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TEST PR: speed up index load through parallelization #20

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TEST PR: speed up index load through parallelization #20

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benpeart
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This patch helps address the CPU cost of loading the index by creating threads to divide the work of loading and converting the cache across all available CPU cores.

It accomplishes this by having the primary thread loop across the index file the offset and (for V4 indexes) expanding the name. It creates a to process each block of entries as it comes to them. Once the are complete and the cache entries are loaded, the rest of the can be loaded and processed normally on the primary thread.

Signed-off-by: Ben Peart [email protected]

gitster and others added 30 commits August 2, 2018 15:30
Test clean-up and corrections.

* es/test-fixes: (26 commits)
  t5608: fix broken &&-chain
  t9119: fix broken &&-chains
  t9000-t9999: fix broken &&-chains
  t7000-t7999: fix broken &&-chains
  t6000-t6999: fix broken &&-chains
  t5000-t5999: fix broken &&-chains
  t4000-t4999: fix broken &&-chains
  t3030: fix broken &&-chains
  t3000-t3999: fix broken &&-chains
  t2000-t2999: fix broken &&-chains
  t1000-t1999: fix broken &&-chains
  t0000-t0999: fix broken &&-chains
  t9814: simplify convoluted check that command correctly errors out
  t9001: fix broken "invoke hook" test
  t7810: use test_expect_code() instead of hand-rolled comparison
  t7400: fix broken "submodule add/reconfigure --force" test
  t7201: drop pointless "exit 0" at end of subshell
  t6036: fix broken "merge fails but has appropriate contents" tests
  t5505: modernize and simplify hard-to-digest test
  t5406: use write_script() instead of birthing shell script manually
  ...
"git diff --color-moved" feature has further been tweaked.

* sb/diff-color-move-more:
  diff.c: offer config option to control ws handling in move detection
  diff.c: add white space mode to move detection that allows indent changes
  diff.c: factor advance_or_nullify out of mark_color_as_moved
  diff.c: decouple white space treatment from move detection algorithm
  diff.c: add a blocks mode for moved code detection
  diff.c: adjust hash function signature to match hashmap expectation
  diff.c: do not pass diff options as keydata to hashmap
  t4015: avoid git as a pipe input
  xdiff/xdiffi.c: remove unneeded function declarations
  xdiff/xdiff.h: remove unused flags
"git checkout" and "git worktree add" learned to honor
checkout.defaultRemote when auto-vivifying a local branch out of a
remote tracking branch in a repository with multiple remotes that
have tracking branches that share the same names.

* ab/checkout-default-remote:
  checkout & worktree: introduce checkout.defaultRemote
  checkout: add advice for ambiguous "checkout <branch>"
  builtin/checkout.c: use "ret" variable for return
  checkout: pass the "num_matches" up to callers
  checkout.c: change "unique" member to "num_matches"
  checkout.c: introduce an *_INIT macro
  checkout.h: wrap the arguments to unique_tracking_name()
  checkout tests: index should be clean after dwim checkout
Code restructuring and a small fix to transport protocol v2 during
fetching.

* jt/fetch-pack-negotiator:
  fetch-pack: introduce negotiator API
  fetch-pack: move common check and marking together
  fetch-pack: make negotiation-related vars local
  fetch-pack: use ref adv. to prune "have" sent
  fetch-pack: directly end negotiation if ACK ready
  fetch-pack: clear marks before re-marking
  fetch-pack: split up everything_local()
Parsing of -L[<N>][,[<M>]] parameters "git blame" and "git log"
take has been tweaked.

* is/parsing-line-range:
  log: prevent error if line range ends past end of file
  blame: prevent error if range ends past end of file
lookup_commit_reference() and friends have been updated to find
in-core object for a specific in-core repository instance.

* sb/object-store-lookup: (32 commits)
  commit.c: allow lookup_commit_reference to handle arbitrary repositories
  commit.c: allow lookup_commit_reference_gently to handle arbitrary repositories
  tag.c: allow deref_tag to handle arbitrary repositories
  object.c: allow parse_object to handle arbitrary repositories
  object.c: allow parse_object_buffer to handle arbitrary repositories
  commit.c: allow get_cached_commit_buffer to handle arbitrary repositories
  commit.c: allow set_commit_buffer to handle arbitrary repositories
  commit.c: migrate the commit buffer to the parsed object store
  commit-slabs: remove realloc counter outside of slab struct
  commit.c: allow parse_commit_buffer to handle arbitrary repositories
  tag: allow parse_tag_buffer to handle arbitrary repositories
  tag: allow lookup_tag to handle arbitrary repositories
  commit: allow lookup_commit to handle arbitrary repositories
  tree: allow lookup_tree to handle arbitrary repositories
  blob: allow lookup_blob to handle arbitrary repositories
  object: allow lookup_object to handle arbitrary repositories
  object: allow object_as_type to handle arbitrary repositories
  tag: add repository argument to deref_tag
  tag: add repository argument to parse_tag_buffer
  tag: add repository argument to lookup_tag
  ...
Various glitches in the heuristics of merge-recursive strategy have
been documented in new tests.

* en/t6042-insane-merge-rename-testcases:
  t6042: add testcase covering long chains of rename conflicts
  t6042: add testcase covering rename/rename(2to1)/delete/delete conflict
  t6042: add testcase covering rename/add/delete conflict type
"git fetch" learned a new option "--negotiation-tip" to limit the
set of commits it tells the other end as "have", to reduce wasted
bandwidth and cycles, which would be helpful when the receiving
repository has a lot of refs that have little to do with the
history at the remote it is fetching from.

* jt/fetch-nego-tip:
  fetch-pack: support negotiation tip whitelist
For a large tree, the index needs to hold many cache entries
allocated on heap.  These cache entries are now allocated out of a
dedicated memory pool to amortize malloc(3) overhead.

* jm/cache-entry-from-mem-pool:
  block alloc: add validations around cache_entry lifecyle
  block alloc: allocate cache entries from mem_pool
  mem-pool: fill out functionality
  mem-pool: add life cycle management functions
  mem-pool: only search head block for available space
  block alloc: add lifecycle APIs for cache_entry structs
  read-cache: teach make_cache_entry to take object_id
  read-cache: teach refresh_cache_entry to take istate
"git gc --auto" opens file descriptors for the packfiles before
spawning "git repack/prune", which would upset Windows that does
not want a process to work on a file that is open by another
process.  The issue has been worked around.

* kg/gc-auto-windows-workaround:
  gc --auto: release pack files before auto packing
"git grep" learned the "--only-matching" option.

