[PATCH 0/3] Fix use of 'cache_bottom' in sparse index

["Victoria Dye via GitGitGadget" <gitgitgadget@gmail.com>]

An issue was found by @stolee after experimenting with the structure of the
test repo used in 't/t1092-sparse-checkout-compatibility.sh' (patch [1/3])
and finding that certain commands (namely those that internally performed a
"cache diff") failed when a sparse directory appeared lexicographically
before the in-cone directory. The root cause was traced to the
'unpack_trees_options.cache_bottom' variable, which was not being advanced
properly for sparse directories. The result was entries being
double-unpacked or improperly unpacked by 'unpack_trees', causing failures
in tests using 'git reset -- <pathspec>' and 'git diff --staged --
<pathspec>' using pathspecs.

The 'cache_bottom' was handled differently in sparse indexes in the first
place based on the reasoning laid out in 17a1bb570b (unpack-trees: preserve
cache_bottom, 2021-07-14). In short, the 'cache_bottom' advancement in
'mark_ce_used(...)' was disabled for sparse directories because
'unpack_callback(...)' would advance the 'cache_bottom' based on the number
of index entries matching or inside the tree being unpacked. If the tree was
a sparse directory, 'cache_bottom' would appropriately advance by 1, and
therefore didn't need the extra advancement in 'mark_ce_used(...)'. However,
this was insufficient to properly advance the 'cache_bottom' for two
reasons:

 1. 'unpack_callback(...)' would only advance the 'cache_bottom' for sparse
    directories if the operation in progress was a "cache diff" (true for
    'git diff --staged' and 'git reset --mixed', but not - for instance -
    'git reset --hard' or 'git read-tree -m').
 2. If sparse directories were unpacked with 'unpack_index_entry(...)'
    (e.g., in 'git reset -- <pathspec>'), the cache tree-based advancement
    of 'cache_bottom' would not happen.

Luckily, the first did not appear to have any behavioral impact. However,
the latter led to incorrect values being returned by 'next_cache_entry(...)'
depending on the structure of the index, causing the test failures observed
in 't1092'.

To fix this, the 'cache_bottom' advancement is reinstated in
'mark_ce_used(...)', and instead it is disabled in 'unpack_callback(...)' if
the tree in question is a sparse directory. This corrects both the
non-"cache diff" cases and the 'unpack_index_entry(...)' cases while
preventing the double-advancement 17a1bb570b originally intended to avoid
(patch [2/3]).

Finally, now that the cache bottom is advanced properly, we can revert the
"performance improvement" introduced in f2a454e0a5 (unpack-trees: improve
performance of next_cache_entry, 2021-11-29) that mitigated performance
issues arising in 'next_cache_entry(...)' from the non-advancing
'cache_bottom' (patch [3/3]). The performance results in
'p2000-sparse-operations.sh' showed expected variability around 0% change in
execution time (+/= 0.04s, depending on the command), with example results
for potentially-affected commands below.

'git reset'                      master            this_series                  
------------------------------------------------------------------------
full-v3                          0.51(0.21+0.27)   0.50(0.21+0.25) -2.0%
full-v4                          0.51(0.22+0.27)   0.50(0.21+0.24) -2.0%
sparse-v3                        0.30(0.04+0.55)   0.28(0.04+0.50) -6.7%
sparse-v4                        0.31(0.04+0.51)   0.29(0.04+0.51) -6.5%

'git reset -- does-not-exist'    master            this_series                  
------------------------------------------------------------------------
full-v3                          0.54(0.23+0.27)   0.55(0.22+0.28) +1.9%
full-v4                          0.56(0.25+0.26)   0.54(0.24+0.26) -3.6%
sparse-v3                        0.31(0.04+0.54)   0.31(0.04+0.50) +0.0%
sparse-v4                        0.31(0.04+0.52)   0.31(0.04+0.50) +0.0%

'git diff --cached'              master            this_series    
-------------------------------------------------------------------------
full-v3                          0.09(0.04+0.04)   0.09(0.04+0.04) +0.0%
full-v4                          0.09(0.04+0.04)   0.09(0.04+0.04) +0.0%
sparse-v3                        0.05(0.01+0.02)   0.05(0.01+0.03) +0.0%
sparse-v4                        0.04(0.01+0.02)   0.04(0.01+0.02) +0.0%


Thanks! -Victoria

Victoria Dye (3):
  t1092: add sparse directory before cone in test repo
  unpack-trees: increment cache_bottom for sparse directories
  Revert "unpack-trees: improve performance of next_cache_entry"

 t/t1092-sparse-checkout-compatibility.sh |  6 +++-
 unpack-trees.c                           | 39 +++++++++---------------
 2 files changed, 19 insertions(+), 26 deletions(-)


base-commit: 1a4874565fa3b6668042216189551b98b4dc0b1b
Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-1179%2Fvdye%2Fbugfix%2Fsparse-index-cache-bottom-v1
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-1179/vdye/bugfix/sparse-index-cache-bottom-v1
Pull-Request: https://github.com/gitgitgadget/git/pull/1179
-- 
gitgitgadget