[PATCH 0/5] Remove the present-despite-SKIP_WORKTREE class of bugs (for sparse-checkouts)

["Elijah Newren via GitGitGadget" <gitgitgadget@gmail.com>]

(Maintainer note: This series builds on (v2 of) vd/sparse-clean-etc, because
it tweaks one of the testcases added there.)

(Note 2: There was a previous RFC round of this series at
https://lore.kernel.org/git/20220109045732.2497526-1-newren@gmail.com/.)

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. And I
figured out how to get the perf cost down to essentially negligible.

Changes since RFC version:

 * updated the commit messages as per suggestions from Victoria, including
   adding performance measurements
 * renamed the new function to use a clearer name
 * replaced the final patch with a different optimization, which is both
   simpler and performs quite a bit better (the cost for my previous patch 5
   was already decent in many cases, but had a few cases where the cost was
   significant).

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 an optimization 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 66b209b ("merge-ort: implement CE_SKIP_WORKTREE handling with
conflicted entries", 2021-03-20) [5] commit ba359fd ("stash: fix stash
application in sparse-checkouts", 2020-12-01) [6]
https://lore.kernel.org/git/CABPp-BH9tju7WVm=QZDOvaMDdZbpNXrVWQdN-jmfN8wC6YVhmw@mail.gmail.com/

Elijah Newren (5):
  t1011: add testcase demonstrating accidental loss of user
    modifications
  unpack-trees: fix accidental loss of user changes
  repo_read_index: clear SKIP_WORKTREE bit from files present in
    worktree
  Update documentation related to sparsity and the skip-worktree bit
  Accelerate clear_skip_worktree_from_present_files() by caching

 Documentation/git-read-tree.txt          | 12 +++-
 Documentation/git-sparse-checkout.txt    | 76 ++++++++++++++----------
 Documentation/git-update-index.txt       | 57 +++++++++++++-----
 repository.c                             |  7 +++
 sparse-index.c                           | 73 +++++++++++++++++++++++
 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, 235 insertions(+), 114 deletions(-)


base-commit: 80697e9259e4a2c12fd0546f5f1895fb3068a66c
Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-1114%2Fnewren%2Ffix-present-despite-skip-worktree-v1
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-1114/newren/fix-present-despite-skip-worktree-v1
Pull-Request: https://github.com/gitgitgadget/git/pull/1114
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