Repository snapshot objects

WARNING: work in progress blueprint

Introduction

A repository snapshot object is a Merkle DAG node used to capture the current state of a VCS repository.

Conceptually, a snapshot object is a complete map from repository entry points ("branches" in Software Heritage terminology, "refs" in Git) to other objects in the repository, including other snapshot objects if repository entry points point to them.
Practically, the map is serialized into a manifest consisting of a list of triples <object type, object ID, branch name>, sorted by branch name.

Entries in a snapshot object can point to the following object kinds:

Software Heritage objects	Git objects
contents	blobs
directories	trees
releases	annotated tags
revisions	commits
snapshots	n/a

Each snapshot object has as its object ID a cryptographic hash (the same used elsewhere in the Merkle DAG) of its manifest.

Git implementation

In the spirit of other Git objects, snapshot objects for Git repositories can be implemented as follows.

# create repo with some commits, branches, and tags
$ git init test
$ cd test/
$ echo foo > foo.txt
$ git add foo.txt 
$ git commit -m 'checkin foo'
$ git branch foo
$ echo bar >> foo.txt 
$ git commit -a -m 'add bar'
$ git tag bar
$ echo baz >> foo.txt 
$ git commit -a -m 'add baz'

# ASSUMPTION: the output of git show-ref is sorted by ref name using
# the usual Git sort algorithm for textual object manifests. This is
# currently the case as of Git 2.8.1, but it is not documented
# behavior in git-show-ref(1).

# repository object in full (the manifest)
$ git show-ref | \
  while read id ref ; do
    type=$(git cat-file -t $id)
    echo $type $id $ref
  done \
  > /tmp/snapshot-object.txt
$ cat /tmp/snapshot-object.txt
commit 585f6e27f540012af621a18d0155aae2a8ec0276 refs/heads/foo
commit 6d976a397fe0b28a5bc59540e64f7f36a861af68 refs/heads/master
commit 521cb6d728f9fa3d6c4d73ddd309c0796ddf6995 refs/tags/bar

# repository object ID, as a Git SHA1
$ git hash-object -w --stdin --literally -t snapshot < /tmp/snapshot-object.txt
470d2daa27715987685708b816bf2b52ba5a47c8

# raw content of the repository object, including Git header
$ zlib-flate -uncompress < .git/objects/47/0d2daa27715987685708b816bf2b52ba5a47c8
snapshot 191commit 585f6e27f540012af621a18d0155aae2a8ec0276 refs/heads/foo
commit 6d976a397fe0b28a5bc59540e64f7f36a861af68 refs/heads/master
commit 521cb6d728f9fa3d6c4d73ddd309c0796ddf6995 refs/tags/bar

# i.e., a 191-byte long object of type "snapshot"
# (note that a "\0" before the first "commit" string has been stripped)