* tb/grep-only-matching:
  grep.c: teach 'git grep --only-matching'
  grep.c: extract show_line_header()
"git rebase --rebase-merges" mode now handles octopus merges as
well.

* js/rebase-merge-octopus:
  rebase --rebase-merges: adjust man page for octopus support
  rebase --rebase-merges: add support for octopus merges
  merge: allow reading the merge commit message from a file
The recursive merge strategy did not properly ensure there was no
change between HEAD and the index before performing its operation,
which has been corrected.

* en/dirty-merge-fixes:
  merge: fix misleading pre-merge check documentation
  merge-recursive: enforce rule that index matches head before merging
  t6044: add more testcases with staged changes before a merge is invoked
  merge-recursive: fix assumption that head tree being merged is HEAD
  merge-recursive: make sure when we say we abort that we actually abort
  t6044: add a testcase for index matching head, when head doesn't match HEAD
  t6044: verify that merges expected to abort actually abort
  index_has_changes(): avoid assuming operating on the_index
  read-cache.c: move index_has_changes() from merge.c
Tests to cover various conflicting cases have been added for
merge-recursive.

* en/t6036-merge-recursive-tests:
  t6036: add a failed conflict detection case: regular files, different modes
  t6036: add a failed conflict detection case with conflicting types
  t6036: add a failed conflict detection case with submodule add/add
  t6036: add a failed conflict detection case with submodule modify/modify
  t6036: add a failed conflict detection case with symlink add/add
  t6036: add a failed conflict detection case with symlink modify/modify
Tests to cover conflict cases that involve submodules have been
added for merge-recursive.

* en/t7405-recursive-submodule-conflicts:
  t7405: verify 'merge --abort' works after submodule/path conflicts
  t7405: add a directory/submodule conflict
  t7405: add a file/submodule conflict
"git rebase" started exporting GIT_DIR environment variable and
exposing it to hook scripts when part of it got rewritten in C.
Instead of matching the old scripted Porcelains' behaviour,
compensate by also exporting GIT_WORK_TREE environment as well to
lessen the damage.  This can harm existing hooks that want to
operate on different repository, but the current behaviour is
already broken for them anyway.

* bc/sequencer-export-work-tree-as-well:
  sequencer: pass absolute GIT_WORK_TREE to exec commands
"git send-email" when using in a batched mode that limits the
number of messages sent in a single SMTP session lost the contents
of the variable used to choose between tls/ssl, unable to send the
second and later batches, which has been fixed.

* jm/send-email-tls-auth-on-batch:
  send-email: fix tls AUTH when sending batch
Add a server-side knob to skip commits in exponential/fibbonacci
stride in an attempt to cover wider swath of history with a smaller
number of iterations, potentially accepting a larger packfile
transfer, instead of going back one commit a time during common
ancestor discovery during the "git fetch" transaction.

* jt/fetch-negotiator-skipping:
  negotiator/skipping: skip commits during fetch
The lazy clone support had a few places where missing but promised
objects were not correctly tolerated, which have been fixed.

* jt/tags-to-promised-blobs-fix:
  tag: don't warn if target is missing but promised
  revision: tolerate promised targets of tags
Look for broken "&&" chains that are hidden in subshell, many of
which have been found and corrected.

* es/chain-lint-in-subshell:
  t/chainlint.sed: drop extra spaces from regex character class
  t/chainlint: add chainlint "specialized" test cases
  t/chainlint: add chainlint "complex" test cases
  t/chainlint: add chainlint "cuddled" test cases
  t/chainlint: add chainlint "loop" and "conditional" test cases
  t/chainlint: add chainlint "nested subshell" test cases
  t/chainlint: add chainlint "one-liner" test cases
  t/chainlint: add chainlint "whitespace" test cases
  t/chainlint: add chainlint "basic" test cases
  t/Makefile: add machinery to check correctness of chainlint.sed
  t/test-lib: teach --chain-lint to detect broken &&-chains in subshells
The singleton commit-graph in-core instance is made per in-core
repository instance.

* jt/commit-graph-per-object-store:
  commit-graph: add repo arg to graph readers
  commit-graph: store graph in struct object_store
  commit-graph: add free_commit_graph
  commit-graph: add missing forward declaration
  object-store: add missing include
  commit-graph: refactor preparing commit graph
Signed-off-by: Junio C Hamano <[email protected]>
The option help text for the force-with-lease option to "git push"
reads like this:

    $ git push -h 2>&1 | grep -e force-with-lease
       --force-with-lease[=<refname>:<expect>]

which comes from having N_("refname>:<expect") as the argument help
text in the source code, with an aparent lack of "<" and ">" at both
ends.

It turns out that parse-options machinery takes the whole string and
encloses it inside a pair of "<>", to make it easier for majority
cases that uses a single token placeholder.

The help string was written in a funnily unbalanced way knowing that
the end result would balance out, by somebody who forgot the
presence of PARSE_OPT_LITERAL_ARGHELP, which is the escape hatch
mechanism designed to help such a case.  We just should use the
official escape hatch instead.

Because ":<expect>" part can be omitted to ask Git to guess, it may
be more correct to spell it as "<refname>[:<expect>]", but that is
not the focus of this topic.

Signed-off-by: Ævar Arnfjörð Bjarmason <[email protected]>
Helped-by: René Scharfe <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>
Don't translate the argument specification for --chmod; "+x" and "-x"
are the literal strings that the commands accept.

Separate alternatives using a pipe character instead of a slash, for
consistency.

Use the flag PARSE_OPT_LITERAL_ARGHELP to prevent parseopt from adding a
pair of angular brackets around the argument help string, as that would
wrongly indicate that users need to replace the literal strings with
some kind of value.

Signed-off-by: Rene Scharfe <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>
Parseopt wraps arguments in a pair of angular brackets by default,
signifying that the user needs to replace it with a value of the
documented type.  Remove the pairs from the option definitions to
duplication and confusion.

Signed-off-by: Rene Scharfe <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>
Wrap both placeholders in the argument help string in angular brackets
to signal that users needs replace them with some actual value.  Use the
flag PARSE_OPT_LITERAL_ARGHELP to prevent parseopt from adding another
pair.

Signed-off-by: Rene Scharfe <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>
Wrap each part of the argument help string in angular brackets to show
that users need to replace them with actual values.  Do that explicitly
to balance the pairs nicely in the code and avoid confusing casual
readers.  Add the flag PARSE_OPT_LITERAL_ARGHELP to keep parseopt from
adding another pair.

Signed-off-by: Rene Scharfe <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>
Wrap the placeholders in the option help string for -w in pairs of
angular brackets to document that users need to replace them with actual
numbers.  Use the flag PARSE_OPT_LITERAL_ARGHELP to prevent parseopt
from adding another pair.

