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References: <20220502170904.2770649-1-calvinwan@google.com>
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From:   Calvin Wan <calvinwan@google.com>
Date:   Mon, 1 Aug 2022 11:28:37 -0700
Message-ID: <CAFySSZAPzO9YzGhsjhG7YRXv+ctTHM2FXzrwRW2bTW4pUt8q7g@mail.gmail.com>
Subject: Re: [PATCH v5 4/6] serve: advertise object-info feature
To:     Junio C Hamano <gitster@pobox.com>
Cc:     git@vger.kernel.org, jonathantanmy@google.com,
        philipoakley@iee.email, johncai86@gmail.com
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> OK.  Now we will no longer advertise a bare "object-info", but
> "object-info=size" (and possibly in the future things other than
> "size").  How would this change affect older clients who knows what
> to do with "object-info" but not "object-info=<values>" yet?

This was a tricky tradeoff that I definitely think I should have
discussed more in the commit message. The issue with how object
info is currently implemented is that it is very inflexible for adding
new parameters.

This is how object-info currently parses a client request:

while (packet_reader_read(request) == PACKET_READ_NORMAL) {
    if (!strcmp("size", request->line)) {
        info.size = 1;
        continue;
    }

    if (parse_oid(request->line, &oid_str_list))
        continue;

    packet_writer_error(&writer,
    "object-info: unexpected line: '%s'",
    request->line);
}

Object-info supports "size" right now but, let's say I want to add
"type" as a parameter. OK I add another if statement like:

if (!strcmp("type", request->line)) {
    info.type = 1;
    continue;
}

And we update the docs to say "type" is now supported by
object-info. If a user now attempts to request "size" and "type"
from a server that has not been updated to support "type",
then the user gets an error message "object-info: unexpected
line: 'type'", which is another situation that is a bad experience
for older clients. The client has no way of knowing that their
failure is caused by a server version issue.

Essentially I think at some point we have to bite the bullet and say
we need to rework some part of the object-info advertisement (or
if anyone has a better idea of achieving the same goal) so that we
can make future incremental changes to object-info. If the supported
parameters are posted in the advertisement, then the client doesn't
have to first make a request to find out that their requested
parameter isn't support by the server. While you noted that we can't
make the assumption now that nobody is using the current
object-info feature, I think the benefit of the change outweighs
the cost of affecting the possibly small amount of users of this
feature. (A quick search on stack overflow for "object-info" tagged
under [git] returned no questions about it so that's what I used as
a cursory estimate for how popular this feature is).

Curious to hear what your thoughts on this are Junio, since as
much as I'd like to create a seamless upgrade experience for
older clients, I'm out of ideas as to how I would do so.