Our basic strategy for taking a ref lock is:
1. Create $ref.lock to take the lock
2. Read the ref again while holding the lock (during which
time we know that nobody else can be updating it).
3. Compare the value we read to the expected "old_sha1"
The value we read in step (2) is returned to the caller via
the lock->old_oid field, who may use it for other purposes
(such as writing a reflog).
If we have no "old_sha1" (i.e., we are unconditionally
taking the lock), then we obviously must omit step 3. But we
_also_ omit step 2. This seems like a nice optimization, but
it means that the caller sees only whatever was left in
lock->old_oid from previous calls to resolve_ref_unsafe(),
which happened outside of the lock.
We can demonstrate this race pretty easily. Imagine you have
three commits, $one, $two, and $three. One script just flips
between $one and $two, without providing an old-sha1:
while true; do
git update-ref -m one refs/heads/foo $one
git update-ref -m two refs/heads/foo $two
done
Meanwhile, another script tries to set the value to $three,
also not using an old-sha1:
while true; do
git update-ref -m three refs/heads/foo $three
done
If these run simultaneously, we'll see a lot of lock
contention, but each of the writes will succeed some of the
time. The reflog may record movements between any of the
three refs, but we would expect it to provide a consistent
log: the "from" field of each log entry should be the same
as the "to" field of the previous one.
But if we check this:
perl -alne '
print "mismatch on line $."
if defined $last && $F[0] ne $last;
$last = $F[1];
' .git/logs/refs/heads/foo
we'll see many mismatches. Why?
Because sometimes, in the time between lock_ref_sha1_basic
filling lock->old_oid via resolve_ref_unsafe() and it taking
the lock, there may be a complete write by another process.
And the "from" field in our reflog entry will be wrong, and
will refer to an older value.
This is probably quite rare in practice. It requires writers
which do not provide an old-sha1 value, and it is a very
quick race. However, it is easy to fix: we simply perform
step (2), the read-under-lock, whether we have an old-sha1
or not. Then the value we hand back to the caller is always
atomic.
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
It's unlikely that we would fail to create or update a
symbolic ref (especially HEAD), but if we do, we should
notice and complain. Note that there's no need to give more
details in our error message; create_symref will already
have done so.
While we're here, let's also fix a minor memory leak in
clone.
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
We generally hold a lock on the matching ref while writing
to its reflog; this prevents two simultaneous writers from
clobbering each other's reflog lines (it does not even have
to be two symref updates; because we don't hold the lock, we
could race with somebody writing to the pointed-to ref via
HEAD, for example).
We can fix this by writing the reflog before we commit the
lockfile. This runs the risk of writing the reflog but
failing the final rename(), but at least we now err on the
same side as the rest of the ref code.
Noticed-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Jeff King <peff@peff.net>
Reviewed-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
The create_symref() function predates the existence of
"struct lock_file", let alone the more recent "struct
ref_lock". Instead, it just does its own manual dot-locking.
Besides being more code, this has a few downsides:
- if git is interrupted while holding the lock, we don't
clean up the lockfile
- we don't do the usual directory/filename conflict check.
So you can sometimes create a symref "refs/heads/foo/bar",
even if "refs/heads/foo" exists (namely, if the refs are
packed and we do not hit the d/f conflict in the
filesystem).
This patch refactors create_symref() to use the "struct
ref_lock" interface, which handles both of these things.
There are a few bonus cleanups that come along with it:
- we leaked ref_path in some error cases
- the symref contents were stored in a fixed-size buffer,
putting an artificial (albeit large) limitation on the
length of the refname. We now write through fprintf, and
handle refnames of any size.
- we called adjust_shared_perm only after the file was
renamed into place, creating a potential race with
readers in a shared repository. The lockfile code now
handles this when creating the lockfile, making it
atomic.
- the legacy prefer_symlink_refs path did not do any
locking at all. Admittedly, it is not atomic from a
reader's perspective (as it unlinks and re-creates the
symlink to overwrite), but at least it cannot conflict
with other writers now.
- the result of this patch is hopefully more readable. It
eliminates three goto labels. Two were for error checking
that is now simplified, and the third was to reach shared
code that has been pulled into its own function.
Signed-off-by: Jeff King <peff@peff.net>
Reviewed-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Once upon a time, create_symref() was used only to point
HEAD at a branch name, and the variable names reflect that
(e.g., calling the path git_HEAD). However, it is much more
generic these days (and has been for some time). Let's
update the variable names to make it easier to follow:
- `ref_target` is now just `refname`. This is closer to
the `ref` that is already in `cache.h`, but with the
extra twist that "name" makes it clear this is the name
and not a ref struct. Dropping "target" hopefully makes
it clear that we are talking about the symref itself,
not what it points to.