Signed-off-by: Rene Scharfe <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>
Parseopt wraps argument help strings in a pair of angular brackets by
default, to tell users that they need to replace it with an actual
value.  This is useful in most cases, because most option arguments
are indeed single values of a certain type.  The option
PARSE_OPT_LITERAL_ARGHELP needs to be used in option definitions with
arguments that have multiple parts or are literal strings.

Stop adding these angular brackets if special characters are present,
as they indicate that we don't deal with a simple placeholder.  This
simplifies the code a bit and makes defining special options slightly
easier.

Remove the flag PARSE_OPT_LITERAL_ARGHELP in the cases where the new
and more cautious handling suffices.

Signed-off-by: Rene Scharfe <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>
want_color_fd() is designed to work only with standard output and
error file descriptors and stores information about each descriptor in
an array. However, it doesn't verify that the passed-in descriptor
lives within that set, which, with a buggy caller, could lead to
access or assignment outside the array bounds.

Signed-off-by: Eric Sunshine <[email protected]>
Acked-by: Johannes Schindelin <[email protected]>
Signed-off-by: Jonathan Nieder <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>
Test fix.

* sg/t5310-empty-input-fix:
  t5310-pack-bitmaps: fix bogus 'pack-objects to file can use bitmap' test
Improve built-in facility to catch broken &&-chain in the tests.

* es/chain-lint-more:
  chainlint: add test of pathological case which triggered false positive
  chainlint: recognize multi-line quoted strings more robustly
  chainlint: let here-doc and multi-line string commence on same line
  chainlint: recognize multi-line $(...) when command cuddled with "$("
  chainlint: match 'quoted' here-doc tags
  chainlint: match arbitrary here-docs tags rather than hard-coded names
The more library-ish parts of the codebase learned to work on the
in-core index-state instance that is passed in by their callers,
instead of always working on the singleton "the_index" instance.

* nd/no-the-index: (24 commits)
  blame.c: remove implicit dependency on the_index
  apply.c: remove implicit dependency on the_index
  apply.c: make init_apply_state() take a struct repository
  apply.c: pass struct apply_state to more functions
  resolve-undo.c: use the right index instead of the_index
  archive-*.c: use the right repository
  archive.c: avoid access to the_index
  grep: use the right index instead of the_index
  attr: remove index from git_attr_set_direction()
  entry.c: use the right index instead of the_index
  submodule.c: use the right index instead of the_index
  pathspec.c: use the right index instead of the_index
  unpack-trees: avoid the_index in verify_absent()
  unpack-trees: convert clear_ce_flags* to avoid the_index
  unpack-trees: don't shadow global var the_index
  unpack-trees: add a note about path invalidation
  unpack-trees: remove 'extern' on function declaration
  ls-files: correct index argument to get_convert_attr_ascii()
  preload-index.c: use the right index instead of the_index
  dir.c: remove an implicit dependency on the_index in pathspec code
  ...
"git tbdiff" that lets us compare individual patches in two
iterations of a topic has been rewritten and made into a built-in
command.

* js/range-diff: (21 commits)
  range-diff: use dim/bold cues to improve dual color mode
  range-diff: make --dual-color the default mode
  range-diff: left-pad patch numbers
  completion: support `git range-diff`
  range-diff: populate the man page
  range-diff --dual-color: skip white-space warnings
  range-diff: offer to dual-color the diffs
  diff: add an internal option to dual-color diffs of diffs
  color: add the meta color GIT_COLOR_REVERSE
  range-diff: use color for the commit pairs
  range-diff: add tests
  range-diff: do not show "function names" in hunk headers
  range-diff: adjust the output of the commit pairs
  range-diff: suppress the diff headers
  range-diff: indent the diffs just like tbdiff
  range-diff: right-trim commit messages
  range-diff: also show the diff between patches
  range-diff: improve the order of the shown commits
  range-diff: first rudimentary implementation
  Introduce `range-diff` to compare iterations of a topic branch
  ...
"git pull --rebase -v" in a repository with a submodule barfed as
an intermediate process did not understand what "-v(erbose)" flag
meant, which has been fixed.

* sb/pull-rebase-submodule:
  git-submodule.sh: accept verbose flag in cmd_update to be non-quiet
Test fix.

* js/chain-lint-attrfix:
  chainlint: fix for core.autocrlf=true
Doc updates.

* jh/partial-clone-doc:
  partial-clone: render design doc using asciidoc
A test prerequisite defined by various test scripts with slightly
different semantics has been consolidated into a single copy and
made into a lazily defined one.

* wc/make-funnynames-shared-lazy-prereq:
  t: factor out FUNNYNAMES as shared lazy prereq
After a partial clone, repeated fetches from promisor remote would
have accumulated many packfiles marked with .promisor bit without
getting them coalesced into fewer packfiles, hurting performance.
"git repack" now learned to repack them.

* jt/repack-promisor-packs:
  repack: repack promisor objects if -a or -A is set
  repack: refactor setup of pack-objects cmd
Code hygiene improvement for the header files.

* en/incl-forward-decl:
  Remove forward declaration of an enum
  compat/precompose_utf8.h: use more common include guard style
  urlmatch.h: fix include guard
  Move definition of enum branch_track from cache.h to branch.h
  alloc: make allocate_alloc_state and clear_alloc_state more consistent
  Add missing includes and forward declarations
Test updates.

* ab/submodule-relative-url-tests:
  submodule: add more exhaustive up-path testing
Recent update to "git config" broke updating variable in a
subsection, which has been corrected.

* sb/config-write-fix:
  git-config: document accidental multi-line setting in deprecated syntax
  config: fix case sensitive subsection names on writing
  t1300: document current behavior of setting options
When "git rebase -i" is told to squash two or more commits into
one, it labeled the log message for each commit with its number.
It correctly called the first one "1st commit", but the next one
was "commit #1", which was off-by-one.  This has been corrected.

* pw/rebase-i-squash-number-fix:
  rebase -i: fix numbering in squash message
"git rebase -i", when a 'merge <branch>' insn in its todo list
fails, segfaulted, which has been (minimally) corrected.

* pw/rebase-i-merge-segv-fix:
  rebase -i: fix SIGSEGV when 'merge <branch>' fails
  t3430: add conflicting commit
A few preliminary minor clean-ups in the area around submodules.

* sb/submodule-cleanup:
  builtin/submodule--helper: remove stray new line
  t7410: update to new style
"git cherry-pick --quit" failed to remove CHERRY_PICK_HEAD even
though we won't be in a cherry-pick session after it returns, which
has been corrected.

