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[RFC/PATCH v2 0/1] Add support for downloading blobs on demand

This patch series enables git to request missing objects when they are
not found in the object store. This is a fault-in model where the
"read-object" sub-process will fetch the missing object and store it in
the object store as a loose, alternate, or pack file. On success, git
will retry the operation and find the requested object.

It utilizes the recent sub-process refactoring to spawn a "read-object"
hook as a sub-process on the first request and then all subsequent
requests are made to that existing sub-process. This significantly
reduces the cost of making multiple request within a single git command.

An earlier version [1] of this patch series is pulled into [2] which
enables registering multiple ODB handlers which can be a simple command
or a sub-process. My hope is that this patch series can be completed to
meet the needs of the various efforts where faulting in missing objects
is required.

In the meantime, now that the sub-process refactoring has made it to
master, I�ve refactored this patch series to be as small and focused as
possible so it can be easily incorporated in other patch series where it
makes sense.

[3] has a need for faulting in missing objects and will be reworked to
take advantage of this patch series for its next iteration.

[4] has a similar function that uses a registered command instead of a
sub-process. Spawning a separate process for every missing object only
works if there are very few missing objects.  It does not scale well
when there are many missing objects (especially small objects like
commits and trees).

Patch is available here:

[RFC] Add support for downloading blobs on demand 
[1] https://public-inbox.org/git/20170113155253.1644-1-benpeart@xxxxxxxxxxxxx/

[RFC/PATCH v4 00/49] Add initial experimental external ODB support 
[2] https://public-inbox.org/git/20170620075523.26961-1-chriscool@xxxxxxxxxxxxx/

[PATCH v2 00/19] WIP object filtering for partial clone
[3] https://public-inbox.org/git/20170713173459.3559-1-git@xxxxxxxxxxxxxxxxx/

[RFC PATCH 0/3] Partial clone: promised blobs (formerly "missing blobs")
[4] https://public-inbox.org/git/cover.1499800530.git.jonathantanmy@xxxxxxxxxx/

Ben Peart (1):
  sha1_file: Add support for downloading blobs on demand

 Documentation/technical/read-object-protocol.txt | 102 ++++++++++++
 cache.h                                          |   1 +
 config.c                                         |   5 +
 contrib/long-running-read-object/example.pl      | 114 +++++++++++++
 environment.c                                    |   1 +
 sha1_file.c                                      | 193 ++++++++++++++++++++++-
 t/t0410-read-object.sh                           |  27 ++++
 t/t0410/read-object                              | 114 +++++++++++++
 8 files changed, 550 insertions(+), 7 deletions(-)
 create mode 100644 Documentation/technical/read-object-protocol.txt
 create mode 100755 contrib/long-running-read-object/example.pl
 create mode 100755 t/t0410-read-object.sh
 create mode 100755 t/t0410/read-object