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Author SHA1 Message Date
Junio C Hamano
d0956cfa8e Merge branch 'mh/safe-create-leading-directories'
Code clean-up and protection against concurrent write access to the
ref namespace.

* mh/safe-create-leading-directories:
  rename_tmp_log(): on SCLD_VANISHED, retry
  rename_tmp_log(): limit the number of remote_empty_directories() attempts
  rename_tmp_log(): handle a possible mkdir/rmdir race
  rename_ref(): extract function rename_tmp_log()
  remove_dir_recurse(): handle disappearing files and directories
  remove_dir_recurse(): tighten condition for removing unreadable dir
  lock_ref_sha1_basic(): if locking fails with ENOENT, retry
  lock_ref_sha1_basic(): on SCLD_VANISHED, retry
  safe_create_leading_directories(): add new error value SCLD_VANISHED
  cmd_init_db(): when creating directories, handle errors conservatively
  safe_create_leading_directories(): introduce enum for return values
  safe_create_leading_directories(): always restore slash at end of loop
  safe_create_leading_directories(): split on first of multiple slashes
  safe_create_leading_directories(): rename local variable
  safe_create_leading_directories(): add explicit "slash" pointer
  safe_create_leading_directories(): reduce scope of local variable
  safe_create_leading_directories(): fix format of "if" chaining
2014-01-27 10:45:33 -08:00
Junio C Hamano
9bb5287098 Merge branch 'mh/retire-ref-fetch-rules'
Code simplification.

* mh/retire-ref-fetch-rules:
  refname_match(): always use the rules in ref_rev_parse_rules
2014-01-27 10:44:07 -08:00
Michael Haggerty
08f555cb82 rename_tmp_log(): on SCLD_VANISHED, retry
If safe_create_leading_directories() fails because a file along the
path unexpectedly vanished, try again from the beginning.  Try at most
4 times.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
2014-01-21 13:47:28 -08:00
Michael Haggerty
f1e9e9a4db rename_tmp_log(): limit the number of remote_empty_directories() attempts
This doesn't seem to be a likely error, but we've got the counter
anyway, so we might as well use it for an added bit of safety.

Please note that the first call to rename() is optimistic, and it is
normal for it to fail if there is a directory in the way.  So bump the
total number of allowed attempts to 4, to be sure that we can still
have at least 3 retries in the case of a race.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
2014-01-21 13:47:24 -08:00
Michael Haggerty
ae4a283e3b rename_tmp_log(): handle a possible mkdir/rmdir race
If a directory vanishes while renaming the temporary reflog file,
retry (up to 3 times).  This could happen if another process deletes
the directory created by safe_create_leading_directories() just before
we rename the file into the directory.

As far as I can tell, this race could not occur internal to git.  The
only time that a directory under $GIT_DIR/logs is deleted is if room
has to be made for a log file for a reference with the same name;
for example, in the following sequence:

    git branch foo/bar    # Creates file .git/logs/refs/heads/foo/bar
    git branch -d foo/bar # Deletes file but leaves .git/logs/refs/heads/foo/
    git branch foo        # Deletes .git/logs/refs/heads/foo/

But the only reason the last command deletes the directory is because
it wants to create a file with the same name.  So if another process
(e.g.,

    git branch foo/baz

) wants to create that directory, one of the two is doomed to failure
anyway because of a D/F conflict.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
2014-01-21 13:47:13 -08:00
Michael Haggerty
fa59ae7971 rename_ref(): extract function rename_tmp_log()
It's about to become a bit more complex.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
2014-01-21 13:46:59 -08:00
Michael Haggerty
e5c223e98b lock_ref_sha1_basic(): if locking fails with ENOENT, retry
If hold_lock_file_for_update() fails with errno==ENOENT, it might be
because somebody else (for example, a pack-refs process) has just
deleted one of the lockfile's ancestor directories.  So if this
condition is detected, try again (up to 3 times).

