[PATCH v2 0/3] upload-pack: support a missing-action

`git pack-objects` already supports a `--missing=<missing-action>` option, so that it can avoid erroring out if some objects aren't available.

It is interesting to have `git upload-pack` support a similar way to avoid sending some objects in case they aren't available on the server.

For example, in case both the server and the client are using a separate promisor remote that contain some objects, it can be better if the server doesn't try to send such objects back to the client, but instead let the client get those objects separately from the promisor remote. (The client needs to have the separate promisor remote configured, for that to work.)

This could work better if there was something, like perhaps a capability, for the client to tell the server something like:

  "I know how to fetch missing objects from this and that
  promisor remotes, so if you choose to, you may omit objects
  that you know are available from these promisor remotes when
  sending objects to me."

But that capability could be added later as other similar capabilities in this area could be very useful. For example in case of a client cloning, something like the following is currently needed:

  GIT_NO_LAZY_FETCH=0 git clone
      -c remote.my_promisor.promisor=true \
      -c remote.my_promisor.fetch="+refs/heads/*:refs/remotes/my_promisor/*" \
      -c remote.my_promisor.url=<MY_PROMISOR_URL> \
      --filter="blob:limit=5k" server

But it would be nice if there was a capability for the client to say that it would like the server to give it information about the promisor that it could use, so that the user doesn't have to pass all the "remote.my_promisor.XXX" config options on the command like. (It would then be a bit similar to the bundle-uri feature where all the bundle related information comes from the server.)

Another example use of this feature could be a server where some objects have been corrupted or deleted. It could still be useful for clients who could get those objects from another source, like perhaps a different client, to be able to fetch or clone from the server.

The fact that the new `uploadpack.missingAction` configuration variable has to be set to a non default value on the server means that regular client users cannot hurt themselves with this feature.

As `git rev-list` also supports a `--missing=<missing-action>` option, the first 2 patches in this series are about refactoring related code from both `git rev-list` and `git pack-objects` into new "missing.{c,h}" files. Patch 3/3 then adds a new `uploadpack.missingAction` configuration variable.

The changes since v1 are the following:

  - In patch 1/3 the refactored parse_missing_action_value() function
    doesn't change the fetch_if_missing global variable anymore. The
    clients of this function change it by themselves instead.

  - In patch 1/3 a minor typo was also fixed.

  - Old patch 2/4 that added a 'int print_ok' argument to
    parse_missing_action_value() was removed, and now callers deal
    with only the values they recognise.

Thanks to Junio for his review and for suggesting of the above changes.

Christian Couder (3):
  rev-list: refactor --missing=<missing-action>
  pack-objects: use the missing action API
  upload-pack: allow configuring a missing-action

 Documentation/config/uploadpack.txt |   9 ++
 Makefile                            |   1 +
 builtin/pack-objects.c              |  48 ++++++-----
 builtin/rev-list.c                  |  43 ++--------
 missing.c                           |  36 ++++++++
 missing.h                           |  19 +++++
 t/t5706-upload-pack-missing.sh      | 125 ++++++++++++++++++++++++++++
 upload-pack.c                       |  19 +++++
 8 files changed, 242 insertions(+), 58 deletions(-)
 create mode 100644 missing.c
 create mode 100644 missing.h
 create mode 100755 t/t5706-upload-pack-missing.sh