On Tue, Oct 15, 2019 at 01:56:02PM +0000, Derrick Stolee via GitGitGadget wrote:

From: Derrick Stolee <redacted>

The sparse-checkout builtin used 'git read-tree -mu HEAD' to update the
skip-worktree bits in the index and to update the working directory.
This extra process is overly complex, and prone to failure. It also
requires that we write our changes to the sparse-checkout file before
trying to update the index.

Remove this extra process call by creating a direct call to
unpack_trees() in the same way 'git read-tree -mu HEAD' does. In
addition, provide an in-memory list of patterns so we can avoid
reading from the sparse-checkout file. This allows us to test a
proposed change to the file before writing to it.

Starting with this patch there is an issue with locking the index:

  $ git init
  Initialized empty Git repository in /home/szeder/src/git/tmp/SC/.git/
  $ >file
  $ git commit -m initial
  [master (root-commit) 5d80b9c] initial
   1 file changed, 0 insertions(+), 0 deletions(-)
   create mode 100644 file
  $ ls .git/index.lock
  ls: cannot access '.git/index.lock': No such file or directory
  $ git sparse-checkout set nope
  warning: core.sparseCheckout is disabled, so changes to the sparse-checkout file will have no effect
  warning: run 'git sparse-checkout init' to enable the sparse-checkout feature
  error: Sparse checkout leaves no entry on working directory
  fatal: Unable to create '/home/szeder/src/git/tmp/SC/.git/index.lock':
  File exists.

  Another git process seems to be running in this repository, e.g.
  an editor opened by 'git commit'. Please make sure all processes
  are terminated then try again. If it still fails, a git process
  may have crashed in this repository earlier:
  remove the file manually to continue.
  $ ls .git/index.lock
  ls: cannot access '.git/index.lock': No such file or directory