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From: Junio C Hamano <gitster@pobox.com>
To: Jonathan Tan <jonathantanmy@google.com>
Cc: git@vger.kernel.org
Subject: Re: [WIP RFC 4/5] upload-pack: refactor writing of "packfile" line
Date: Fri, 07 Dec 2018 09:22:04 +0900	[thread overview]
Message-ID: <xmqqmupi89ub.fsf@gitster-ct.c.googlers.com> (raw)
In-Reply-To: <20181206232538.141378-1-jonathantanmy@google.com> (Jonathan Tan's message of "Thu, 6 Dec 2018 15:25:38 -0800")

Jonathan Tan <jonathantanmy@google.com> writes:

>> Jonathan Tan <jonathantanmy@google.com> writes:
>> 
>> > @@ -126,6 +129,12 @@ static int read_pack_objects_stdout(int outfd, struct output_state *os)
>> >  	}
>> >  	os->used += readsz;
>> >  
>> > +	if (!os->packfile_started) {
>> > +		os->packfile_started = 1;
>> > +		if (use_protocol_v2)
>> > +			packet_write_fmt(1, "packfile\n");
>> 
>> If we fix this function so that the only byte in the buffer is held
>> back without emitted when os->used == 1 as I alluded to, this may
>> have to be done a bit later, as with such a change, it is no longer
>> guaranteed that send_client_data() will be called after this point.
>
> I'm not sure what you mean about there being no guarantee that
> send_client_data() is not called - in create_pack_file(), there is an
> "if (output_state.used > 0)" line (previously "if (0 <= buffered)") that
> outputs anything remaining.

I was referring to this part of the review on the previous step,
which you may not yet have read.

    OK, this corresponds to the "*cp++ = buffered"  in the original just
    before xread().

    > +		os->used = 1;
    > +	} else {
    > +		send_client_data(1, os->buffer, os->used);
    > +		os->used = 0;

    I am not sure if the code is correct when os->used happens to be 1
    (shouldn't we hold the byte, skip the call to send_client_data(),
    and go back to poll() to expect more data?), but this is a faithful
    code movement and rewrite of the original.

The point of this logic is to make sure we always hold back some
bytes and do not feed *all* the bytes to the other side by calling
"send-client-data" until we made sure the upstream of what we are
relaying (pack-objects?) successfully exited, but it looks to me
that the "else" clause above ends up flushing everything when
os->used is 1, which goes against the whole purpose of the code.

And the "fix" I was alluding to was to update that "else" clause to
make it a no-op that keeps os->used non-zero, which would not call
send-client-data.

When that fix happens, the part that early in the function this
patch added "now we know we will call send-client-data, so let's say
'here comes packdata' unless we have already said that" is making
the decision too early.  Depending on the value of os->used when we
enter the code and the number of bytes xread() reads from the
upstream, we might not call send-client-data yet (namely, when we
have no buffered data and we happened to get only one byte).

> ... it might be
> better if the server can send sideband throughout the whole response -
> perhaps that should be investigated first.

Yup.  It just looked quite crazy, and it is even more crazy to
buffer keepalives ;-)

  reply	other threads:[~2018-12-07  0:22 UTC|newest]

Thread overview: 30+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-12-03 23:37 [WIP RFC 0/5] Design for offloading part of packfile response to CDN Jonathan Tan
2018-12-03 23:37 ` [WIP RFC 1/5] Documentation: order protocol v2 sections Jonathan Tan
2018-12-05  4:10   ` Junio C Hamano
2018-12-06 22:54     ` Jonathan Tan
2018-12-09  0:15       ` Junio C Hamano
2018-12-03 23:37 ` [WIP RFC 2/5] Documentation: add Packfile URIs design doc Jonathan Tan
2018-12-04  0:21   ` Stefan Beller
2018-12-04  1:54   ` brian m. carlson
2018-12-04 19:29     ` Jonathan Tan
2019-02-19 13:22       ` Christian Couder
2019-02-19 20:10         ` Jonathan Tan
2019-02-22 11:35           ` Christian Couder
2019-02-19 13:44     ` Ævar Arnfjörð Bjarmason
2019-02-21  1:09       ` brian m. carlson
2019-02-22  9:34         ` Ævar Arnfjörð Bjarmason
2018-12-05  5:02   ` Junio C Hamano
2018-12-05  5:55     ` Junio C Hamano
2018-12-06 23:16     ` Jonathan Tan
2019-02-19 14:28   ` Ævar Arnfjörð Bjarmason
2019-02-19 22:06     ` Jonathan Tan
2018-12-03 23:37 ` [WIP RFC 3/5] upload-pack: refactor reading of pack-objects out Jonathan Tan
2018-12-04  0:30   ` Stefan Beller
2018-12-05  6:30   ` Junio C Hamano
2018-12-03 23:37 ` [WIP RFC 4/5] upload-pack: refactor writing of "packfile" line Jonathan Tan
2018-12-06  6:35   ` Junio C Hamano
2018-12-06 23:25     ` Jonathan Tan
2018-12-07  0:22       ` Junio C Hamano [this message]
2018-12-03 23:37 ` [WIP RFC 5/5] upload-pack: send part of packfile response as uri Jonathan Tan
2018-12-04 20:09   ` Stefan Beller
2018-12-04  0:01 ` [WIP RFC 0/5] Design for offloading part of packfile response to CDN Stefan Beller

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