[PATCH 00/18] Partial clone (from clone to lazy fetch in 18 patches)

These patches are also available online: https://github.com/jonathantanmy/git/commits/partialclone3

(I've announced it in another e-mail, but am now sending the patches to the mailing list too.)

Here's an update of my work so far. Notable features:
 - These 18 patches allow a user to clone with --blob-max-bytes=<bytes>,
   creating a partial clone that is automatically configured to lazily
   fetch missing objects from the origin. The local repo also has fsck
   working offline, and GC working (albeit only on locally created
   objects).
 - Cloning and fetching is currently only able to exclude blobs by a
   size threshold, but the local repository is already capable of
   fetching missing objects of any type. For example, if a repository
   with missing trees or commits is generated by any tool (for example,
   a future version of Git), current Git with my patches will still be
   able to operate on them, automatically fetching those missing trees
   and commits when needed.
 - Missing blobs are fetched all at once during checkout.

Jeff Hostetler has sent out some object-filtering patches [1] that is a superset of the object-filtering functionality that I have (in the pack-objects patches). I have gone for the minimal approach here, but if his patches are merged, I'll update my patch set to use those.

[1] https://public-inbox.org/git/20170922203017.53986-6-git@jeffhostetler.com/

Demo ====

Obtain a repository.

    $ make prefix=$HOME/local install
    $ cd $HOME/tmp
    $ git clone https://github.com/git/git

Make it advertise the new feature and allow requests for arbitrary blobs.

    $ git -C git config uploadpack.advertiseblobmaxbytes 1
    $ git -C git config uploadpack.allowanysha1inwant 1

Perform the partial clone and check that it is indeed smaller. Specify "file://" in order to test the partial clone mechanism. (If not, Git will perform a local clone, which unselectively copies every object.)

    $ git clone --blob-max-bytes=0 "file://$(pwd)/git" git2
    $ git clone "file://$(pwd)/git" git3
    $ du -sh git2 git3
    85M	git2
    130M	git3

Observe that the new repo is automatically configured to fetch missing objects from the original repo. Subsequent fetches will also be partial.

    $ cat git2/.git/config
    [core]
    	repositoryformatversion = 1
    	filemode = true
    	bare = false
    	logallrefupdates = true
    [remote "origin"]
    	url = [snip]
    	fetch = +refs/heads/*:refs/remotes/origin/*
    	blobmaxbytes = 0
    [extensions]
    	partialclone = origin
    [branch "master"]
    	remote = origin
    	merge = refs/heads/master

Design ======

Local repository layout -----------------------

A repository declares its dependence on a *promisor remote* (a remote that declares that it can serve certain objects when requested) by a repository extension "partialclone". `extensions.partialclone` must be set to the name of the remote ("origin" in the demo above).

A packfile can be annotated as originating from the promisor remote by the existence of a "(packfile name).promisor" file with arbitrary contents (similar to the ".keep" file). Whenever a promisor remote sends an object, it declares that it can serve every object directly or indirectly referenced by the sent object.

A promisor packfile is a packfile annotated with the ".promisor" file. A promisor object is an object that the promisor remote is known to be able to serve, because it is an object in a promisor packfile or directly referred to by one.

(In the future, we might need to add ".promisor" support to loose objects.)

Connectivity check and gc -------------------------

The object walk done by the connectivity check (as used by fsck and fetch) stops at all promisor objects.

The object walk done by gc also stops at all promisor objects. Only non-promisor packfiles are deleted (if pack deletion is requested); promisor packfiles are left alone. This maintains the distinction between promisor packfiles and non-promisor packfiles. (In the future, we might need to do something more sophisticated with promisor packfiles.)

Fetching of missing objects ---------------------------

When `sha1_object_info_extended()` (or similar) is invoked, it will automatically attempt to fetch a missing object from the promisor remote if that object is not in the local repository. For efficiency, no check is made as to whether that object is known to be a promisor object or not.

