Re: [PATCH v4 03/35] pkt-line: add delim packet support

From: Junio C Hamano <gitster@pobox.com>
Date: Mar 1, 2018, 23:43 UTC
Message-ID: <xmqqefl3e505.fsf@gitster-ct.c.googlers.com>
In-Reply-To: <20180301224916.GB241240@google.com>

Brandon Williams <bmwill@google.com> writes:

Show 10 quoted lines

> On 03/01, Junio C Hamano wrote:
> ...
>> Hmph, strictly speaking, the "delim" does not have to be a part of
>> how packetized stream is defined.  As long as we stop abusing flush
>> as "This is merely an end of one segment of what I say." and make it
>> always mean "I am done speaking, it is your turn.", the application
>> payload can define its own syntax to separate groups of packets.
>
> Thanks actually a good point.  We could just as easily have the delim
> packet to be an empty packet-line "0004" or something like that.

Yes. As long as there is an easy and obvious "cannot be a value" constant, you can use it as a delimiter defined at the application level. For example, your command-request uses delim, like so:

+    request = empty-request | command-request
+    empty-request = flush-pkt
+    command-request = command
+		      capability-list
+		      (command-args)
+		      flush-pkt
+    command = PKT-LINE("command=" key LF)
+    command-args = delim-pkt
+		   *PKT-Line(arg LF)

to mark the end of cap list, but if an empty packet does not make sense as a member of a cap list and a commmand args list, then an empty packet between cap list and command arg can be used instead. A protocol-ignorant proxy can still work just fine.

Having a defined delim at the protocol level is often convenient, of course, but once the application starts calling for multi-level delimiters (i.e. maybe there are chapters and sections inside each chapter in a single request message), it would not be sufficient to define a single delim packet type. The application layer needs to define its own convention (e.g. if no "empty" section is allowed, then "two consecutive delim is a chapter break; one delim is a section break" can become a viable way to emulate multi-level delimiters).