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
2014-01-21 13:46:30 -08:00
Michael Haggerty
c4c61c763e lock_ref_sha1_basic(): on SCLD_VANISHED, retry
If safe_create_leading_directories() fails because a file along the
path unexpectedly vanished, try again (up to 3 times).

This can occur if another process is deleting directories at the same
time as we are trying to make them.  For example, "git pack-refs
--all" tries to delete the loose refs and any empty directories that
are left behind.  If a pack-refs process is running, then it might
delete a directory that we need to put a new loose reference in.

If safe_create_leading_directories() thinks this might have happened,
then take its advice and try again (maximum three attempts).

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
2014-01-21 13:46:07 -08:00
Michael Haggerty
54457fe509 refname_match(): always use the rules in ref_rev_parse_rules
We used to use two separate rules for the normal ref resolution
dwimming and dwimming done to decide which remote ref to grab.  The
third parameter to refname_match() selected which rules to use.

When these two rules were harmonized in

    2011-11-04 dd621df9cd refs DWIMmery: use the same rule for both "git fetch" and others

, ref_fetch_rules was #defined to avoid potential breakages for
in-flight topics.

It is now safe to remove the backwards-compatibility code, so remove
refname_match()'s third parameter, make ref_rev_parse_rules private to
refs.c, and remove ref_fetch_rules entirely.

Suggested-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
2014-01-14 13:58:06 -08:00
Junio C Hamano
540cc75f38 Merge branch 'mh/shorten-unambigous-ref'
* mh/shorten-unambigous-ref:
  shorten_unambiguous_ref(): tighten up pointer arithmetic
  gen_scanf_fmt(): delete function and use snprintf() instead
  shorten_unambiguous_ref(): introduce a new local variable
2014-01-13 11:34:08 -08:00
Michael Haggerty
7902fe03f9 shorten_unambiguous_ref(): tighten up pointer arithmetic
As long as we're being pathologically stingy with mallocs, we might as
well do the math right and save 6 (!) bytes.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
2014-01-09 15:02:36 -08:00
Michael Haggerty
4346663a14 gen_scanf_fmt(): delete function and use snprintf() instead
To replace "%.*s" with "%s", all we have to do is use snprintf()
to interpolate "%s" into the pattern.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
2014-01-09 14:56:06 -08:00
Michael Haggerty
84d5633f98 shorten_unambiguous_ref(): introduce a new local variable
When filling the scanf_fmts array, use a separate variable to keep
track of the offset to avoid clobbering total_len (which we will need
in the next commit).

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
2014-01-09 14:52:44 -08:00
Christian Couder
5955654823 replace {pre,suf}fixcmp() with {starts,ends}_with()
Leaving only the function definitions and declarations so that any
new topic in flight can still make use of the old functions, replace
existing uses of the prefixcmp() and suffixcmp() with new API
functions.

The change can be recreated by mechanically applying this:

    $ git grep -l -e prefixcmp -e suffixcmp -- \*.c |
      grep -v strbuf\\.c |
      xargs perl -pi -e '
        s|!prefixcmp\(|starts_with\(|g;
        s|prefixcmp\(|!starts_with\(|g;
        s|!suffixcmp\(|ends_with\(|g;
        s|suffixcmp\(|!ends_with\(|g;
      '

on the result of preparatory changes in this series.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
2013-12-05 14:13:21 -08:00
Junio C Hamano
e0fd1e3841 Merge branch 'sb/refs-code-cleanup'
* sb/refs-code-cleanup:
  cache: remove unused function 'have_git_dir'
  refs: remove unused function invalidate_ref_cache
2013-11-01 07:38:58 -07:00
Junio C Hamano
149a8134a7 Merge branch 'jk/refs-c-squelch-gcc'
* jk/refs-c-squelch-gcc:
  silence gcc array-bounds warning
2013-10-30 12:11:04 -07:00
Stefan Beller
746593bdca refs: remove unused function invalidate_ref_cache
The function 'invalidate_ref_cache' was introduced in 79c7ca5 (2011-10-17,
invalidate_ref_cache(): rename function from invalidate_cached_refs())
by a rename and elevated to be publicly usable in 8be8bde (2011-10-17,
invalidate_ref_cache(): expose this function in the refs API)

However it is not used anymore, as 8bf90dc (2011-10-17, write_ref_sha1():
only invalidate the loose ref cache) and (much) later 506a760 (2013-04-22,
refs: change how packed refs are deleted) removed any calls to this
function. So it seems as if we don't need that function any more,
good bye!

