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

From: Jeff Hostetler <git@jeffhostetler.com>
Date: Mar 19, 2018, 10:19 UTC
Message-ID: <e2a9d2a9-f2e2-f285-52c3-be11668da543@jeffhostetler.com>
In-Reply-To: <20180316211837.GB12333@sigill.intra.peff.net>

On 3/16/2018 5:18 PM, Jeff King wrote:

> On Fri, Mar 16, 2018 at 07:40:55PM +0000, git@jeffhostetler.com wrote:
>

[...]

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> 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. That
> may be OK for telemetry data, but it would probably lead to problems for
> something like status porcelain, since Git's view of paths is just a
> string of bytes (not to mention possible uses elsewhere like author
> names, subject lines, etc).

[...]

I'll come back to the UTF-8/YAML questions in a separate response.

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>> Documentation for the new API is given in json-writer.h at the bottom of
>> the first patch.
> 
> The API generally looks pleasant, but the nesting surprised me.  E.g.,
> I'd have expected:
> 
>    jw_array_begin(out);
>    jw_array_begin(out);
>    jw_array_append_int(out, 42);
>    jw_array_end(out);
>    jw_array_end(out);
> 
> to result in an array containing an array containing an integer. But
> array_begin() actually resets the strbuf, so you can't build up nested
> items like this internally. Ditto for objects within objects. You have
> to use two separate strbufs and copy the data an extra time.
> 
> To make the above work, I think you'd have to store a little more state.
> E.g., the "array_append" functions check "out->len" to see if they need
> to add a separating comma. That wouldn't work if we might be part of a
> nested array. So I think you'd need a context struct like:
> 
>    struct json_writer {
>      int first_item;
>      struct strbuf out;
>    };
>    #define JSON_WRITER_INIT { 1, STRBUF_INIT }
> 
> to store the state and the output. As a bonus, you could also use it to
> store some other sanity checks (e.g., keep a "depth" counter and BUG()
> when somebody tries to access the finished strbuf with a hanging-open
> object or array).

Yeah, I thought about that, but I think it gets more complex than that. I'd need a stack of "first_item" values. Or maybe the _begin() needs to increment a depth and set first_item and the _end() needs to always unset first_item. I'll look at this gain.

The thing I liked about the bottom-up construction is that it is easier to collect multiple sets in parallel and combine them during the final roll-up. With the in-line nesting, you're tempted to try to construct the resulting JSON in a single series and that may not fit what the code is trying to do. For example, if I wanted to collect an array of error messages as they are generated and an array of argv arrays and any alias expansions, then put together a final JSON string containing them and the final exit code, I'd need to build it in parts. I can build these parts in pieces of JSON and combine them at the end -- or build up other similar data structures (string arrays, lists, or whatever) and then have a JSON conversion step. But we can make it work both ways, I just wanted to keep it simpler.

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>> I wasn't sure how to unit test the API from a shell script, so I added a
>> helper command that does most of the work in the second patch.
> 
> In general I'd really prefer to keep the shell script as the driver for
> the tests, and have t/helper programs just be conduits. E.g., something
> like:
> 
>    cat >expect <<-\EOF &&
>    {"key": "value", "foo": 42}
>    EOF
>    test-json-writer >actual \
>      object_begin \
>      object_append_string key value \
>      object_append_int foo 42 \
>      object_end &&
>    test_cmp expect actual
> 
> It's a bit tedious (though fairly mechanical) to expose the API in this
> way, but it makes it much easier to debug, modify, or add tests later
> on (for example, I had to modify the C program to find out that my
> append example above wouldn't work).

Yeah, I wasn't sure if such a simple api required exposing all that machinery to the shell or not. And the api is fairly self-contained and not depending on a lot of disk/repo setup or anything, so my tests would be essentially static WRT everything else.

With my t0019 script you should have been able use -x -v to see what was failing.

> 
> -Peff
>

thanks for the quick review Jeff