When a packed ref is deleted, the whole packed-refs file is
rewritten to omit the ref that no longer exists. However if another
gc command is running and calls `pack-refs --all` simultaneously,
there is a chance that a ref that was just updated lose the newly
created commits.
Through these steps, losing commits on newly updated refs can be
demonstrated:
# step 1: compile git without `USE_NSEC` option
Some kernel releases do enable it by default while some do
not. And if we compile git without `USE_NSEC`, it will be easier
demonstrated by the following steps.
# step 2: setup a repository and add the first commit
git init repo &&
(cd repo &&
git config core.logallrefupdates true &&
git commit --allow-empty -m foo)
# step 3: in one terminal, repack the refs repeatedly
cd repo &&
while true
do
git pack-refs --all
done
# step 4: in another terminal, simultaneously update the
# master with update-ref, and create and delete an
# unrelated ref also with update-ref
cd repo &&
while true
do
us=$(git commit-tree -m foo -p HEAD HEAD^{tree}) &&
git update-ref refs/heads/newbranch $us &&
git update-ref refs/heads/master $us &&
git update-ref -d refs/heads/newbranch &&
them=$(git rev-parse master) &&
if test "$them" != "$us"
then
echo >&2 "lost commit: $us"
exit 1
fi
# eye candy
printf .
done
Though we have the packed-refs lock file and loose refs lock
files to avoid updating conflicts, a ref will lost its newly
commits if racy stat-validity of `packed-refs` file happens
(which is quite same as the racy-git described in
`Documentation/technical/racy-git.txt`), the following
specific set of operations demonstrates the problem:
1. Call `pack-refs --all` to pack all the loose refs to
packed-refs, and let say the modify time of the
packed-refs is DATE_M.
2. Call `update-ref` to update a new commit to master while
it is already packed. the old value (let us call it
OID_A) remains in the packed-refs file and write the new
value (let us call it OID_B) to $GIT_DIR/refs/heads/master.
3. Call `update-ref -d` within the same DATE_M from the 1th
step to delete a different ref newbranch which is packed
in the packed-refs file. It check newbranch's oid from
packed-refs file without locking it.
Meanwhile it keeps a snapshot of the packed-refs file in
memory and record the file's attributes with the snapshot.
The oid of master in the packed-refs's snapshot is OID_A.
4. Call a new `pack-refs --all` to pack the loose refs, the
oid of master in packe-refs file is OID_B, and the loose
refs $GIT_DIR/refs/heads/master is removed. Let's say
the `pack-refs --all` is very quickly done and the new
packed-refs file's modify time is still DATE_M, and it
has the same file size, even the same inode.
5. 3th step now goes on after checking the newbranch, it
begin to rewrite the packed-refs file. After get the
lock file of packed-ref file, it checks it's on-disk
file attributes with the snapshot, suck as the timestamp,
the file size and the inode value. If they are both the
same values, and the snapshot is not refreshed.
Because the loose ref of master is removed by 4th step,
`update-ref -d` will updates the new packed-ref to disk
which contains master with the oid OID_A. So now the
newly commit OID_B of master is lost.
The best path forward is just always refreshing after take
the lock file of `packed-refs` file. Traditionally we avoided
that because refreshing it implied parsing the whole file.
But these days we mmap it, so it really is just an extra
open()/mmap() and a quick read of the header. That doesn't seem
like an outrageous cost to pay when we're already taking the lock.
Signed-off-by: Sun Chao <sunchao9@huawei.com>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Sun Chao <sunchao9@huawei.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
We have been trying out a few language features outside c89; the
coding guidelines document did not talk about them and instead had
a blanket ban against them.
* jc/post-c89-rules-doc:
CodingGuidelines: spell out post-C89 rules
Code restructuring during 2.20 period broke fetching tags via
"import" based transports.
* fc/fetch-with-import-fix:
fetch: fix regression with transport helpers
fetch: make the code more understandable
fetch: trivial cleanup
t5801 (remote-helpers): add test to fetch tags
t5801 (remote-helpers): cleanup refspec stuff
The commit-graph file is now part of the "files that the runtime
may keep open file descriptors on, all of which would need to be
closed when done with the object store", and the file descriptor to
an existing commit-graph file now is closed before "gc" finalizes a
new instance to replace it.
* ds/close-object-store:
packfile: rename close_all_packs to close_object_store
packfile: close commit-graph in close_all_packs
commit-graph: use raw_object_store when closing
commit-graph: extract write_commit_graph_file()
commit-graph: extract copy_oids_to_commits()
commit-graph: extract count_distinct_commits()
commit-graph: extract fill_oids_from_all_packs()
commit-graph: extract fill_oids_from_commit_hex()
commit-graph: extract fill_oids_from_packs()
commit-graph: create write_commit_graph_context
commit-graph: remove Future Work section
commit-graph: collapse parameters into flags
commit-graph: return with errors during write
commit-graph: fix the_repository reference
"git checkout -p" needs to selectively apply a patch in reverse,
which did not work well.