This automatic fetching can be toggled on and off by the `fetch_if_missing` global variable, and it is on by default.

The actual fetch is done through the fetch-pack/upload-pack protocol. Right now, this uses the fact that upload-pack allows blob and tree "want"s, and this incurs the overhead of the unnecessary ref advertisement. I hope that protocol v2 will allow us to declare that blob and tree "want"s are allowed, and allow the client to declare that it does not want the ref advertisement. All packfiles downloaded in this way are annotated with ".promisor".

Fetching with `git fetch` -------------------------

The fetch-pack/upload-pack protocol has also been extended to support omission of blobs above a certain size. The client only allows this when fetching from the promisor remote, and will annotate any packs received from the promisor remote with ".promisor".

Jonathan Tan (18):
  fsck: introduce partialclone extension
  fsck: support refs pointing to promisor objects
  fsck: support referenced promisor objects
  fsck: support promisor objects as CLI argument
  index-pack: refactor writing of .keep files
  introduce fetch-object: fetch one promisor object
  sha1_file: support lazily fetching missing objects
  rev-list: support termination at promisor objects
  gc: do not repack promisor packfiles
  pack-objects: rename want_.* to ignore_.*
  pack-objects: support --blob-max-bytes
  fetch-pack: support excluding large blobs
  fetch: refactor calculation of remote list
  fetch: support excluding large blobs
  clone: support excluding large blobs
  clone: configure blobmaxbytes in created repos
  unpack-trees: batch fetching of missing blobs
  fetch-pack: restore save_commit_buffer after use

 Documentation/git-pack-objects.txt                |  12 +-
 Documentation/technical/pack-protocol.txt         |   9 +
 Documentation/technical/protocol-capabilities.txt |   7 +
 Documentation/technical/repository-version.txt    |  12 +
 Makefile                                          |   1 +
 builtin/cat-file.c                                |   2 +
 builtin/clone.c                                   |  24 +-
 builtin/fetch-pack.c                              |  21 ++
 builtin/fetch.c                                   |  36 ++-
 builtin/fsck.c                                    |  26 +-
 builtin/gc.c                                      |   3 +
 builtin/index-pack.c                              | 113 ++++---
 builtin/pack-objects.c                            |  97 ++++--
 builtin/prune.c                                   |   7 +
 builtin/repack.c                                  |   7 +-
 builtin/rev-list.c                                |  13 +
 cache.h                                           |  13 +-
 connected.c                                       |   1 +
 environment.c                                     |   1 +
 fetch-object.c                                    |  45 +++
 fetch-object.h                                    |  11 +
 fetch-pack.c                                      |  23 +-
 fetch-pack.h                                      |   3 +
 list-objects.c                                    |  16 +-
 object.c                                          |   2 +-
 packfile.c                                        |  77 ++++-
 packfile.h                                        |  13 +
 remote-curl.c                                     |  21 +-
 remote.c                                          |   2 +
 remote.h                                          |   2 +
 revision.c                                        |  33 ++-
 revision.h                                        |   5 +-
 setup.c                                           |   7 +-
 sha1_file.c                                       |  38 ++-
 t/t0410-partial-clone.sh                          | 343 ++++++++++++++++++++++
 t/t5300-pack-object.sh                            |  45 +++
 t/t5500-fetch-pack.sh                             | 115 ++++++++
 t/t5601-clone.sh                                  | 101 +++++++
 t/test-lib-functions.sh                           |  12 +
 transport-helper.c                                |   4 +
 transport.c                                       |  18 ++
 transport.h                                       |  12 +
 unpack-trees.c                                    |  22 ++
 upload-pack.c                                     |  16 +-
 44 files changed, 1278 insertions(+), 113 deletions(-)
 create mode 100644 fetch-object.c
 create mode 100644 fetch-object.h
 create mode 100755 t/t0410-partial-clone.sh