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Provide a mechanism to turn off symlink resolution in ceiling paths

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Commit 1b77d83cab 'setup_git_directory_gently_1(): resolve symlinks
in ceiling paths' changed the setup code to resolve symlinks in the
entries in GIT_CEILING_DIRECTORIES.  Because those entries are
compared textually to the symlink-resolved current directory, an
entry in GIT_CEILING_DIRECTORIES that contained a symlink would have
no effect.  It was known that this could cause performance problems
if the symlink resolution *itself* touched slow filesystems, but it
was thought that such use cases would be unlikely.  The intention of
the earlier change was to deal with a case when the user has this:

	GIT_CEILING_DIRECTORIES=/home/gitster

but in reality, /home/gitster is a symbolic link to somewhere else,
e.g. /net/machine/home4/gitster. A textual comparison between the
specified value /home/gitster and the location getcwd(3) returns
would not help us, but readlink("/home/gitster") would still be
fast.

After this change was released, Anders Kaseorg <andersk@mit.edu>
reported:

> [...] my computer has been acting so slow when I’m not connected to
> the network.  I put various network filesystem paths in
> $GIT_CEILING_DIRECTORIES, such as
> /afs/athena.mit.edu/user/a/n/andersk (to avoid hitting its parents
> /afs/athena.mit.edu, /afs/athena.mit.edu/user/a, and
> /afs/athena.mit.edu/user/a/n which all live in different AFS
> volumes).  Now when I’m not connected to the network, every
> invocation of Git, including the __git_ps1 in my shell prompt, waits
> for AFS to timeout.

To allow users to work around this problem, give them a mechanism to
turn off symlink resolution in GIT_CEILING_DIRECTORIES entries.  All
the entries that follow an empty entry will not be checked for symbolic
links and used literally in comparison.  E.g. with these:

	GIT_CEILING_DIRECTORIES=:/foo/bar:/xyzzy or
	GIT_CEILING_DIRECTORIES=/foo/bar::/xyzzy

we will not readlink("/xyzzy") because it comes after an empty entry.

With the former (but not with the latter), "/foo/bar" comes after an
empty entry, and we will not readlink it, either.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
This commit is contained in:
Michael Haggerty 2013-02-20 10:09:24 +01:00 committed by Junio C Hamano
parent 1b77d83cab
commit 7ec30aaa5b
3 changed files with 52 additions and 16 deletions
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19
Documentation/git.txt
View file

@ -653,12 +653,19 @@ git so take care if using Cogito etc.
The '--namespace' command-line option also sets this value.
'GIT_CEILING_DIRECTORIES'::
This should be a colon-separated list of absolute paths.
If set, it is a list of directories that git should not chdir
up into while looking for a repository directory.
It will not exclude the current working directory or
a GIT_DIR set on the command line or in the environment.
(Useful for excluding slow-loading network directories.)
This should be a colon-separated list of absolute paths. If
set, it is a list of directories that git should not chdir up
into while looking for a repository directory (useful for
excluding slow-loading network directories). It will not
exclude the current working directory or a GIT_DIR set on the
command line or in the environment. Normally, Git has to read
the entries in this list and resolve any symlink that
might be present in order to compare them with the current
directory. However, if even this access is slow, you
can add an empty entry to the list to tell Git that the
subsequent entries are not symlinks and needn't be resolved;
e.g.,
'GIT_CEILING_DIRECTORIES=/maybe/symlink::/very/slow/non/symlink'.
'GIT_DISCOVERY_ACROSS_FILESYSTEM'::
When run in a directory that does not have ".git" repository

32
setup.c
View file

@ -624,22 +624,32 @@ static dev_t get_device_or_die(const char *path, const char *prefix, int prefix_
/*
* A "string_list_each_func_t" function that canonicalizes an entry
* from GIT_CEILING_DIRECTORIES using real_path_if_valid(), or
* discards it if unusable.
* discards it if unusable. The presence of an empty entry in
* GIT_CEILING_DIRECTORIES turns off canonicalization for all
* subsequent entries.
*/
static int canonicalize_ceiling_entry(struct string_list_item *item,
void *unused)
void *cb_data)
{
int *empty_entry_found = cb_data;
char *ceil = item->string;
const char *real_path;
if (!*ceil || !is_absolute_path(ceil))
if (!*ceil) {
*empty_entry_found = 1;
return 0;
real_path = real_path_if_valid(ceil);
if (!real_path)
} else if (!is_absolute_path(ceil)) {
return 0;
free(item->string);
item->string = xstrdup(real_path);
return 1;
} else if (*empty_entry_found) {
/* Keep entry but do not canonicalize it */
return 1;
} else {
const char *real_path = real_path_if_valid(ceil);
if (!real_path)
return 0;
free(item->string);
item->string = xstrdup(real_path);
return 1;
}
}
/*
@ -679,9 +689,11 @@ static const char *setup_git_directory_gently_1(int *nongit_ok)
return setup_explicit_git_dir(gitdirenv, cwd, len, nongit_ok);
if (env_ceiling_dirs) {
int empty_entry_found = 0;
string_list_split(&ceiling_dirs, env_ceiling_dirs, PATH_SEP, -1);
filter_string_list(&ceiling_dirs, 0,
canonicalize_ceiling_entry, NULL);
canonicalize_ceiling_entry, &empty_entry_found);
ceil_offset = longest_ancestor_length(cwd, &ceiling_dirs);
string_list_clear(&ceiling_dirs, 0);
}

17
t/t1504-ceiling-dirs.sh
View file

@ -44,6 +44,10 @@ test_prefix ceil_at_sub ""
GIT_CEILING_DIRECTORIES="$TRASH_ROOT/sub/"
test_prefix ceil_at_sub_slash ""
if test_have_prereq SYMLINKS
then
ln -s sub top
fi
mkdir -p sub/dir || exit 1
cd sub/dir || exit 1
@ -68,6 +72,19 @@ test_fail subdir_ceil_at_sub
GIT_CEILING_DIRECTORIES="$TRASH_ROOT/sub/"
test_fail subdir_ceil_at_sub_slash
if test_have_prereq SYMLINKS
then
GIT_CEILING_DIRECTORIES="$TRASH_ROOT/top"
test_fail subdir_ceil_at_top
GIT_CEILING_DIRECTORIES="$TRASH_ROOT/top/"
test_fail subdir_ceil_at_top_slash
GIT_CEILING_DIRECTORIES=":$TRASH_ROOT/top"
test_prefix subdir_ceil_at_top_no_resolve "sub/dir/"
GIT_CEILING_DIRECTORIES=":$TRASH_ROOT/top/"
test_prefix subdir_ceil_at_top_slash_no_resolve "sub/dir/"
fi
GIT_CEILING_DIRECTORIES="$TRASH_ROOT/sub/dir"
test_prefix subdir_ceil_at_subdir "sub/dir/"