upload-pack: implement ref-in-want

Currently, while performing packfile negotiation, clients are only
allowed to specify their desired objects using object ids.  This causes
a vulnerability to failure when an object turns non-existent during
negotiation, which may happen if, for example, the desired repository is
provided by multiple Git servers in a load-balancing arrangement and
there exists replication delay.

In order to eliminate this vulnerability, implement the ref-in-want
feature for the 'fetch' command in protocol version 2.  This feature
enables the 'fetch' command to support requests in the form of ref names
through a new "want-ref <ref>" parameter.  At the conclusion of
negotiation, the server will send a list of all of the wanted references
(as provided by "want-ref" lines) in addition to the generated packfile.

Signed-off-by: Brandon Williams <bmwill@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

2 files changed, 34 insertions, 1 deletions

diff --git a/Documentation/config.txt b/Documentation/config.txt
index ab641bf5a9..fb1dd7428a 100644
--- a/Documentation/config.txt
+++ b/Documentation/config.txt
@@ -3479,6 +3479,13 @@ Note that this configuration variable is ignored if it is seen in the
 repository-level config (this is a safety measure against fetching from
 untrusted repositories).
 
+uploadpack.allowRefInWant::
+	If this option is set, `upload-pack` will support the `ref-in-want`
+	feature of the protocol version 2 `fetch` command.  This feature
+	is intended for the benefit of load-balanced servers which may
+	not have the same view of what OIDs their refs point to due to
+	replication delay.
+
 url.<base>.insteadOf::
 	Any URL that starts with this value will be rewritten to
 	start, instead, with <base>. In cases where some site serves a
diff --git a/Documentation/technical/protocol-v2.txt b/Documentation/technical/protocol-v2.txt
index 49bda76d23..1da71d0ddb 100644
--- a/Documentation/technical/protocol-v2.txt
+++ b/Documentation/technical/protocol-v2.txt
@@ -299,12 +299,21 @@ included in the client's request:
 	for use with partial clone and partial fetch operations. See
 	`rev-list` for possible "filter-spec" values.
 
+If the 'ref-in-want' feature is advertised, the following argument can
+be included in the client's request as well as the potential addition of
+the 'wanted-refs' section in the server's response as explained below.
+
+    want-ref <ref>
+	Indicates to the server that the client wants to retrieve a
+	particular ref, where <ref> is the full name of a ref on the
+	server.
+
 The response of `fetch` is broken into a number of sections separated by
 delimiter packets (0001), with each section beginning with its section
 header.
 
     output = *section
-    section = (acknowledgments | shallow-info | packfile)
+    section = (acknowledgments | shallow-info | wanted-refs | packfile)
 	      (flush-pkt | delim-pkt)
 
     acknowledgments = PKT-LINE("acknowledgments" LF)
@@ -319,6 +328,10 @@ header.
     shallow = "shallow" SP obj-id
     unshallow = "unshallow" SP obj-id
 
+    wanted-refs = PKT-LINE("wanted-refs" LF)
+		  *PKT-LINE(wanted-ref LF)
+    wanted-ref = obj-id SP refname
+
     packfile = PKT-LINE("packfile" LF)
 	       *PKT-LINE(%x01-03 *%x00-ff)
 
@@ -379,6 +392,19 @@ header.
 	* This section is only included if a packfile section is also
 	  included in the response.
 
+    wanted-refs section
+	* This section is only included if the client has requested a
+	  ref using a 'want-ref' line and if a packfile section is also
+	  included in the response.
+
+	* Always begins with the section header "wanted-refs".
+
+	* The server will send a ref listing ("<oid> <refname>") for
+	  each reference requested using 'want-ref' lines.
+
+	* The server MUST NOT send any refs which were not requested
+	  using 'want-ref' lines.
+
     packfile section
 	* This section is only included if the client has sent 'want'
 	  lines in its request and either requested that no more