* nd/cherry-pick-quit-fix:
  cherry-pick: fix --quit not deleting CHERRY_PICK_HEAD
The sideband code learned to optionally paint selected keywords at
the beginning of incoming lines on the receiving end.

* hn/highlight-sideband-keywords:
  sideband: do not read beyond the end of input
  sideband: highlight keywords in remote sideband output
* ab/checkout-default-remote:
  t2024: mark test using "checkout -p" with PERL prerequisite
Signed-off-by: Junio C Hamano <[email protected]>
read-cache.c Outdated

if (istate->version == 4) {
previous_name = &previous_name_buf;
mem_pool_init(&istate->ce_mem_pool, 0, 0);
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This mem_pool_init code is common to both the if and the else branch.

gitster and others added 4 commits August 22, 2018 11:17
In a recent update in 2.18 era, "git pack-objects" started
producing a larger than necessary packfiles by missing
opportunities to use large deltas.

* nd/pack-deltify-regression-fix:
  pack-objects: fix performance issues on packing large deltas
This patch helps address the CPU cost of loading the index by creating
multiple threads to divide the work of loading and converting the cache
entries across all available CPU cores.

It accomplishes this by having the primary thread loop across the index file
tracking the offset and (for V4 indexes) expanding the name. It creates a
thread to process each block of entries as it comes to them. Once the
threads are complete and the cache entries are loaded, the rest of the
extensions can be loaded and processed normally on the primary thread.

I used p0002-read-cache.sh to generate some performance data:

100,000 entries

Test                                HEAD~3           HEAD~2
---------------------------------------------------------------------------
read_cache/discard_cache 1000 times 14.02(0.01+0.12) 9.81(0.01+0.07) -30.0%

1,000,000 entries

Test                                HEAD~3            HEAD~2
------------------------------------------------------------------------------
read_cache/discard_cache 1000 times 202.06(0.06+0.09) 155.72(0.03+0.06) -22.9%

Signed-off-by: Ben Peart <[email protected]>
This patch helps address the CPU cost of loading the index by loading
the cache extensions on a worker thread in parallel with loading the cache
entries.

This is possible because the current extensions don't access the cache
entries in the index_state structure so are OK that they don't all exist
yet.

The CACHE_EXT_TREE, CACHE_EXT_RESOLVE_UNDO, and CACHE_EXT_UNTRACKED
extensions don't even get a pointer to the index so don't have access to the
cache entries.

CACHE_EXT_LINK only uses the index_state to initialize the split index.
CACHE_EXT_FSMONITOR only uses the index_state to save the fsmonitor last
update and dirty flags.

I used p0002-read-cache.sh to generate some performance data on the
cumulative impact:

100,000 entries

Test                                HEAD~3           HEAD~2
---------------------------------------------------------------------------
read_cache/discard_cache 1000 times 14.08(0.01+0.10) 9.72(0.03+0.06) -31.0%

1,000,000 entries

Test                                HEAD~3            HEAD~2
------------------------------------------------------------------------------
read_cache/discard_cache 1000 times 202.95(0.01+0.07) 154.14(0.03+0.06) -24.1%

Signed-off-by: Ben Peart <[email protected]>
…arsing.

 - strbuf_remove() in expand_name_field() is not exactly a good fit
   for stripping a part at the end, _setlen() would do the same job
   and is much cheaper.

 - the open-coded loop to find the end of the string in
   expand_name_field() can't beat an optimized strlen()

I used p0002-read-cache.sh to generate some performance data on the
cumulative impact:

100,000 files

Test                                HEAD~3           HEAD
---------------------------------------------------------------------------
read_cache/discard_cache 1000 times 14.08(0.03+0.09) 8.71(0.01+0.09) -38.1%

1,000,000 files

Test                                HEAD~3            HEAD
------------------------------------------------------------------------------
read_cache/discard_cache 1000 times 201.77(0.03+0.07) 149.68(0.04+0.07) -25.8%

Suggested by: Nguyễn Thái Ngọc Duy <[email protected]>
Signed-off-by: Ben Peart <[email protected]>
@benpeart benpeart closed this Aug 30, 2018
derrickstolee pushed a commit that referenced this pull request Oct 7, 2021
In a sparse index it is possible for the tree that is being verified
to be freed while it is being verified. This happens when the index is
sparse but the cache tree is not and index_name_pos() looks up a path
from the cache tree that is a descendant of a sparse index entry. That
triggers a call to ensure_full_index() which frees the cache tree that
is being verified.  Carrying on trying to verify the tree after this
results in a use-after-free bug. Instead restart the verification if a
sparse index is converted to a full index. This bug is triggered by a
call to reset_head() in "git rebase --apply". Thanks to René Scharfe
and Derrick Stolee for their help analyzing the problem.

==74345==ERROR: AddressSanitizer: heap-use-after-free on address 0x606000001b20 at pc 0x557cbe82d3a2 bp 0x7ffdfee08090 sp 0x7ffdfee08080
READ of size 4 at 0x606000001b20 thread T0
    #0 0x557cbe82d3a1 in verify_one /home/phil/src/git/cache-tree.c:863
    #1 0x557cbe82ca9d in verify_one /home/phil/src/git/cache-tree.c:840
    #2 0x557cbe82ca9d in verify_one /home/phil/src/git/cache-tree.c:840
    #3 0x557cbe82ca9d in verify_one /home/phil/src/git/cache-tree.c:840
    #4 0x557cbe830a2b in cache_tree_verify /home/phil/src/git/cache-tree.c:910
    #5 0x557cbea53741 in write_locked_index /home/phil/src/git/read-cache.c:3250
    #6 0x557cbeab7fdd in reset_head /home/phil/src/git/reset.c:87
    #7 0x557cbe72147f in cmd_rebase builtin/rebase.c:2074
    #8 0x557cbe5bd151 in run_builtin /home/phil/src/git/git.c:461
    #9 0x557cbe5bd151 in handle_builtin /home/phil/src/git/git.c:714
    #10 0x557cbe5c0503 in run_argv /home/phil/src/git/git.c:781
    #11 0x557cbe5c0503 in cmd_main /home/phil/src/git/git.c:912
    #12 0x557cbe5bad28 in main /home/phil/src/git/common-main.c:52
    #13 0x7fdd4b82eb24 in __libc_start_main (/usr/lib/libc.so.6+0x27b24)
    #14 0x557cbe5bcb8d in _start (/home/phil/src/git/git+0x1b9b8d)

