repo: Add a checkout option to not hardlink zero-sized files

In rpm-ostree we've hit a few cases where hardlinking zero-sized
files causes us problems.  The most prominent is lock files in
`/usr/etc`, such as `/usr/etc/selinux/semanage.LOCK`.  If there
are two zero-sized lock files to grab, but they're hardlinked,
then locking will fail.

Another case here is if one is using ostree inside a container
and don't have access to FUSE (i.e. `rofiles-fuse`), then the
ostree hardlinking can cause files that aren't ordinarily hardlinked
to become so, and mutation of one mutates all.  An example where
this is concerning is Python `__init__.py` files.

Now, these lock files should clearly not be in the tree to begin
with, but - we're not gaining a huge amount by hardlinking these
files either, so let's add an option to disable it.

Closes: #1752
Approved by: jlebon

src/libostree/ostree-repo-checkout.c

@@ -586,6 +586,7 @@ checkout_one_file_at (OstreeRepo                        *repo,
   const gboolean is_symlink = (g_file_info_get_file_type (source_info) == G_FILE_TYPE_SYMBOLIC_LINK);
   const gboolean is_whiteout = (!is_symlink && options->process_whiteouts &&
                                 g_str_has_prefix (destination_name, WHITEOUT_PREFIX));
+  const gboolean is_reg_zerosized = (!is_symlink && g_file_info_get_size (source_info) == 0);
 
   /* First, see if it's a Docker whiteout,
    * https://github.com/docker/docker/blob/1a714e76a2cb9008cd19609059e9988ff1660b78/pkg/archive/whiteouts.go
@@ -604,6 +605,10 @@ checkout_one_file_at (OstreeRepo                        *repo,
 
       need_copy = FALSE;
     }
+  else if (options->force_copy_zerosized && is_reg_zerosized)
+    {
+      need_copy = TRUE;
+    }
   else if (!options->force_copy)
     {
       HardlinkResult hardlink_res = HARDLINK_RESULT_NOT_SUPPORTED;
@@ -699,6 +704,7 @@ checkout_one_file_at (OstreeRepo                        *repo,
   if (can_cache
       && !is_whiteout
       && !is_symlink
+      && !is_reg_zerosized
       && need_copy
       && repo->mode == OSTREE_REPO_MODE_ARCHIVE
       && options->mode == OSTREE_REPO_CHECKOUT_MODE_USER)
@@ -762,7 +768,7 @@ checkout_one_file_at (OstreeRepo                        *repo,
        * succeeded at hardlinking above.
        */
       if (options->no_copy_fallback)
-        g_assert (is_bare_user_symlink);
+        g_assert (is_bare_user_symlink || is_reg_zerosized);
       if (!ostree_repo_load_file (repo, checksum, &input, NULL, &xattrs,
                                   cancellable, error))
         return FALSE;

src/libostree/ostree-repo.h

@@ -933,7 +933,8 @@ typedef struct {
   gboolean no_copy_fallback;
   gboolean force_copy; /* Since: 2017.6 */
   gboolean bareuseronly_dirs; /* Since: 2017.7 */
-  gboolean unused_bools[5];
+  gboolean force_copy_zerosized; /* Since: 2018.9 */
+  gboolean unused_bools[4];
   /* 4 byte hole on 64 bit */
 
   const char *subpath;