Re: [RFC PATCH 19/21] usage API: use C99 macros for {usage,usagef,die,error,warning,die}*()

["Ævar Arnfjörð Bjarmason" <avarab@gmail.com>]

On Mon, Dec 27 2021, Jeff Hostetler wrote:

> On 11/15/21 5:18 PM, Ævar Arnfjörð Bjarmason wrote:
> [...]
>> It might be a good change to remove the "fmt" key from the "error"
>> events as a follow-up change. As these few examples from running the
>> test suite show it's sometimes redundant (same as the "msg"), rather
>> useless (just a "%s"), or something we could now mostly aggregate by
>> file/line instead of the normalized printf format:
>>        1 file":"builtin/gc.c","line":1391,"msg":"'bogus' is not a
>> valid task","fmt":"'%s' is not a valid task"}
>>        1 file":"builtin/for-each-ref.c","line":89,"msg":"format: %(then) atom used more than once","fmt":"%s"}
>>        1 file":"builtin/fast-import.c","line":411,"msg":"Garbage after mark: N :202 :302x","fmt":"Garbage after mark: %s"}
>> "Mostly" here assumes that it would be OK if the aggregation changed
>> between git versions, which may be what all users of trace2 want. The
>> change that introduced the "fmt" code was ee4512ed481 (trace2: create
>> new combined trace facility, 2019-02-22), and the documentation change
>> was e544221d97a (trace2: Documentation/technical/api-trace2.txt,
>> 2019-02-22).
>> Both are rather vague on what problem "fmt" solved exactly, aside
>> from
>> the obvious one of it being impossible to do meaningful aggregations
>> due to the "file" and "line" being the same everywhere, which isn't
>> the case now.
>> In any case, let's leave "fmt" be for now, the above summary was
>> given
>> in case it's interesting to remove it in the future, e.g. to save
>> space in trace2 payloads.
>
> I added the "fmt" field so that we could do aggregations
> of error messages across multiple users without regard
> to what branch or filename or percentage or whatever was
> formatted into the actual "msg" written to stderr.
>
> The actual file:line wasn't useful (primarily because it
> was probably something in usage.c), but even if we fix that
> it might not be useful if we have users running 10 different
> versions of Git (because some people don't upgrade immediately).
>
> So I'd rather not kill it right now.

Thanks. I'm not trying to kill it, but just poking at what it was for
exactly.

Depending on the answer to that perhaps we didn't need it anymore, but
the explanation you provide (mostly) makes sense.

The "mostly" being because I'm assuming that you only need to deal with
LC_ALL=C users?

I.e. the documented promise that you can group things by "fmt" doesn't
hold if you're processing even streams from users who are using a
translated git, because we'll get the translated format string, not the
original.

For that we'd need to change the API from/to to:

    - error(_("some format %s"), ...)
    + error(N_("some format %s"), ...)

So being able to say "just group on file/line" would be simpler.

And also "mostly" because the "fmt" case also won't handle these and
other duplicate formats (but maybe you haven't run into them in
practice):

    $ git grep -E '\b(usage|die|error|warning)(_errno)?\("%s\"' -- '*.[ch]' | wc -l
    90

So I was somewhat hoping for future work that you'd be OK with the new
file/line grouping.

Because keeping "fmt" would eventually need some massive coccinelle
search/replacement for "_(...)" -> "N_(...)" per the above, even then
consumers of the stream would get duplicate grouping for the likes of
"%s".

Do you think if as a follow-up we had "__func__"[1] along with
"file/line" that the "file/__func__" combination would be good enough?
The advantage of that would be that we could punt that "fmt"
change/complexity and document:

    If you'd like to group errors the "file/line" pair will be unique
    enough within a given git version to do so (sans a few codepaths that
    relay errors from elsewhere).

    If you'd like a semi-stable grouping across similar git versions the
    "file/func" pair should be Good Enough for most purposes. Some functions
    might emit multiple errors, but you'd probably want to group them as
    similar enough anyway.

But I realize that those things don't give you exactly the same things
that "fmt" does, but maybe they're good enough (or even better?), or
not.

1. https://gcc.gnu.org/onlinedocs/gcc/Function-Names.html