0x606000001b20 is located 0 bytes inside of 56-byte region [0x606000001b20,0x606000001b58)
freed by thread T0 here:
    #0 0x7fdd4bacff19 in __interceptor_free /build/gcc/src/gcc/libsanitizer/asan/asan_malloc_linux.cpp:127
    #1 0x557cbe82af60 in cache_tree_free /home/phil/src/git/cache-tree.c:35
    #2 0x557cbe82aee5 in cache_tree_free /home/phil/src/git/cache-tree.c:31
    #3 0x557cbe82aee5 in cache_tree_free /home/phil/src/git/cache-tree.c:31
    #4 0x557cbe82aee5 in cache_tree_free /home/phil/src/git/cache-tree.c:31
    #5 0x557cbeb2557a in ensure_full_index /home/phil/src/git/sparse-index.c:310
    #6 0x557cbea45c4a in index_name_stage_pos /home/phil/src/git/read-cache.c:588
    #7 0x557cbe82ce37 in verify_one /home/phil/src/git/cache-tree.c:850
    #8 0x557cbe82ca9d in verify_one /home/phil/src/git/cache-tree.c:840
    #9 0x557cbe82ca9d in verify_one /home/phil/src/git/cache-tree.c:840
    #10 0x557cbe82ca9d in verify_one /home/phil/src/git/cache-tree.c:840
    #11 0x557cbe830a2b in cache_tree_verify /home/phil/src/git/cache-tree.c:910
    #12 0x557cbea53741 in write_locked_index /home/phil/src/git/read-cache.c:3250
    #13 0x557cbeab7fdd in reset_head /home/phil/src/git/reset.c:87
    #14 0x557cbe72147f in cmd_rebase builtin/rebase.c:2074
    #15 0x557cbe5bd151 in run_builtin /home/phil/src/git/git.c:461
    #16 0x557cbe5bd151 in handle_builtin /home/phil/src/git/git.c:714
    #17 0x557cbe5c0503 in run_argv /home/phil/src/git/git.c:781
    #18 0x557cbe5c0503 in cmd_main /home/phil/src/git/git.c:912
    #19 0x557cbe5bad28 in main /home/phil/src/git/common-main.c:52
    #20 0x7fdd4b82eb24 in __libc_start_main (/usr/lib/libc.so.6+0x27b24)

previously allocated by thread T0 here:
    #0 0x7fdd4bad0459 in __interceptor_calloc /build/gcc/src/gcc/libsanitizer/asan/asan_malloc_linux.cpp:154
    #1 0x557cbebc1807 in xcalloc /home/phil/src/git/wrapper.c:140
    #2 0x557cbe82b7d8 in cache_tree /home/phil/src/git/cache-tree.c:17
    #3 0x557cbe82b7d8 in prime_cache_tree_rec /home/phil/src/git/cache-tree.c:763
    #4 0x557cbe82b837 in prime_cache_tree_rec /home/phil/src/git/cache-tree.c:764
    #5 0x557cbe82b837 in prime_cache_tree_rec /home/phil/src/git/cache-tree.c:764
    #6 0x557cbe8304e1 in prime_cache_tree /home/phil/src/git/cache-tree.c:779
    #7 0x557cbeab7fa7 in reset_head /home/phil/src/git/reset.c:85
    #8 0x557cbe72147f in cmd_rebase builtin/rebase.c:2074
    #9 0x557cbe5bd151 in run_builtin /home/phil/src/git/git.c:461
    #10 0x557cbe5bd151 in handle_builtin /home/phil/src/git/git.c:714
    #11 0x557cbe5c0503 in run_argv /home/phil/src/git/git.c:781
    #12 0x557cbe5c0503 in cmd_main /home/phil/src/git/git.c:912
    #13 0x557cbe5bad28 in main /home/phil/src/git/common-main.c:52
    #14 0x7fdd4b82eb24 in __libc_start_main (/usr/lib/libc.so.6+0x27b24)

Signed-off-by: Phillip Wood <[email protected]>
Signed-off-by: Derrick Stolee <[email protected]>
neerajsi-msft pushed a commit to neerajsi-msft/git that referenced this pull request Oct 26, 2021
In a sparse index it is possible for the tree that is being verified
to be freed while it is being verified. This happens when the index is
sparse but the cache tree is not and index_name_pos() looks up a path
from the cache tree that is a descendant of a sparse index entry. That
triggers a call to ensure_full_index() which frees the cache tree that
is being verified.  Carrying on trying to verify the tree after this
results in a use-after-free bug. Instead restart the verification if a
sparse index is converted to a full index. This bug is triggered by a
call to reset_head() in "git rebase --apply". Thanks to René Scharfe
and Derrick Stolee for their help analyzing the problem.

==74345==ERROR: AddressSanitizer: heap-use-after-free on address 0x606000001b20 at pc 0x557cbe82d3a2 bp 0x7ffdfee08090 sp 0x7ffdfee08080
READ of size 4 at 0x606000001b20 thread T0
    #0 0x557cbe82d3a1 in verify_one /home/phil/src/git/cache-tree.c:863
    #1 0x557cbe82ca9d in verify_one /home/phil/src/git/cache-tree.c:840
    microsoft#2 0x557cbe82ca9d in verify_one /home/phil/src/git/cache-tree.c:840
    microsoft#3 0x557cbe82ca9d in verify_one /home/phil/src/git/cache-tree.c:840
    microsoft#4 0x557cbe830a2b in cache_tree_verify /home/phil/src/git/cache-tree.c:910
    microsoft#5 0x557cbea53741 in write_locked_index /home/phil/src/git/read-cache.c:3250
    microsoft#6 0x557cbeab7fdd in reset_head /home/phil/src/git/reset.c:87
    microsoft#7 0x557cbe72147f in cmd_rebase builtin/rebase.c:2074
    microsoft#8 0x557cbe5bd151 in run_builtin /home/phil/src/git/git.c:461
    microsoft#9 0x557cbe5bd151 in handle_builtin /home/phil/src/git/git.c:714
    microsoft#10 0x557cbe5c0503 in run_argv /home/phil/src/git/git.c:781
    microsoft#11 0x557cbe5c0503 in cmd_main /home/phil/src/git/git.c:912
    microsoft#12 0x557cbe5bad28 in main /home/phil/src/git/common-main.c:52
    microsoft#13 0x7fdd4b82eb24 in __libc_start_main (/usr/lib/libc.so.6+0x27b24)
    microsoft#14 0x557cbe5bcb8d in _start (/home/phil/src/git/git+0x1b9b8d)