- `git_HEAD` is now `ref_path`; the on-disk path
corresponding to `ref`.
- `refs_heads_master` is now just `target`; i.e., what the
symref points at. This term also matches what is in
the symlink(2) manpage (at least on Linux).
- the buffer to hold the symref file's contents was simply
called `ref`. It's now `buf` (admittedly also generic,
but at least not actively introducing confusion with the
other variable holding the refname).
Signed-off-by: Jeff King <peff@peff.net>
Reviewed-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
"git symbolic-ref" forgot to report a failure with its exit status.
* jk/symbolic-ref-maint:
t1401: test reflog creation for git-symbolic-ref
symbolic-ref: propagate error code from create_symref()
The write(2) emulation for Windows learned to set errno to EPIPE
when necessary.
* js/emu-write-epipe-on-windows:
mingw: emulate write(2) that fails with a EPIPE
The current code writes a reflog entry whenever we update a
symbolic ref, but we never test that this is so. Let's add a
test to make sure upcoming refactoring doesn't cause a
regression.
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
If create_symref() fails, git-symbolic-ref will still exit
with code 0, and our caller has no idea that the command did
nothing.
This appears to have been broken since the beginning of time
(e.g., it is not a regression where create_symref() stopped
calling die() or something similar).
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
"format-patch" has learned a new option to zero-out the commit
object name on the mbox "From " line.
* bc/format-patch-null-from-line:
format-patch: check that header line has expected format
format-patch: add an option to suppress commit hash
sha1_file.c: introduce a null_oid constant
When getpwuid() on the system returned NULL (e.g. the user is not
in the /etc/passwd file or other uid-to-name mappings), the
codepath to find who the user is to record it in the reflog barfed
and died. Loosen the check in this codepath, which already accepts
questionable ident string (e.g. host part of the e-mail address is
obviously bogus), and in general when we operate fmt_ident() function
in non-strict mode.
* jk/ident-loosen-getpwuid:
ident: loosen getpwuid error in non-strict mode
ident: keep a flag for bogus default_email
ident: make xgetpwuid_self() a static local helper
The completion script (in contrib/) used to list "git column"
(which is not an end-user facing command) as one of the choices
* sg/completion-no-column:
completion: remove 'git column' from porcelain commands
Add new config to avoid typing "--recurse-submodules" on each push.
* mc/push-recurse-submodules-config:
push: follow the "last one wins" convention for --recurse-submodules
push: test that --recurse-submodules on command line overrides config
push: add recurseSubmodules config option
On Windows, when writing to a pipe fails, errno is always
EINVAL. However, Git expects it to be EPIPE.
According to the documentation, there are two cases in which write()
triggers EINVAL: the buffer is NULL, or the length is odd but the mode
is 16-bit Unicode (the broken pipe is not mentioned as possible cause).
Git never sets the file mode to anything but binary, therefore we know
that errno should actually be EPIPE if it is EINVAL and the buffer is
not NULL.
See https://msdn.microsoft.com/en-us/library/1570wh78.aspx for more
details.
This works around t5571.11 failing with v2.6.4 on Windows.
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Acked-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
* git://ozlabs.org/~paulus/gitk:
gitk: sv.po: Update Swedish translation (311t)
gitk: Let .bleft.mid widgets 'breathe'
gitk: Match ttk fonts to gitk fonts
gitk: Update revision date in Japanese PO file
gitk: Update "Language:" header
gitk: Improve translation message
gitk: Remove unused line
gitk: Update year
gitk: Change last translator line
gitk: Update fuzzy messages
gitk: Update Japanese translation
gitk: Fix translation around copyright sign
gitk: Update Japanese translation
gitk: Fix wrong translation
gitk: Translate Japanese catalog
gitk: Translate more to Japanese catalog
gitk: Update Japanese message catalog
gitk: Re-sync line number in Japanese message catalogue
gitk: Color name update
"git rebase -i" started with merge strategy options did not
propagate them upon "git rebase --continue".
* fr/rebase-i-continue-preserve-options:
rebase -i: remember merge options beyond continue actions
The format of the "From " header line is very specific to allow
utilities to detect Git-style patches. Add a test that the patches
created are in the expected format.
Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Oftentimes, patches created by git format-patch will be stored in
version control or compared with diff. In these cases, two otherwise
identical patches can have different commit hashes, leading to diff
noise. Teach git format-patch a --zero-commit option that instead
produces an all-zero hash to avoid this diff noise.
Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Jeff King