This is v2 of changes [1] to remove some mkdir/rmdir races around
safe_create_leading_directories() and a couple of its callers. Thanks
to Jonathan Nieder for his thorough review of v1 and to Ramsay Jones
for pointing out a typo. I think I have addressed all of their
suggestions.
This patch series has a bigger scope than v1. The main differences is
that I took Jonathan's (implicit?) suggestion to move the retrying to
a level higher, where files are actually created in the directories
and therefore the directories are definitely no longer subject to
pruning.
Differences from v1:
* Improve some commit messages
* Break up some changes to safe_create_leading_directories() into
smaller steps.
* Fix a problem in safe_create_leading_directories() when handling
paths with multiple slashes (e.g., "foo//bar"). (I noticed this
pre-existing problem while making the other changes.)
* Change how retrying is done:
* safe_create_leading_directories() doesn't retry internally;
instead, its return value is turned into an enum with a new value,
SCLD_VANISHED, that indicates that a directory along the path
vanished while it was working. This return value is an indication
that its caller might want to try calling the function again.
* Change rename_ref() to retry if either
safe_create_leading_directories() returns SCLD_VANISHED or if its
own call to rename() returns ENOENT.
* Fix a similar mkdir/rmdir race in lock_ref_sha1_basic(). This one
is actually more interesting than the one in rename_ref() because it
can be triggered internal to git.
* Fix a race in remove_dir_recurse(): if somebody else deletes a file
or directory that the function wanted to delete anyway, don't treat
it as an error.
The main git-internal race that I know to be fixed by these changes is
when pack-refs is trying to delete empty directories at the same time
as another process is trying to create a new reference. It can be
reproduced by this script:
BRANCHES="foo/bar/xyzzy foo/bar/plugh"
HEADS="h1 h2"
rm -rf test-repo
$GIT init test-repo
cd test-repo
$GIT config user.name 'Lou User'
$GIT config user.email 'lu...@example.com'
for h in $HEADS
do
$GIT commit --allow-empty -m $h
$GIT branch $h
done
(
while true
do
for b in $BRANCHES
do
for h in $HEADS
do
$GIT update-ref refs/heads/$b $h
done
done
done
) &
pid1=$!
(
while true
do
$GIT pack-refs --all
done
) &
pid2=$!
The script has to fail if update-ref and pack-refs try to lock the
same reference at the same time, because we don't handle lock
contention:
fatal: Unable to create
'/home/mhagger/tmp/test-repo/.git/refs/heads/foo/bar/plugh.lock': File exists.
If no other git process is currently running, this probably means a
git process crashed in this repository earlier. Make sure no other git
process is running and remove the file manually to continue.
Also, if update-ref changes the value of a reference while pack-refs
is running, then pack-refs emits a warning and leaves the new value of
the loose reference in place:
error: Ref refs/heads/foo/bar/xyzzy is at
003a9cedc3ec79b8f589b158ffe91177f0a611b3 but expected
34bf1e34d27a639732a01fef0a791e58c2c0882f
But it used to have other unnecessary failures, too, related to
mkdir/rmdir races between the two programs:
error: unable to create directory for .git/refs/heads/foo/bar/plugh
and
fatal: Unable to create
'/home/mhagger/tmp/test-repo/.git/refs/heads/foo/bar/plugh.lock': No such file
or directory
This patch series eliminates the last two types of failures. Here are
tallies of failures from running the above script for 60 seconds
before and after this change. Before:
Total updates: 46900
Unable to create '*.lock': File exists: 1992
* is at * but expected *: 940
unable to create directory: 187
Unable to create '*.lock': No such file or directory: 197
After:
Total updates: 47892
Unable to create '*.lock': File exists: 2105
* is at * but expected *: 835
unable to create directory: 0
Unable to create '*.lock': No such file or directory: 0
Clearly, more work is still needed in this area. For example, the "*
is at * but expected *" errors are not really errors at all and should
not be reported as such. And we might want to consider adding a short
delay and then a retry when acquiring locks. If I run the same test
script with the following patch, then most or all of the "Unable to
create '*.lock': File exists" errors go away too.
======================================================================
diff --git a/refs.c b/refs.c
index 810f802..b3ff1f5 100644
--- a/refs.c
+++ b/refs.c
@@ -2117,7 +2117,10 @@ static struct ref_lock *lock_ref_sha1_basic(const char
*refname,
* again:
*/
goto retry;
- else
+ else if (errno == EEXIST && --attempts > 0) {
+ usleep(1000);
+ goto retry;
+ } else
unable_to_lock_index_die(ref_file, errno);
}
return old_sha1 ? verify_lock(lock, old_sha1, mustexist) : lock;
======================================================================
I hope to do some more work here in the near future.
[1] Version 1: http://thread.gmane.org/gmane.comp.version-control.git/239638
Michael Haggerty (17):
safe_create_leading_directories(): fix format of "if" chaining
safe_create_leading_directories(): reduce scope of local variable
safe_create_leading_directories(): add explicit "slash" pointer
safe_create_leading_directories(): rename local variable
safe_create_leading_directories(): split on first of multiple slashes
safe_create_leading_directories(): always restore slash at end of loop
safe_create_leading_directories(): introduce enum for return values
cmd_init_db(): when creating directories, handle errors conservatively
safe_create_leading_directories(): add new error value SCLD_VANISHED
lock_ref_sha1_basic(): on SCLD_VANISHED, retry
lock_ref_sha1_basic(): if locking fails with ENOENT, retry
remove_dir_recurse(): tighten condition for removing unreadable dir
remove_dir_recurse(): handle disappearing files and directories
rename_ref(): extract function rename_tmp_log()
rename_tmp_log(): handle a possible mkdir/rmdir race
rename_tmp_log(): limit the number of remote_empty_directories()
attempts
rename_tmp_log(): on SCLD_VANISHED, retry
builtin/init-db.c | 9 +++---
cache.h | 25 +++++++++++++--
dir.c | 27 +++++++++++-----
merge-recursive.c | 2 +-
refs.c | 92 ++++++++++++++++++++++++++++++++++++++++---------------
sha1_file.c | 67 ++++++++++++++++++++++------------------
6 files changed, 155 insertions(+), 67 deletions(-)
--
1.8.5.2
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