0x606000001b20 is located 0 bytes inside of 56-byte region [0x606000001b20,0x606000001b58)
freed by thread T0 here:
    #0 0x7fdd4bacff19 in __interceptor_free /build/gcc/src/gcc/libsanitizer/asan/asan_malloc_linux.cpp:127
    #1 0x557cbe82af60 in cache_tree_free /home/phil/src/git/cache-tree.c:35
    microsoft#2 0x557cbe82aee5 in cache_tree_free /home/phil/src/git/cache-tree.c:31
    microsoft#3 0x557cbe82aee5 in cache_tree_free /home/phil/src/git/cache-tree.c:31
    microsoft#4 0x557cbe82aee5 in cache_tree_free /home/phil/src/git/cache-tree.c:31
    microsoft#5 0x557cbeb2557a in ensure_full_index /home/phil/src/git/sparse-index.c:310
    microsoft#6 0x557cbea45c4a in index_name_stage_pos /home/phil/src/git/read-cache.c:588
    microsoft#7 0x557cbe82ce37 in verify_one /home/phil/src/git/cache-tree.c:850
    microsoft#8 0x557cbe82ca9d in verify_one /home/phil/src/git/cache-tree.c:840
    microsoft#9 0x557cbe82ca9d in verify_one /home/phil/src/git/cache-tree.c:840
    microsoft#10 0x557cbe82ca9d in verify_one /home/phil/src/git/cache-tree.c:840
    microsoft#11 0x557cbe830a2b in cache_tree_verify /home/phil/src/git/cache-tree.c:910
    microsoft#12 0x557cbea53741 in write_locked_index /home/phil/src/git/read-cache.c:3250
    microsoft#13 0x557cbeab7fdd in reset_head /home/phil/src/git/reset.c:87
    microsoft#14 0x557cbe72147f in cmd_rebase builtin/rebase.c:2074
    microsoft#15 0x557cbe5bd151 in run_builtin /home/phil/src/git/git.c:461
    microsoft#16 0x557cbe5bd151 in handle_builtin /home/phil/src/git/git.c:714
    microsoft#17 0x557cbe5c0503 in run_argv /home/phil/src/git/git.c:781
    microsoft#18 0x557cbe5c0503 in cmd_main /home/phil/src/git/git.c:912
    microsoft#19 0x557cbe5bad28 in main /home/phil/src/git/common-main.c:52
    microsoft#20 0x7fdd4b82eb24 in __libc_start_main (/usr/lib/libc.so.6+0x27b24)

previously allocated by thread T0 here:
    #0 0x7fdd4bad0459 in __interceptor_calloc /build/gcc/src/gcc/libsanitizer/asan/asan_malloc_linux.cpp:154
    #1 0x557cbebc1807 in xcalloc /home/phil/src/git/wrapper.c:140
    microsoft#2 0x557cbe82b7d8 in cache_tree /home/phil/src/git/cache-tree.c:17
    microsoft#3 0x557cbe82b7d8 in prime_cache_tree_rec /home/phil/src/git/cache-tree.c:763
    microsoft#4 0x557cbe82b837 in prime_cache_tree_rec /home/phil/src/git/cache-tree.c:764
    microsoft#5 0x557cbe82b837 in prime_cache_tree_rec /home/phil/src/git/cache-tree.c:764
    microsoft#6 0x557cbe8304e1 in prime_cache_tree /home/phil/src/git/cache-tree.c:779
    microsoft#7 0x557cbeab7fa7 in reset_head /home/phil/src/git/reset.c:85
    microsoft#8 0x557cbe72147f in cmd_rebase builtin/rebase.c:2074
    microsoft#9 0x557cbe5bd151 in run_builtin /home/phil/src/git/git.c:461
    microsoft#10 0x557cbe5bd151 in handle_builtin /home/phil/src/git/git.c:714
    microsoft#11 0x557cbe5c0503 in run_argv /home/phil/src/git/git.c:781
    microsoft#12 0x557cbe5c0503 in cmd_main /home/phil/src/git/git.c:912
    microsoft#13 0x557cbe5bad28 in main /home/phil/src/git/common-main.c:52
    microsoft#14 0x7fdd4b82eb24 in __libc_start_main (/usr/lib/libc.so.6+0x27b24)

Signed-off-by: Phillip Wood <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>
dscho pushed a commit that referenced this pull request Oct 30, 2021
In a sparse index it is possible for the tree that is being verified
to be freed while it is being verified. This happens when the index is
sparse but the cache tree is not and index_name_pos() looks up a path
from the cache tree that is a descendant of a sparse index entry. That
triggers a call to ensure_full_index() which frees the cache tree that
is being verified.  Carrying on trying to verify the tree after this
results in a use-after-free bug. Instead restart the verification if a
sparse index is converted to a full index. This bug is triggered by a
call to reset_head() in "git rebase --apply". Thanks to René Scharfe
and Derrick Stolee for their help analyzing the problem.

==74345==ERROR: AddressSanitizer: heap-use-after-free on address 0x606000001b20 at pc 0x557cbe82d3a2 bp 0x7ffdfee08090 sp 0x7ffdfee08080
READ of size 4 at 0x606000001b20 thread T0
    #0 0x557cbe82d3a1 in verify_one /home/phil/src/git/cache-tree.c:863
    #1 0x557cbe82ca9d in verify_one /home/phil/src/git/cache-tree.c:840
    #2 0x557cbe82ca9d in verify_one /home/phil/src/git/cache-tree.c:840
    #3 0x557cbe82ca9d in verify_one /home/phil/src/git/cache-tree.c:840
    #4 0x557cbe830a2b in cache_tree_verify /home/phil/src/git/cache-tree.c:910
    #5 0x557cbea53741 in write_locked_index /home/phil/src/git/read-cache.c:3250
    #6 0x557cbeab7fdd in reset_head /home/phil/src/git/reset.c:87
    #7 0x557cbe72147f in cmd_rebase builtin/rebase.c:2074
    #8 0x557cbe5bd151 in run_builtin /home/phil/src/git/git.c:461
    #9 0x557cbe5bd151 in handle_builtin /home/phil/src/git/git.c:714
    #10 0x557cbe5c0503 in run_argv /home/phil/src/git/git.c:781
    #11 0x557cbe5c0503 in cmd_main /home/phil/src/git/git.c:912
    #12 0x557cbe5bad28 in main /home/phil/src/git/common-main.c:52
    #13 0x7fdd4b82eb24 in __libc_start_main (/usr/lib/libc.so.6+0x27b24)
    #14 0x557cbe5bcb8d in _start (/home/phil/src/git/git+0x1b9b8d)

