bundle: add new version for use with SHA-256

Currently we detect the hash algorithm in use by the length of the
object ID.  This is inelegant and prevents us from using a different
hash algorithm that is also 256 bits in length.

Since we cannot extend the v2 format in a backward-compatible way, let's
add a v3 format, which is identical, except for the addition of
capabilities, which are prefixed by an at sign.  We add "object-format"
as the only capability and reject unknown capabilities, since we do not
have a network connection and therefore cannot negotiate with the other
side.

For compatibility, default to the v2 format for SHA-1 and require v3
for SHA-256.

In t5510, always use format v3 so we can be sure we produce consistent
results across hash algorithms.  Since head -n N lists the top N lines
instead of the Nth line, let's run our output through sed to normalize
it and compare it against a fixed value, which will make sure we get
exactly what we're expecting.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Reviewed-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

2 files changed, 37 insertions, 2 deletions

diff --git a/Documentation/git-bundle.txt b/Documentation/git-bundle.txt
index d34b0964be..53804cad4b 100644
--- a/Documentation/git-bundle.txt
+++ b/Documentation/git-bundle.txt
@@ -9,7 +9,8 @@ git-bundle - Move objects and refs by archive
 SYNOPSIS
 --------
 [verse]
-'git bundle' create [-q | --quiet | --progress | --all-progress] [--all-progress-implied] <file> <git-rev-list-args>
+'git bundle' create [-q | --quiet | --progress | --all-progress] [--all-progress-implied]
+		    [--version=<version>] <file> <git-rev-list-args>
 'git bundle' verify [-q | --quiet] <file>
 'git bundle' list-heads <file> [<refname>...]
 'git bundle' unbundle <file> [<refname>...]
@@ -102,6 +103,12 @@ unbundle <file>::
 	is activated.  Unlike --all-progress this flag doesn't actually
 	force any progress display by itself.
 
+--version=<version>::
+	Specify the bundle version.  Version 2 is the older format and can only be
+	used with SHA-1 repositories; the newer version 3 contains capabilities that
+	permit extensions. The default is the oldest supported format, based on the
+	hash algorithm in use.
+
 -q::
 --quiet::
 	This flag makes the command not to report its progress
diff --git a/Documentation/technical/bundle-format.txt b/Documentation/technical/bundle-format.txt
index 0e828151a5..bac558d049 100644
--- a/Documentation/technical/bundle-format.txt
+++ b/Documentation/technical/bundle-format.txt
@@ -7,6 +7,8 @@ The Git bundle format is a format that represents both refs and Git objects.
 We will use ABNF notation to define the Git bundle format. See
 protocol-common.txt for the details.
 
+A v2 bundle looks like this:
+
 ----
 bundle    = signature *prerequisite *reference LF pack
 signature = "# v2 git bundle" LF
@@ -18,9 +20,28 @@ reference    = obj-id SP refname LF
 pack         = ... ; packfile
 ----
 
+A v3 bundle looks like this:
+
+----
+bundle    = signature *capability *prerequisite *reference LF pack
+signature = "# v3 git bundle" LF
+
+capability   = "@" key ["=" value] LF
+prerequisite = "-" obj-id SP comment LF
+comment      = *CHAR
+reference    = obj-id SP refname LF
+key          = 1*(ALPHA / DIGIT / "-")
+value        = *(%01-09 / %0b-FF)
+
+pack         = ... ; packfile
+----
+
 == Semantics
 
-A Git bundle consists of three parts.
+A Git bundle consists of several parts.
+
+* "Capabilities", which are only in the v3 format, indicate functionality that
+	the bundle requires to be read properly.
 
 * "Prerequisites" lists the objects that are NOT included in the bundle and the
   reader of the bundle MUST already have, in order to use the data in the
@@ -46,3 +67,10 @@ put any string here. The reader of the bundle MUST ignore the comment.
 Note that the prerequisites does not represent a shallow-clone boundary. The
 semantics of the prerequisites and the shallow-clone boundaries are different,
 and the Git bundle v2 format cannot represent a shallow clone repository.
+
+== Capabilities
+
+Because there is no opportunity for negotiation, unknown capabilities cause 'git
+bundle' to abort.  The only known capability is `object-format`, which specifies
+the hash algorithm in use, and can take the same values as the
+`extensions.objectFormat` configuration value.