Re: [PATCH 0/2] routines to generate JSON data

From: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Date: Mar 16, 2018, 23:00 UTC
Message-ID: <87tvtfd3sl.fsf@evledraar.gmail.com>
In-Reply-To: <20180316211837.GB12333@sigill.intra.peff.net>

On Fri, Mar 16 2018, Jeff King jotted:

> I really like the idea of being able to send our machine-readable output
> in some "standard" syntax for which people may already have parsers. But
> one big hangup with JSON is that it assumes all strings are UTF-8.

FWIW It's not UTF-8 but "Unicode characters", i.e. any Unicode encoding is valid, not that it changes anything you're pointing out, but people on Win32 could use UTF-16 as-is if their filenames were in that format.

I'm just going to use UTF-8 synonymously with "Unicode encoding" for the rest of this mail...

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> Some possible solutions I can think of:
>
>   1. Ignore the UTF-8 requirement, making a JSON-like output (which I
>      think is what your patches do). I'm not sure what problems this
>      might cause on the parsing side.

Maybe some JSON parsers are more permissive, but they'll commonly just die on non-Unicode (usually UTF-8) input, e.g.:

    $ (echo -n '{"str ": "'; head -c 3 /dev/urandom ; echo -n '"}') | perl -0666 -MJSON::XS -wE 'say decode_json(<>)->{str}'
    malformed UTF-8 character in JSON string, at character offset 10 (before "\x{fffd}e\x{fffd}"}") at -e line 1, <> chunk 1.

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>   2. Specially encode non-UTF-8 bits. I'm not familiar enough with JSON
>      to know the options here, but my understanding is that numeric
>      escapes are just for inserting unicode code points. _Can_ you
>      actually transport arbitrary binary data across JSON without
>      base64-encoding it (yech)?

There's no way to transfer binary data in JSON without it being shoved into a UTF-8 encoding, so you'd need to know on the other side that such-and-such a field has binary in it, i.e. you'll need to invent your own schema.

E.g.:

    head -c 10 /dev/urandom | perl -MDevel::Peek -MJSON::XS -wE 'my $in = <STDIN>; my $roundtrip = decode_json(encode_json({str => $in}))->{str}; utf8::decode($roundtrip) if $ARGV[0]; say Dump [$in, $roundtrip]' 0

You can tweak that trailing "0" to "1" to toggle the ad-hoc schema, i.e. after we decode the JSON we go and manually UTF-8 decode it to get back at the same binary data, otherwise we end up with an UTF-8 escaped version of what we put in.

>   3. Some other similar format. YAML comes to mind. Last time I looked
>      (quite a while ago), it seemed insanely complex, but I think you
>      could implement only a reasonable subset. OTOH, I think the tools
>      ecosystem for parsing JSON (e.g., jq) is much better.

The lack of fast schema-less formats that supported arrays, hashes etc. and didn't suck when it came to mixed binary/UTF-8 led us to implementing our own at work: https://github.com/Sereal/Sereal

I think for git's use-case we're probably best off with JSON. It's going to work almost all of the time, and when it doesn't it's going to be on someone's weird non-UTF-8 repo, and those people are probably used to dealing with crap because of that anyway and can just manually decode their thing after it gets double-encoded.

That sucks, but given that we'll be using this either for just ASCII (telemetry) or UTF-8 most of the time, and that realistically other formats either suck more or aren't nearly as ubiquitous...