* pw/add-p-recount:
add -p: fix checkout -p with pathological context
Code clean-up to avoid signed integer overlaps during binary search.
* rs/avoid-overflow-in-midpoint-computation:
cleanup: fix possible overflow errors in binary search, part 2
"git interpret-trailers" always treated '#' as the comment
character, regardless of core.commentChar setting, which has been
corrected.
* jk/trailers-use-config:
interpret-trailers: load default config
"git stash show 23" used to work, but no more after getting
rewritten in C; this regression has been corrected.
* tg/stash-ref-by-index-fix:
stash: fix show referencing stash index
"git rebase --abort" used to leave refs/rewritten/ when concluding
"git rebase -r", which has been corrected.
* pw/rebase-abort-clean-rewritten:
rebase --abort/--quit: cleanup refs/rewritten
sequencer: return errors from sequencer_remove_state()
rebase: warn if state directory cannot be removed
rebase: fix a memory leak
An incorrect list of options was cached after command line
completion failed (e.g. trying to complete a command that requires
a repository outside one), which has been corrected.
* nd/completion-no-cache-failure:
completion: do not cache if --git-completion-helper fails
The code to parse scaled numbers out of configuration files has
been made more robust and also easier to follow.
* rs/config-unit-parsing:
config: simplify parsing of unit factors
config: don't multiply in parse_unit_factor()
config: use unsigned_mult_overflows to check for overflows
The codepath to compute delta islands used to spew progress output
without giving the callers any way to squelch it, which has been
fixed.
* jk/delta-islands-progress-fix:
delta-islands: respect progress flag
Use "Erase in Line" CSI sequence that is already used in the editor
support to clear cruft in the progress output.
* sg/rebase-progress:
progress: use term_clear_line()
rebase: fix garbled progress display with '-x'
pager: add a helper function to clear the last line in the terminal
t3404: make the 'rebase.missingCommitsCheck=ignore' test more focused
t3404: modernize here doc style
"git submodule foreach" did not protect command line options passed
to the command to be run in each submodule correctly, when the
"--recursive" option was in use.
* ms/submodule-foreach-fix:
submodule foreach: fix recursion of options
The configuration variable rebase.rescheduleFailedExec should be
effective only while running an interactive rebase and should not
affect anything when running an non-interactive one, which was not
the case. This has been corrected.
* js/rebase-reschedule-applies-only-to-interactive:
rebase --am: ignore rebase.rescheduleFailedExec
The "git clone" documentation refers to command line options in its
description in the short form; they have been replaced with long
forms to make them more recognisable.
* qn/clone-doc-use-long-form:
docs: git-clone: list short form of options first
docs: git-clone: refer to long form of options
"git rm" to resolve a conflicted path leaked an internal message
"needs merge" before actually removing the path, which was
confusing. This has been corrected.
* jc/denoise-rm-to-resolve:
rm: resolving by removal is not a warning-worthy event
A codepath that reads from GPG for signed object verification read
past the end of allocated buffer, which has been fixed.
* sr/gpg-interface-stop-at-the-end:
gpg-interface: do not scan past the end of buffer
"git clean" silently skipped a path when it cannot lstat() it; now
it gives a warning.
* js/clean-report-too-long-a-path:
clean: show an error message when the path is too long
"git push --atomic" that goes over the transport-helper (namely,
the smart http transport) failed to prevent refs to be pushed when
it can locally tell that one of the ref update will fail without
having to consult the other end, which has been corrected.
* es/local-atomic-push-failure-with-http:
transport-helper: avoid var decl in for () loop control
transport-helper: enforce atomic in push_refs_with_push
Dev support update.
* sg/ci-brew-gcc-workaround:
ci/lib.sh: update a comment about installed P4 and Git-LFS versions
ci: disable Homebrew's auto cleanup
ci: don't update Homebrew
Code clean-up for new compilers.
The 'kwset' one may get a wholesale replacement, either with new
version of kwset from upstream or removal of its users, but in the
meantime, it is probably OK to merge it down.
* js/gcc-8-and-9:
config: avoid calling `labs()` on too-large data type
winansi: simplify loading the GetCurrentConsoleFontEx() function
kwset: allow building with GCC 8
poll (mingw): allow compiling with GCC 8 and DEVELOPER=1
The list of for-each like macros used by clang-format has been
updated.
* mo/clang-format-for-each-update:
clang-format: use git grep to generate the ForEachMacros list
The URL decoding code has been updated to avoid going past the end
of the string while parsing %-<hex>-<hex> sequence.
* md/url-parse-harden:
url: do not allow %00 to represent NUL in URLs
url: do not read past end of buffer
The description about slashes in gitignore patterns (used to
indicate things like "anchored to this level only" and "only
matches directories") has been revamped.
* an/ignore-doc-update:
gitignore.txt: make slash-rules more readable