Signed-off-by: Stefan Beller <stefanbeller@googlemail.com>
Acked-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
2013-10-28 08:55:56 -07:00
Jeff King
a4165851e7 silence gcc array-bounds warning
In shorten_unambiguous_ref, we build and cache a reverse-map of the
rev-parse rules like this:

  static char **scanf_fmts;
  static int nr_rules;
  if (!nr_rules) {
	  for (; ref_rev_parse_rules[nr_rules]; nr_rules++)
		  ... generate scanf_fmts ...
  }

where ref_rev_parse_rules is terminated with a NULL pointer.
Compiling with "gcc -O2 -Wall" does not cause any problems, but
compiling with "-O3 -Wall" generates:

  $ make CFLAGS='-O3 -Wall' refs.o
  refs.c: In function ‘shorten_unambiguous_ref’:
  refs.c:3379:29: warning: array subscript is above array bounds [-Warray-bounds]
     for (; ref_rev_parse_rules[nr_rules]; nr_rules++)

Curiously, we can silence this by explicitly nr_rules to 0
in the beginning of the loop, even though the compiler
should be able to tell that we follow this code path only
when nr_rules is already 0.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
2013-10-24 15:41:56 -07:00
Ramsay Jones
ce1e846207 refs.c: spell NULL pointer as NULL
A call to update_ref_lock() passes '0' to the 'int *type_p' parameter.
Noticed by sparse.  ("Using plain integer as NULL pointer")

Signed-off-by: Ramsay Jones <ramsay@ramsay1.demon.co.uk>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
2013-10-14 16:10:50 -07:00
Junio C Hamano
f406140baa Merge branch 'fc/at-head'
Instead of typing four capital letters "HEAD", you can say "@" now,
e.g. "git log @".

* fc/at-head:
  Add new @ shortcut for HEAD
  sha1-name: pass len argument to interpret_branch_name()
2013-09-20 12:38:10 -07:00
Junio C Hamano
9a86b89941 Merge branch 'bk/refs-multi-update'
Give "update-refs" a "--stdin" option to read multiple update
requests and perform them in an all-or-none fashion.

* bk/refs-multi-update:
  update-ref: add test cases covering --stdin signature
  update-ref: support multiple simultaneous updates
  refs: add update_refs for multiple simultaneous updates
  refs: add function to repack without multiple refs
  refs: factor delete_ref loose ref step into a helper
  refs: factor update_ref steps into helpers
  refs: report ref type from lock_any_ref_for_update
  reset: rename update_refs to reset_refs
2013-09-20 12:36:12 -07:00
Felipe Contreras
9ba89f484e Add new @ shortcut for HEAD
Typing 'HEAD' is tedious, especially when we can use '@' instead.

The reason for choosing '@' is that it follows naturally from the
ref@op syntax (e.g. HEAD@{u}), except we have no ref, and no
operation, and when we don't have those, it makes sens to assume
'HEAD'.

So now we can use 'git show @~1', and all that goody goodness.

Until now '@' was a valid name, but it conflicts with this idea, so
let's make it invalid. Probably very few people, if any, used this name.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
2013-09-12 14:39:34 -07:00
Junio C Hamano
2233ad4534 Merge branch 'jc/push-cas'
Allow a safer "rewind of the remote tip" push than blind "--force",
by requiring that the overwritten remote ref to be unchanged since
the new history to replace it was prepared.