0x606000001b20 is located 0 bytes inside of 56-byte region [0x606000001b20,0x606000001b58)
freed by thread T0 here:
    #0 0x7fdd4bacff19 in __interceptor_free /build/gcc/src/gcc/libsanitizer/asan/asan_malloc_linux.cpp:127
    #1 0x557cbe82af60 in cache_tree_free /home/phil/src/git/cache-tree.c:35
    #2 0x557cbe82aee5 in cache_tree_free /home/phil/src/git/cache-tree.c:31
    #3 0x557cbe82aee5 in cache_tree_free /home/phil/src/git/cache-tree.c:31
    #4 0x557cbe82aee5 in cache_tree_free /home/phil/src/git/cache-tree.c:31
    #5 0x557cbeb2557a in ensure_full_index /home/phil/src/git/sparse-index.c:310
    #6 0x557cbea45c4a in index_name_stage_pos /home/phil/src/git/read-cache.c:588
    #7 0x557cbe82ce37 in verify_one /home/phil/src/git/cache-tree.c:850
    #8 0x557cbe82ca9d in verify_one /home/phil/src/git/cache-tree.c:840
    #9 0x557cbe82ca9d in verify_one /home/phil/src/git/cache-tree.c:840
    #10 0x557cbe82ca9d in verify_one /home/phil/src/git/cache-tree.c:840
    #11 0x557cbe830a2b in cache_tree_verify /home/phil/src/git/cache-tree.c:910
    #12 0x557cbea53741 in write_locked_index /home/phil/src/git/read-cache.c:3250
    #13 0x557cbeab7fdd in reset_head /home/phil/src/git/reset.c:87
    #14 0x557cbe72147f in cmd_rebase builtin/rebase.c:2074
    #15 0x557cbe5bd151 in run_builtin /home/phil/src/git/git.c:461
    #16 0x557cbe5bd151 in handle_builtin /home/phil/src/git/git.c:714
    #17 0x557cbe5c0503 in run_argv /home/phil/src/git/git.c:781
    #18 0x557cbe5c0503 in cmd_main /home/phil/src/git/git.c:912
    #19 0x557cbe5bad28 in main /home/phil/src/git/common-main.c:52
    #20 0x7fdd4b82eb24 in __libc_start_main (/usr/lib/libc.so.6+0x27b24)

previously allocated by thread T0 here:
    #0 0x7fdd4bad0459 in __interceptor_calloc /build/gcc/src/gcc/libsanitizer/asan/asan_malloc_linux.cpp:154
    #1 0x557cbebc1807 in xcalloc /home/phil/src/git/wrapper.c:140
    #2 0x557cbe82b7d8 in cache_tree /home/phil/src/git/cache-tree.c:17
    #3 0x557cbe82b7d8 in prime_cache_tree_rec /home/phil/src/git/cache-tree.c:763
    #4 0x557cbe82b837 in prime_cache_tree_rec /home/phil/src/git/cache-tree.c:764
    #5 0x557cbe82b837 in prime_cache_tree_rec /home/phil/src/git/cache-tree.c:764
    #6 0x557cbe8304e1 in prime_cache_tree /home/phil/src/git/cache-tree.c:779
    #7 0x557cbeab7fa7 in reset_head /home/phil/src/git/reset.c:85
    #8 0x557cbe72147f in cmd_rebase builtin/rebase.c:2074
    #9 0x557cbe5bd151 in run_builtin /home/phil/src/git/git.c:461
    #10 0x557cbe5bd151 in handle_builtin /home/phil/src/git/git.c:714
    #11 0x557cbe5c0503 in run_argv /home/phil/src/git/git.c:781
    #12 0x557cbe5c0503 in cmd_main /home/phil/src/git/git.c:912
    #13 0x557cbe5bad28 in main /home/phil/src/git/common-main.c:52
    #14 0x7fdd4b82eb24 in __libc_start_main (/usr/lib/libc.so.6+0x27b24)

Signed-off-by: Phillip Wood <[email protected]>
Signed-off-by: Derrick Stolee <[email protected]>
dscho added a commit that referenced this pull request Oct 9, 2024
An internal customer reported a segfault when running `git
sparse-checkout set` with the `index.sparse` config enabled. I was
unable to reproduce it locally, but with their help we debugged into the
failing process and discovered the following stacktrace:

```
#0  0x00007ff6318fb7b0 in rehash (map=0x3dfb00d0440, newsize=1048576) at hashmap.c:125
#1  0x00007ff6318fbc66 in hashmap_add (map=0x3dfb00d0440, entry=0x3dfb5c58bc8) at hashmap.c:247
#2  0x00007ff631937a70 in hash_index_entry (istate=0x3dfb00d0400, ce=0x3dfb5c58bc8) at name-hash.c:122
#3  0x00007ff631938a2f in add_name_hash (istate=0x3dfb00d0400, ce=0x3dfb5c58bc8) at name-hash.c:638
#4  0x00007ff631a064de in set_index_entry (istate=0x3dfb00d0400, nr=8291, ce=0x3dfb5c58bc8) at sparse-index.c:255
#5  0x00007ff631a06692 in add_path_to_index (oid=0x5ff130, base=0x5ff580, path=0x3dfb4b725da "<redacted>", mode=33188, context=0x5ff570)    at sparse-index.c:307
#6  0x00007ff631a3b48c in read_tree_at (r=0x7ff631c026a0 <the_repo>, tree=0x3dfb5b41f60, base=0x5ff580, depth=2, pathspec=0x5ff5a0,    fn=0x7ff631a064e5 <add_path_to_index>, context=0x5ff570) at tree.c:46
#7  0x00007ff631a3b60b in read_tree_at (r=0x7ff631c026a0 <the_repo>, tree=0x3dfb5b41e80, base=0x5ff580, depth=1, pathspec=0x5ff5a0,    fn=0x7ff631a064e5 <add_path_to_index>, context=0x5ff570) at tree.c:80
#8  0x00007ff631a3b60b in read_tree_at (r=0x7ff631c026a0 <the_repo>, tree=0x3dfb5b41ac8, base=0x5ff580, depth=0, pathspec=0x5ff5a0,    fn=0x7ff631a064e5 <add_path_to_index>, context=0x5ff570) at tree.c:80
#9  0x00007ff631a06a95 in expand_index (istate=0x3dfb00d0100, pl=0x0) at sparse-index.c:422
#10 0x00007ff631a06cbd in ensure_full_index (istate=0x3dfb00d0100) at sparse-index.c:456
#11 0x00007ff631990d08 in index_name_stage_pos (istate=0x3dfb00d0100, name=0x3dfb0020080 "algorithm/levenshtein", namelen=21, stage=0,    search_mode=EXPAND_SPARSE) at read-cache.c:556
#12 0x00007ff631990d6c in index_name_pos (istate=0x3dfb00d0100, name=0x3dfb0020080 "algorithm/levenshtein", namelen=21) at read-cache.c:566
#13 0x00007ff63180dbb5 in sanitize_paths (argc=185, argv=0x3dfb0030018, prefix=0x0, skip_checks=0) at builtin/sparse-checkout.c:756
#14 0x00007ff63180de50 in sparse_checkout_set (argc=185, argv=0x3dfb0030018, prefix=0x0) at builtin/sparse-checkout.c:860
#15 0x00007ff63180e6c5 in cmd_sparse_checkout (argc=186, argv=0x3dfb0030018, prefix=0x0) at builtin/sparse-checkout.c:1063
#16 0x00007ff6317234cb in run_builtin (p=0x7ff631ad9b38 <commands+2808>, argc=187, argv=0x3dfb0030018) at git.c:548
#17 0x00007ff6317239c0 in handle_builtin (argc=187, argv=0x3dfb0030018) at git.c:808
#18 0x00007ff631723c7d in run_argv (argcp=0x5ffdd0, argv=0x5ffd78) at git.c:877
#19 0x00007ff6317241d1 in cmd_main (argc=187, argv=0x3dfb0030018) at git.c:1017
#20 0x00007ff631838b60 in main (argc=190, argv=0x3dfb0030000) at common-main.c:64 
```

