[RFC PATCH 0/5] Remove the present-despite-SKIP_WORKTREE class of bugs

[Elijah Newren <newren@gmail.com>]

(Maintainer note: This series builds on vd/sparse-clean-etc, because it
tweaks one of the testcases added there.)

Files in the present-despite-SKIP_WORKTREE state have caused no ends of
discussions and bugs[1,2,3,4,5,6,...and lots of others].  Trying to
address the big issue of discovering & recovering from this state has
befuddled me for over a year because I was worried we'd need additional
code at every skip_worktree-checking path in the code (and they are all
over the place), and that we'd make the code significantly slower unless
we plumbed a bunch of additional information all over the place to allow
some reasonable optimizations.

This series tries to solve the problem a bit differently by automatic
early discovery and recovery; as it result, it greatly simplifies the
landscape, reduces our testing matrix burden, and fixes a large swath of
bugs.  It does have a cost, though.  See the commit message of patch 3
(the crux of this series) for the details.

Quick overview:

  * Patches 1 & 2 add a test to demonstrate accidental deletion of
    possibly-modified files, and then fix the bug.
  * Patch 3 is the crux of this series; a small amount of code with a
    huge commit message
  * Patch 4 updates the documentation
  * Patch 5 adds some optimizations to reduce the performance impact of
    patch 3

[1] https://lore.kernel.org/git/CABPp-BFnFpzwGC11TLoLs8YK5yiisA5D5-fFjXnJsbESVDwZsA@mail.gmail.com/
[2] https://lore.kernel.org/git/CABPp-BGJ_Nvi5TmgriD9Bh6eNXE2EDq2f8e8QKXAeYG3BxZafA@mail.gmail.com/
[3] https://lore.kernel.org/git/pull.809.git.git.1592356884310.gitgitgadget@gmail.com/
[4] commit 66b209b86a ("merge-ort: implement CE_SKIP_WORKTREE handling with conflicted entries", 2021-03-20)
[5] commit ba359fd507 ("stash: fix stash application in sparse-checkouts", 2020-12-01)
[6] https://lore.kernel.org/git/CABPp-BH9tju7WVm=QZDOvaMDdZbpNXrVWQdN-jmfN8wC6YVhmw@mail.gmail.com/

[Final note: This series and test results can also be seen at
https://github.com/gitgitgadget/git/pull/1113; gitgitgadget wanted me to
"rewrap" the lines with >80 character URLS in commit messages, and I
refuse to remove the URLs or break them by inserting newlines, so I'm
sending it manually instead.]

Elijah Newren (5):
  t1011: add testcase demonstrating accidental loss of user
    modifications
  unpack-trees: fix accidental loss of user changes
  repo_read_index: ensure SKIP_WORKTREE means skip worktree
  Update documentation related to sparsity and the skip-worktree bit
  Accelerate ensure_skip_worktree_means_skip_worktree by caching

 Documentation/git-read-tree.txt          |  12 ++-
 Documentation/git-sparse-checkout.txt    |  76 ++++++++------
 Documentation/git-update-index.txt       |  55 +++++++---
 repository.c                             |   7 ++
 sparse-index.c                           | 123 +++++++++++++++++++++++
 sparse-index.h                           |   1 +
 t/t1011-read-tree-sparse-checkout.sh     |  23 ++++-
 t/t1092-sparse-checkout-compatibility.sh |  16 +--
 t/t3705-add-sparse-checkout.sh           |   2 +
 t/t6428-merge-conflicts-sparse.sh        |  23 +----
 t/t7012-skip-worktree-writing.sh         |  44 ++------
 t/t7817-grep-sparse-checkout.sh          |  11 +-
 unpack-trees.c                           |   4 +-
 13 files changed, 284 insertions(+), 113 deletions(-)

-- 
2.34.1.442.ge63c19bdd2.dirty