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pack-protocol.txt: accept error packets in any context

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In the Git pack protocol definition, an error packet may appear only in
a certain context. However, servers can face a runtime error (e.g. I/O
error) at an arbitrary timing. This patch changes the protocol to allow
an error packet to be sent instead of any packet.

Without this protocol spec change, when a server cannot process a
request, there's no way to tell that to a client. Since the server
cannot produce a valid response, it would be forced to cut a connection
without telling why. With this protocol spec change, the server can be
more gentle in this situation. An old client may see these error packets
as an unexpected packet, but this is not worse than having an unexpected
EOF.

Following this protocol spec change, the error packet handling code is
moved to pkt-line.c. Implementation wise, this implementation uses
pkt-line to communicate with a subprocess. Since this is not a part of
Git protocol, it's possible that a packet that is not supposed to be an
error packet is mistakenly parsed as an error packet. This error packet
handling is enabled only for the Git pack protocol parsing code
considering this.

Signed-off-by: Masaya Suzuki <masayasuzuki@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
This commit is contained in:
Masaya Suzuki
2018-12-29 13:19:15 -08:00
committed by Junio C Hamano
parent 01f9ec64c8
commit 2d103c31c2
15 changed files with 54 additions and 34 deletions
Download Patch File Download Diff File Expand all files Collapse all files

20
Documentation/technical/pack-protocol.txt
View File

@ -22,6 +22,16 @@ protocol-common.txt. When the grammar indicate `PKT-LINE(...)`, unless
otherwise noted the usual pkt-line LF rules apply: the sender SHOULD
include a LF, but the receiver MUST NOT complain if it is not present.
An error packet is a special pkt-line that contains an error string.
----
error-line = PKT-LINE("ERR" SP explanation-text)
----
Throughout the protocol, where `PKT-LINE(...)` is expected, an error packet MAY
be sent. Once this packet is sent by a client or a server, the data transfer
process defined in this protocol is terminated.
Transports
----------
There are three transports over which the packfile protocol is
@ -89,13 +99,6 @@ process on the server side over the Git protocol is this:
"0039git-upload-pack /schacon/gitbook.git\0host=example.com\0" |
nc -v example.com 9418
If the server refuses the request for some reasons, it could abort
gracefully with an error message.
----
error-line = PKT-LINE("ERR" SP explanation-text)
----
SSH Transport
-------------
@ -398,12 +401,11 @@ from the client).
Then the server will start sending its packfile data.
----
server-response = *ack_multi ack / nak / error-line
server-response = *ack_multi ack / nak
ack_multi = PKT-LINE("ACK" SP obj-id ack_status)
ack_status = "continue" / "common" / "ready"
ack = PKT-LINE("ACK" SP obj-id)
nak = PKT-LINE("NAK")
error-line = PKT-LINE("ERR" SP explanation-text)
----
A simple clone may look like this (with no 'have' lines):