The machinery is more or less ready.  The "--force" option is again
the big red button to override any safety, thanks to J6t's sanity
(the original round allowed --lockref to defeat --force).

The logic to choose the default implemented here is fragile
(e.g. "git fetch" after seeing a failure will update the
remote-tracking branch and will make the next "push" pass,
defeating the safety pretty easily).  It is suitable only for the
simplest workflows, and it may hurt users more than it helps them.

* jc/push-cas:
  push: teach --force-with-lease to smart-http transport
  send-pack: fix parsing of --force-with-lease option
  t5540/5541: smart-http does not support "--force-with-lease"
  t5533: test "push --force-with-lease"
  push --force-with-lease: tie it all together
  push --force-with-lease: implement logic to populate old_sha1_expect[]
  remote.c: add command line option parser for "--force-with-lease"
  builtin/push.c: use OPT_BOOL, not OPT_BOOLEAN
  cache.h: move remote/connect API out of it
2013-09-09 14:30:29 -07:00
Brad King
98aee92d5c refs: add update_refs for multiple simultaneous updates
Add 'struct ref_update' to encode the information needed to update or
delete a ref (name, new sha1, optional old sha1, no-deref flag).  Add
function 'update_refs' accepting an array of updates to perform.  First
sort the input array to order locks consistently everywhere and reject
multiple updates to the same ref.  Then acquire locks on all refs with
verified old values.  Then update or delete all refs accordingly.  Fail
if any one lock cannot be obtained or any one old value does not match.

Though the refs themselves cannot be modified together in a single
atomic transaction, this function does enable some useful semantics.
For example, a caller may create a new branch starting from the head of
another branch and rewind the original branch at the same time.  This
transfers ownership of commits between branches without risk of losing
commits added to the original branch by a concurrent process, or risk of
a concurrent process creating the new branch first.

Signed-off-by: Brad King <brad.king@kitware.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
2013-09-04 11:10:28 -07:00
Brad King
61cee0dbac refs: add function to repack without multiple refs
Generalize repack_without_ref as repack_without_refs to support a list
of refs and implement the former in terms of the latter.

Signed-off-by: Brad King <brad.king@kitware.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
2013-09-04 11:09:55 -07:00
Brad King
2ddb5d170a refs: factor delete_ref loose ref step into a helper
Factor loose ref deletion into helper function delete_ref_loose to allow
later use elsewhere.

Signed-off-by: Brad King <brad.king@kitware.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
2013-09-04 11:09:09 -07:00
Brad King
4738a33338 refs: factor update_ref steps into helpers
Factor the lock and write steps and error handling into helper functions
update_ref_lock and update_ref_write to allow later use elsewhere.
Expose lock_any_ref_for_update's type_p to update_ref_lock callers.

While at it, drop "static" from the local "lock" variable as it is not
necessary to keep across invocations.

Signed-off-by: Brad King <brad.king@kitware.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
2013-09-04 11:08:36 -07:00
Felipe Contreras
cf99a761d3 sha1-name: pass len argument to interpret_branch_name()
This is useful to make sure we don't step outside the boundaries of what
we are interpreting at the moment. For example while interpreting
foobar@{u}~1, the job of interpret_branch_name() ends right before ~1,
but there's no way to figure that out inside the function, unless the
len argument is passed.

So let's do that.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
2013-09-03 11:33:00 -07:00
Brad King
9bbb0fa1fd refs: report ref type from lock_any_ref_for_update
Expose lock_ref_sha1_basic's type_p argument to callers of
lock_any_ref_for_update.  Update all call sites to ignore it by passing
NULL for now.

Signed-off-by: Brad King <brad.king@kitware.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
2013-08-30 14:57:28 -07:00
Junio C Hamano
2c2b6646c2 Revert "Add new @ shortcut for HEAD"
This reverts commit cdfd94837b, as it
does not just apply to "@" (and forms with modifiers like @{u}
applied to it), but also affects e.g. "refs/heads/@/foo", which it
shouldn't.