The very bottom of the stack being the `rehash()` method from
`hashmap.c` as called within the `name-hash` API made me look at where
these hashmaps were being used in the sparse index logic. These were
being copied across indexes, which seems dangerous. Indeed, clearing
these hashmaps and setting them as not initialized fixes the segfault.

The second commit is a response to a test failure that happens in
`t1092-sparse-checkout-compatibility.sh` where `git stash pop` starts to
fail because the underlying `git checkout-index` process fails due to
colliding files. Passing the `-f` flag appears to work, but it's unclear
why this name-hash change causes that change in behavior.
dscho added a commit that referenced this pull request Oct 9, 2024
An internal customer reported a segfault when running `git
sparse-checkout set` with the `index.sparse` config enabled. I was
unable to reproduce it locally, but with their help we debugged into the
failing process and discovered the following stacktrace:

```
#0  0x00007ff6318fb7b0 in rehash (map=0x3dfb00d0440, newsize=1048576) at hashmap.c:125
#1  0x00007ff6318fbc66 in hashmap_add (map=0x3dfb00d0440, entry=0x3dfb5c58bc8) at hashmap.c:247
#2  0x00007ff631937a70 in hash_index_entry (istate=0x3dfb00d0400, ce=0x3dfb5c58bc8) at name-hash.c:122
#3  0x00007ff631938a2f in add_name_hash (istate=0x3dfb00d0400, ce=0x3dfb5c58bc8) at name-hash.c:638
#4  0x00007ff631a064de in set_index_entry (istate=0x3dfb00d0400, nr=8291, ce=0x3dfb5c58bc8) at sparse-index.c:255
#5  0x00007ff631a06692 in add_path_to_index (oid=0x5ff130, base=0x5ff580, path=0x3dfb4b725da "<redacted>", mode=33188, context=0x5ff570)    at sparse-index.c:307
#6  0x00007ff631a3b48c in read_tree_at (r=0x7ff631c026a0 <the_repo>, tree=0x3dfb5b41f60, base=0x5ff580, depth=2, pathspec=0x5ff5a0,    fn=0x7ff631a064e5 <add_path_to_index>, context=0x5ff570) at tree.c:46
#7  0x00007ff631a3b60b in read_tree_at (r=0x7ff631c026a0 <the_repo>, tree=0x3dfb5b41e80, base=0x5ff580, depth=1, pathspec=0x5ff5a0,    fn=0x7ff631a064e5 <add_path_to_index>, context=0x5ff570) at tree.c:80
#8  0x00007ff631a3b60b in read_tree_at (r=0x7ff631c026a0 <the_repo>, tree=0x3dfb5b41ac8, base=0x5ff580, depth=0, pathspec=0x5ff5a0,    fn=0x7ff631a064e5 <add_path_to_index>, context=0x5ff570) at tree.c:80
#9  0x00007ff631a06a95 in expand_index (istate=0x3dfb00d0100, pl=0x0) at sparse-index.c:422
#10 0x00007ff631a06cbd in ensure_full_index (istate=0x3dfb00d0100) at sparse-index.c:456
#11 0x00007ff631990d08 in index_name_stage_pos (istate=0x3dfb00d0100, name=0x3dfb0020080 "algorithm/levenshtein", namelen=21, stage=0,    search_mode=EXPAND_SPARSE) at read-cache.c:556
#12 0x00007ff631990d6c in index_name_pos (istate=0x3dfb00d0100, name=0x3dfb0020080 "algorithm/levenshtein", namelen=21) at read-cache.c:566
#13 0x00007ff63180dbb5 in sanitize_paths (argc=185, argv=0x3dfb0030018, prefix=0x0, skip_checks=0) at builtin/sparse-checkout.c:756
#14 0x00007ff63180de50 in sparse_checkout_set (argc=185, argv=0x3dfb0030018, prefix=0x0) at builtin/sparse-checkout.c:860
#15 0x00007ff63180e6c5 in cmd_sparse_checkout (argc=186, argv=0x3dfb0030018, prefix=0x0) at builtin/sparse-checkout.c:1063
#16 0x00007ff6317234cb in run_builtin (p=0x7ff631ad9b38 <commands+2808>, argc=187, argv=0x3dfb0030018) at git.c:548
#17 0x00007ff6317239c0 in handle_builtin (argc=187, argv=0x3dfb0030018) at git.c:808
#18 0x00007ff631723c7d in run_argv (argcp=0x5ffdd0, argv=0x5ffd78) at git.c:877
#19 0x00007ff6317241d1 in cmd_main (argc=187, argv=0x3dfb0030018) at git.c:1017
#20 0x00007ff631838b60 in main (argc=190, argv=0x3dfb0030000) at common-main.c:64 
```

The very bottom of the stack being the `rehash()` method from
`hashmap.c` as called within the `name-hash` API made me look at where
these hashmaps were being used in the sparse index logic. These were
being copied across indexes, which seems dangerous. Indeed, clearing
these hashmaps and setting them as not initialized fixes the segfault.

The second commit is a response to a test failure that happens in
`t1092-sparse-checkout-compatibility.sh` where `git stash pop` starts to
fail because the underlying `git checkout-index` process fails due to
colliding files. Passing the `-f` flag appears to work, but it's unclear
why this name-hash change causes that change in behavior.
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