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lib/checkout: Do UNION_FILES via atomic renameat() 

I was looking at fixing an `rpm-ostree livefs` bug where we need to replace
`/usr/lib/passwd`. It's obviously bad if that temporarily disappears 😉. My plan
is to do a subpath checkout of just `/usr/lib/{passwd,group}`.

Make this atomic (i.e. file always exists) by changing the logic to create a
temporary link in repo/tmp, then rename() it into place.

A bonus here is we kill one of the very few (only?) non-error-cleanup i.e.
"non-linear" `goto`s in the ostree codebase.

Closes: #1171
Approved by: jlebon
This commit is contained in:
Colin Walters 2017-09-13 16:22:18 -04:00 committed by Atomic Bot
parent 7499620254
commit c32f234e9a
1 changed files with 59 additions and 17 deletions
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76
src/libostree/ostree-repo-checkout.c
View File

@ -338,6 +338,41 @@ typedef enum {
HARDLINK_RESULT_LINKED
} HardlinkResult;
/* Used for OSTREE_REPO_CHECKOUT_OVERWRITE_UNION_FILES. In
* order to atomically replace a target, we add a new link
* in self->tmp_dir_fd, with a name placed into the mutable
* buffer @tmpname.
*/
static gboolean
hardlink_add_tmp_name (OstreeRepo *self,
int srcfd,
const char *loose_path,
char *tmpname,
GCancellable *cancellable,
GError **error)
{
const int max_attempts = 128;
guint i;
for (i = 0; i < max_attempts; i++)
{
glnx_gen_temp_name (tmpname);
if (linkat (srcfd, loose_path, self->tmp_dir_fd, tmpname, 0) < 0)
{
if (errno == EEXIST)
continue;
else
return glnx_throw_errno_prefix (error, "linkat");
}
else
break;
}
if (i == max_attempts)
return glnx_throw (error, "Exhausted attempts to make temporary hardlink");
return TRUE;
}
static gboolean
checkout_file_hardlink (OstreeRepo *self,
OstreeRepoCheckoutAtOptions *options,
@ -353,7 +388,6 @@ checkout_file_hardlink (OstreeRepo *self,
int srcfd = _ostree_repo_mode_is_bare (self->mode) ?
self->objects_dir_fd : self->uncompressed_objects_dir_fd;
again:
if (linkat (srcfd, loose_path, destination_dfd, destination_name, 0) == 0)
ret_result = HARDLINK_RESULT_LINKED;
else if (!options->no_copy_fallback && (errno == EMLINK || errno == EXDEV || errno == EPERM))
@ -380,27 +414,20 @@ checkout_file_hardlink (OstreeRepo *self,
ret_result = HARDLINK_RESULT_SKIP_EXISTED;
break;
case OSTREE_REPO_CHECKOUT_OVERWRITE_UNION_FILES:
{
/* Idiocy, from man rename(2)
*
* "If oldpath and newpath are existing hard links referring to
* the same file, then rename() does nothing, and returns a
* success status."
*
* So we can't make this atomic.
*/
(void) unlinkat (destination_dfd, destination_name, 0);
goto again;
}
case OSTREE_REPO_CHECKOUT_OVERWRITE_UNION_IDENTICAL:
{
/* In this mode, we error out on EEXIST *unless* the files are already
* hardlinked, which is what rpm-ostree wants for package layering.
* https://github.com/projectatomic/rpm-ostree/issues/982
/* In both union-files and union-identical, see if the src/target are
* already hardlinked. If they are, we're done.
*
* If not, for union-identical we error out, which is what
* rpm-ostree wants for package layering.
* https://github.com/projectatomic/rpm-ostree/issues/982
* This should be similar to the librpm version:
* https://github.com/rpm-software-management/rpm/blob/e3cd2bc85e0578f158d14e6f9624eb955c32543b/lib/rpmfi.c#L921
* in rpmfilesCompare().
*
* For union-files, we make a temporary link, then rename() it
* into place.
*/
struct stat src_stbuf;
if (!glnx_fstatat (srcfd, loose_path, &src_stbuf,
@ -413,10 +440,25 @@ checkout_file_hardlink (OstreeRepo *self,
const gboolean is_identical =
(src_stbuf.st_dev == dest_stbuf.st_dev &&
src_stbuf.st_ino == dest_stbuf.st_ino);
if (is_identical)
ret_result = HARDLINK_RESULT_SKIP_EXISTED;
else if (options->overwrite_mode == OSTREE_REPO_CHECKOUT_OVERWRITE_UNION_FILES)
{
char *tmpname = strdupa ("checkout-union-XXXXXX");
/* Make a link with a temp name */
if (!hardlink_add_tmp_name (self, srcfd, loose_path, tmpname, cancellable, error))
return FALSE;
/* Rename it into place */
if (!glnx_renameat (self->tmp_dir_fd, tmpname, destination_dfd, destination_name, error))
return FALSE;
ret_result = HARDLINK_RESULT_LINKED;
}
else
return glnx_throw_errno_prefix (error, "Hardlinking %s to %s", loose_path, destination_name);
{
g_assert_cmpint (options->overwrite_mode, ==, OSTREE_REPO_CHECKOUT_OVERWRITE_UNION_IDENTICAL);
return glnx_throw_errno_prefix (error, "Hardlinking %s to %s", loose_path, destination_name);
}
break;
}
}