The basic idea of giving a short-hand might be good, and the topic
can be retried later, but let's revert to avoid affecting existing
use cases for now for the upcoming release.
2013-08-14 15:04:24 -07:00
Junio C Hamano
f1093b0f60 Merge branch 'mh/packed-refs-do-one-ref-recursion'
Fix a NULL-pointer dereference during nested iterations over
references (for example, when replace references are being used).

* mh/packed-refs-do-one-ref-recursion:
  do_one_ref(): save and restore value of current_ref
2013-07-31 12:38:12 -07:00
Junio C Hamano
29143fc4e3 Merge branch 'mh/ref-races-optim-invalidate-cached'
* mh/ref-races-optim-invalidate-cached:
  refs: do not invalidate the packed-refs cache unnecessarily
2013-07-24 19:21:02 -07:00
Michael Haggerty
d0cf51e940 do_one_ref(): save and restore value of current_ref
If do_one_ref() is called recursively, then the inner call should not
permanently overwrite the value stored in current_ref by the outer
call.  Aside from the tiny optimization loss, peel_ref() expects the
value of current_ref not to change across a call to peel_entry().  But
in the presence of replace references that assumption could be
violated by a recursive call to do_one_ref:

do_for_each_entry()
  do_one_ref()
    builtin/describe.c:get_name()
      peel_ref()
        peel_entry()
          peel_object ()
            deref_tag_noverify()
              parse_object()
                lookup_replace_object()
                  do_lookup_replace_object()
                    prepare_replace_object()
                      do_for_each_ref()
                        do_for_each_entry()
                          do_for_each_entry_in_dir()
                            do_one_ref()

The inner call to do_one_ref() was unconditionally setting current_ref
to NULL when it was done, causing peel_ref() to perform an invalid
memory access.

So change do_one_ref() to save the old value of current_ref before
overwriting it, and restore the old value afterward rather than
setting it to NULL.

Reported-by: Mantas Mikulėnas <grawity@gmail.com>
Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
2013-07-17 18:19:16 -07:00
Junio C Hamano
47a5918536 cache.h: move remote/connect API out of it
The definition of "struct ref" in "cache.h", a header file so
central to the system, always confused me.  This structure is not
about the local ref used by sha1-name API to name local objects.

It is what refspecs are expanded into, after finding out what refs
the other side has, to define what refs are updated after object
transfer succeeds to what values.  It belongs to "remote.h" together
with "struct refspec".

While we are at it, also move the types and functions related to the
Git transport connection to a new header file connect.h

Signed-off-by: Junio C Hamano <gitster@pobox.com>
2013-07-08 14:34:24 -07:00
Junio C Hamano
079424a2cf Merge branch 'mh/ref-races'
"git pack-refs" that races with new ref creation or deletion have
been susceptible to lossage of refs under right conditions, which
has been tightened up.

* mh/ref-races:
  for_each_ref: load all loose refs before packed refs
  get_packed_ref_cache: reload packed-refs file when it changes
  add a stat_validity struct
  Extract a struct stat_data from cache_entry
  packed_ref_cache: increment refcount when locked
  do_for_each_entry(): increment the packed refs cache refcount
  refs: manage lifetime of packed refs cache via reference counting
  refs: implement simple transactions for the packed-refs file
  refs: wrap the packed refs cache in a level of indirection
  pack_refs(): split creation of packed refs and entry writing
  repack_without_ref(): split list curation and entry writing
2013-06-30 15:40:05 -07:00
Michael Haggerty
5d478f5ca1 refs: do not invalidate the packed-refs cache unnecessarily
Now that we keep track of the packed-refs file metadata, we can detect
when the packed-refs file has been modified since we last read it, and
we do so automatically every time that get_packed_ref_cache() is
called.  So there is no need to invalidate the cache automatically
when lock_packed_refs() is called; usually the old copy will still be
valid.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
2013-06-20 15:50:17 -07:00
Jeff King
98eeb09e8a for_each_ref: load all loose refs before packed refs
If we are iterating through the refs using for_each_ref (or
any of its sister functions), we can get into a race
condition with a simultaneous "pack-refs --prune" that looks
like this:

  0. We have a large number of loose refs, and a few packed
     refs. refs/heads/z/foo is loose, with no matching entry
     in the packed-refs file.

