sparse: vendor Facebook-developed extension

Authored by indygreg.


sparse: vendor Facebook-developed extension

Facebook has developed an extension to enable "sparse" checkouts -
a working directory with a subset of files. This feature is a critical
component in enabling repositories to scale to infinite number of
files while retaining reasonable performance. It's worth noting
that sparse checkout is only one possible solution to this problem:
another is virtual filesystems that realize files on first access.
But given that virtual filesystems may not be accessible to all
users, sparse checkout is necessary as a fallback.

Per mailing list discussion at
we want to add sparse checkout to the Mercurial distribution via
roughly the following mechanism:

  1. Vendor extension as-is with minimal modifications (this patch)
  2. Refactor extension so it is more clearly experimental and inline with Mercurial practices
  3. Move code from extension into core where possible
  4. Drop experimental labeling and/or move feature into core after sign-off from narrow clone feature owners

This commit essentially copies the sparse extension and tests
from revision 71e0a2aeca92a4078fe1b8c76e32c88ff1929737 of the repository.

A list of modifications made as part of vendoring is as follows:

  • "EXPERIMENTAL" added to module docstring
  • Imports were changed to match Mercurial style conventions
  • "testedwith" value was updated to core Mercurial special value and comment boilerplate was inserted
  • A "clone_sparse" function was renamed to "clonesparse" to appease the style checker
  • Paths to the sparse extension in tests reflect built-in location
  • test-sparse-extensions.t was renamed to test-sparse-fsmonitor.t and references to "simplecache" were removed. The test always skips because it isn't trivial to run it given the way we currently run fsmonitor tests
  • A double empty line was removed from test-sparse-profiles.t

There are aspects of the added code that are obviously not ideal.
The goal is to make a minimal number of modifications as part of
the vendoring to make it easier to track changes from the original
implementation. Refactoring will occur in subsequent patches.


indygregJul 1 2017, 1:43 PM
rHGf08a178adadf: contrib: widen "direct use of `python`" net again