Re: [RFC/WIP PATCH 11/11] Document protocol version 2

From: Stefan Beller <sbeller@google.com>
Date: Jun 1, 2015, 23:40 UTC
Message-ID: <CAGZ79kYD--dZ_V=_X_Eo31KYTKXt2njuf56XqRRdaTJeLhDjaQ@mail.gmail.com>
In-Reply-To: <CAGZ79kaRTLX7eBCOA=yQHVwcN-H-o_aZFfQ1gw7Nx-NC82pbag@mail.gmail.com>

On Mon, Jun 1, 2015 at 4:14 PM, Stefan Beller <sbeller@google.com> wrote:

Show 67 quoted lines

> On Fri, May 29, 2015 at 3:21 PM, Jeff King <peff@peff.net> wrote:
>> On Fri, May 29, 2015 at 02:52:14PM -0700, Junio C Hamano wrote:
>>
>>> > Currently we can do a = as part of the line after the first ref, such as
>>> >
>>> >     symref=HEAD:refs/heads/master agent=git/2:2.4.0
>>> >
>>> > so I thought we want to keep this.
>>>
>>> I do not understand that statement.
>>>
>>> Capability exchange in v2 is one packet per cap, so the above
>>> example would be expressed as:
>>>
>>>       symref=HEAD:refs/heads/master
>>>         agent=git/2:2.4.0
>>>
>>> right?  Your "keyvaluepair" is limited to [a-z0-9-_=]*, and neither
>>> of the above two can be expressed with that, which was why I said
>>> you need two different set of characters before and after "=".  Left
>>> hand side of "=" is tightly limited and that is OK.  Right hand side
>>> may contain characters like ':', '.' and '/', so your alphabet need
>>> to be more lenient, even in v1 (which I would imagine would be "any
>>> octet other than SP, LF and NUL").
>
> I think the recent issue with the push certificates shows that having arbitrary
> data after the = is a bad idea. So we need to be very cautious when to allow
> which data after the =.
>
> I'll try split up the patch.
>
>>
>> Yes. See git_user_agent_sanitized(), for example, which allows basically
>> any printable ASCII except for SP.
>>
>> I think the v2 capabilities do not even need to have that restriction.
>> It can allow arbitrary binary data, because it has an 8bit-clean framing
>> mechanism (pkt-lines). Of course, that means such capabilities cannot be
>> represented in a v1 conversation (whose framing mechanism involves SP
>> and NUL). But it's probably acceptable to introduce new capabilities
>> which are only available in a v2 conversation. Old clients that do not
>> understand v2 would not understand the capability either. It does
>> require new clients implementing the capability to _also_ implement v2
>> if they have not done so, but I do not mind pushing people in that
>> direction.
>>
>> The initial v2 client implementation should probably do a few cautionary
>> things, then:
>>
>>   1. Do _not_ fold the per-pkt capabilities into a v1 string; that loses
>>      the robust framing. I suggested string_list earlier, but probably
>>      we want a list of ptr/len pair, so that it can remain NUL-clean.
>>
>>   2. Avoid holding on to unknown packets longer than necessary. Some
>>      capability pkt-lines may be arbitrarily large (up to 64K). If we do
>>      not understand them during the v2 read of the capabilities, there
>>      is no point hanging on to them. It's not _wrong_ to do so, but just
>>      inefficient; if we know that clients will just throw away unknown
>>      packets, then we can later introduce new packets with large data,
>>      without worrying about wasting the client's resources.
>>
>>      I suspect it's not that big a deal either way, though. I have no
>>      plans for sending a bunch of large packets, and anyway network
>>      bandwidth is probably more precious than client memory.
>
> That's very sensible thoughts after rereading this email. The version
> I'll be sending out today will not follow those suggestions though. :(

Thinking about this further, maybe it is a good idea to restrict the capabilities advertising to alphabetical order?

The exchange would look like this:

server:
  for capability in list:
      pkt_write(capability)
  pkt_flush

client:
  do
    line = recv_pkt()
    parse_capability(line)
  while line != flush

with parse_capability checking if we know the capability and maybe setting some internal field if we know this capability.

Now if we assume the number of capabilities grows over time a lot (someone may "abuse" it for a cool feature, similar to the refs currently. Nobody thought about having so many refs in advance)

So how does parse_capability scale w.r.t the number of capabilities? If parse_capability is just a linear search then it is O(n) and with n capabilities the client faces an O(n^2) computation which is bad. So if we were to require alphabetic capabilities, you could internally keep track and the whole operation is O(n). I just wonder if this is premature optimization or some thought we need to think of.

To prevent this problem from popping up, it must be easier to introduce a new phase after the capabilities exchange than to just abuse the capabilities phase for whatever you plan on doing.

Thanks, Stefan

>
>>
>> -Peff