  1. Process A starts iterating through the refs. It loads
     the packed-refs file from disk, then starts lazily
     traversing through the loose ref directories.

  2. Process B, running "pack-refs --prune", writes out the
     new packed-refs file. It then deletes the newly packed
     refs, including refs/heads/z/foo.

  3. Meanwhile, process A has finally gotten to
     refs/heads/z (it traverses alphabetically). It
     descends, but finds nothing there.  It checks its
     cached view of the packed-refs file, but it does not
     mention anything in "refs/heads/z/" at all (it predates
     the new file written by B in step 2).

The traversal completes successfully without mentioning
refs/heads/z/foo at all (the name, of course, isn't
important; but the more refs you have and the farther down
the alphabetical list a ref is, the more likely it is to hit
the race). If refs/heads/z/foo did exist in the packed refs
file at state 0, we would see an entry for it, but it would
show whatever sha1 the ref had the last time it was packed
(which could be an arbitrarily long time ago).

This can be especially dangerous when process A is "git
prune", as it means our set of reachable tips will be
incomplete, and we may erroneously prune objects reachable
from that tip (the same thing can happen if "repack -ad" is
used, as it simply drops unreachable objects that are
packed).

This patch solves it by loading all of the loose refs for
our traversal into our in-memory cache, and then refreshing
the packed-refs cache. Because a pack-refs writer will
always put the new packed-refs file into place before
starting the prune, we know that any loose refs we fail to
see will either truly be missing, or will have already been
put in the packed-refs file by the time we refresh.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
2013-06-20 15:50:17 -07:00
Jeff King
ca9199300e get_packed_ref_cache: reload packed-refs file when it changes
Once we read the packed-refs file into memory, we cache it
to save work on future ref lookups. However, our cache may
be out of date with respect to what is on disk if another
process is simultaneously packing the refs. Normally it
is acceptable for us to be a little out of date, since there
is no guarantee whether we read the file before or after the
simultaneous update. However, there is an important special
case: our packed-refs file must be up to date with respect
to any loose refs we read. Otherwise, we risk the following
race condition:

  0. There exists a loose ref refs/heads/master.

  1. Process A starts and looks up the ref "master". It
     first checks $GIT_DIR/master, which does not exist. It
     then loads (and caches) the packed-refs file to see if
     "master" exists in it, which it does not.

  2. Meanwhile, process B runs "pack-refs --all --prune". It
     creates a new packed-refs file which contains
     refs/heads/master, and removes the loose copy at
     $GIT_DIR/refs/heads/master.

  3. Process A continues its lookup, and eventually tries
     $GIT_DIR/refs/heads/master.  It sees that the loose ref
     is missing, and falls back to the packed-refs file. But
     it examines its cached version, which does not have
     refs/heads/master. After trying a few other prefixes,
     it reports master as a non-existent ref.

There are many variants (e.g., step 1 may involve process A
looking up another ref entirely, so even a fully qualified
refname can fail). One of the most interesting ones is if
"refs/heads/master" is already packed. In that case process
A will not see it as missing, but rather will report
whatever value happened to be in the packed-refs file before
process B repacked (which might be an arbitrarily old
value).

We can fix this by making sure we reload the packed-refs
file from disk after looking at any loose refs. That's
unacceptably slow, so we can check its stat()-validity as a
proxy, and read it only when it appears to have changed.

