is still that this should be off by default; it is not a vulnerability for a parser to parse what you give it in its entirety, only for an HTTP server to parse input from the network with no limitations. (And I'm still unconvinced that request body size is not a sufficient limitation!) But I understand others might feel differently, so as long as it's possible to disable I'm fine being outvoted on that one.