Reading the packed-refs file after performing any loose-ref
system calls is sufficient because we know the ordering of
the pack-refs process: it always makes sure the newly
written packed-refs file is installed into place before
pruning any loose refs. As long as those operations by B
appear in their executed order to process A, by the time A
sees the missing loose ref, the new packed-refs file must be
in place.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
2013-06-20 15:50:17 -07:00
Michael Haggerty
4f6b83e370 packed_ref_cache: increment refcount when locked
Increment the packed_ref_cache reference count while it is locked to
prevent its being freed.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
2013-06-20 15:50:17 -07:00
Michael Haggerty
8baf2bb99a do_for_each_entry(): increment the packed refs cache refcount
This function calls a user-supplied callback function which could do
something that causes the packed refs cache to be invalidated.  So
acquire a reference count on the data structure to prevent our copy
from being freed while we are iterating over it.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
2013-06-20 15:50:17 -07:00
Michael Haggerty
5f5e2a8868 refs: manage lifetime of packed refs cache via reference counting
In struct packed_ref_cache, keep a count of the number of users of the
data structure.  Only free the packed ref cache when the reference
count goes to zero rather than when the packed ref cache is cleared.
This mechanism will be used to prevent the cache data structure from
being freed while it is being iterated over.

So far, only the reference in struct ref_cache::packed is counted;
other users will be adjusted in separate commits.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
2013-06-20 15:50:17 -07:00
Michael Haggerty
9f69d29770 refs: implement simple transactions for the packed-refs file
Handle simple transactions for the packed-refs file at the
packed_ref_cache level via new functions lock_packed_refs(),
commit_packed_refs(), and rollback_packed_refs().

Only allow the packed ref cache to be modified (via add_packed_ref())
while the packed refs file is locked.

Change clone to add the new references within a transaction.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
2013-06-20 15:50:17 -07:00
Michael Haggerty
2fff781290 refs: wrap the packed refs cache in a level of indirection
As we know, we can solve any problem in this manner.  In this case,
the problem is to avoid freeing a packed refs cache while somebody is
using it.  So add a level of indirection as a prelude to
reference-counting the packed refs cache.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
2013-06-20 15:50:17 -07:00
Michael Haggerty
267f9a8cc8 pack_refs(): split creation of packed refs and entry writing
Split pack_refs() into multiple passes:

* Iterate over loose refs.  For each one that can be turned into a
  packed ref, create a corresponding entry in the packed refs cache.

* Write the packed refs to the packed-refs file.

This change isolates the mutation of the packed-refs file to a single
place.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
2013-06-20 15:50:17 -07:00
Michael Haggerty
7b40d39638 repack_without_ref(): split list curation and entry writing
The repack_without_ref() function first removes the deleted ref from
the internal packed-refs list, then writes the packed-refs list to
disk, omitting any broken or stale entries.  This patch splits that
second step into multiple passes:

* collect the list of refnames that should be deleted from packed_refs

* delete those refnames from the cache

* write the remainder to the packed-refs file

The purpose of this change is to make the "write the remainder" part
reusable.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
2013-06-20 15:50:16 -07:00
Michael Haggerty
fcb7c76274 resolve_ref_unsafe(): close race condition reading loose refs
We read loose references in two steps.  The code is roughly:

    lstat()
    if error ENOENT:
        loose ref is missing; look for corresponding packed ref
    else if S_ISLNK:
        readlink()
        if error:
            report failure
    else if S_ISDIR:
        report failure
    else
        open()
        if error:
            report failure
        read()

The problem is that the first filesystem call, to lstat(), is not
atomic with the second filesystem call, to readlink() or open().
Therefore it is possible for another process to change the file
between our two calls, for example:

* If the other process deletes the file, our second call will fail
  with ENOENT, which we *should* interpret as "loose ref is missing;
  look for corresponding packed ref".  This can arise if the other
  process is pack-refs; it might have just written a new packed-refs
  file containing the old contents of the reference then deleted the
  loose ref.

* If the other process changes a symlink into a plain file, our call
  to readlink() will fail with EINVAL, which we *should* respond to by
  trying to open() and read() the file.

The old code treats the reference as missing in both of these cases,
which is incorrect.

So instead, handle errors more selectively: if the result of
readline()/open() is a failure that is inconsistent with the result of
the previous lstat(), then something is fishy.  In this case jump back
and start over again with a fresh call to lstat().

One race is still possible and undetected: another process could
change the file from a regular file into a symlink between the call to
lstat and the call to open().  The open() call would silently follow
the symlink and not know that something is wrong.  This situation
could be detected in two ways:

* On systems that support O_NOFOLLOW, pass that option to the open().

* On other systems, call fstat() on the fd returned by open() and make
  sure that it agrees with the stat info from the original lstat().

However, we don't use symlinks anymore, so this situation is unlikely.
Moreover, it doesn't appear that treating a symlink as a regular file
would have grave consequences; after all, this is exactly how the code
handles non-relative symlinks.  So this commit leaves that race
unaddressed.

Note that this solves only the part of the race within
resolve_ref_unsafe. In the situation described above, we may still be
depending on a cached view of the packed-refs file; that race will be
dealt with in a future patch.

This problem was reported and diagnosed by Jeff King <peff@peff.net>,
and this solution is derived from his patch.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Reviewed-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
2013-06-19 10:23:04 -07:00
Michael Haggerty
2884c06ae7 resolve_ref_unsafe(): handle the case of an SHA-1 within loop
There is only one "break" statement within the loop, which jumps to
the code after the loop that handles the case of a file that holds a
SHA-1.  So move that code from below the loop into the if statement
where the break was previously located.  This makes the logic flow
more local.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Reviewed-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
2013-06-19 10:23:03 -07:00
Michael Haggerty
47f534bf92 resolve_ref_unsafe(): extract function handle_missing_loose_ref()
The nesting was getting a bit out of hand, and it's about to get
worse.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Reviewed-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
2013-06-19 10:23:03 -07:00
Junio C Hamano
bb1c8fbcc8 Merge branch 'fc/at-head'
Instead of typing four capital letters "HEAD", you can say "@"
instead.

* fc/at-head:
  sha1_name: compare variable with constant, not constant with variable
  Add new @ shortcut for HEAD
  sha1_name: refactor reinterpret()
  sha1_name: check @{-N} errors sooner
  sha1_name: reorganize get_sha1_basic()
  sha1_name: don't waste cycles in the @-parsing loop
  sha1_name: remove unnecessary braces
  sha1_name: remove no-op
  tests: at-combinations: @{N} versus HEAD@{N}
  tests: at-combinations: increase coverage
  tests: at-combinations: improve nonsense()
  tests: at-combinations: check ref names directly
  tests: at-combinations: simplify setup
2013-06-11 13:31:23 -07:00
Junio C Hamano
2f1ef15070 Merge branch 'mh/packed-refs-various'
Update reading and updating packed-refs file, correcting corner case
bugs.

* mh/packed-refs-various: (33 commits)
  refs: handle the main ref_cache specially
  refs: change do_for_each_*() functions to take ref_cache arguments
  pack_one_ref(): do some cheap tests before a more expensive one
  pack_one_ref(): use write_packed_entry() to do the writing
  pack_one_ref(): use function peel_entry()
  refs: inline function do_not_prune()
  pack_refs(): change to use do_for_each_entry()
  refs: use same lock_file object for both ref-packing functions
  pack_one_ref(): rename "path" parameter to "refname"
  pack-refs: merge code from pack-refs.{c,h} into refs.{c,h}
  pack-refs: rename handle_one_ref() to pack_one_ref()
  refs: extract a function write_packed_entry()
  repack_without_ref(): write peeled refs in the rewritten file
  t3211: demonstrate loss of peeled refs if a packed ref is deleted
  refs: change how packed refs are deleted
  search_ref_dir(): return an index rather than a pointer
  repack_without_ref(): silence errors for dangling packed refs
  t3210: test for spurious error messages for dangling packed refs
  refs: change the internal reference-iteration API
  refs: extract a function peel_entry()
  ...
2013-05-29 14:23:49 -07:00