Add stub for new libglnx tmpfile API, port simpler callers to it

It's hard right now to do a full port to the new libglnx tmpfile
API since there are complex cases in the commit path which deal
with symlinks as well.

Let's make things more gradual by introducing the important part (struct with
autocleanup) here in libotutil, port what we can. This will make a future
complete port easier.

Closes: #871
Approved by: jlebon

src/libostree/ostree-repo-checkout.c

@@ -60,13 +60,11 @@ checkout_object_for_uncompressed_cache (OstreeRepo      *self,
   guint32 file_mode = g_file_info_get_attribute_uint32 (src_info, "unix::mode");
   file_mode &= ~(S_ISUID|S_ISGID);
 
-  glnx_fd_close int fd = -1;
-  g_autofree char *temp_filename = NULL;
-  if (!glnx_open_tmpfile_linkable_at (self->tmp_dir_fd, ".", O_WRONLY | O_CLOEXEC,
-                                      &fd, &temp_filename,
-                                      error))
+  g_auto(OtTmpfile) tmpf = { 0, };
+  if (!ot_open_tmpfile_linkable_at (self->tmp_dir_fd, ".", O_WRONLY | O_CLOEXEC,
+                                    &tmpf, error))
     return FALSE;
-  g_autoptr(GOutputStream) temp_out = g_unix_output_stream_new (fd, FALSE);
+  g_autoptr(GOutputStream) temp_out = g_unix_output_stream_new (tmpf.fd, FALSE);
 
   if (g_output_stream_splice (temp_out, content, 0, cancellable, error) < 0)
     return FALSE;
@@ -76,14 +74,14 @@ checkout_object_for_uncompressed_cache (OstreeRepo      *self,
 
   if (!self->disable_fsync)
     {
-      if (TEMP_FAILURE_RETRY (fsync (fd)) < 0)
+      if (TEMP_FAILURE_RETRY (fsync (tmpf.fd)) < 0)
         return glnx_throw_errno (error);
     }
 
   if (!g_output_stream_close (temp_out, cancellable, error))
     return FALSE;
 
-  if (fchmod (fd, file_mode) < 0)
+  if (fchmod (tmpf.fd, file_mode) < 0)
     return glnx_throw_errno (error);
 
   if (!_ostree_repo_ensure_loose_objdir_at (self->uncompressed_objects_dir_fd,
@@ -91,10 +89,9 @@ checkout_object_for_uncompressed_cache (OstreeRepo      *self,
                                             cancellable, error))
     return FALSE;
 
-  if (!glnx_link_tmpfile_at (self->tmp_dir_fd, GLNX_LINK_TMPFILE_NOREPLACE_IGNORE_EXIST,
-                             fd, temp_filename,
-                             self->uncompressed_objects_dir_fd, loose_path,
-                             error))
+  if (!ot_link_tmpfile_at (&tmpf, GLNX_LINK_TMPFILE_NOREPLACE_IGNORE_EXIST,
+                           self->uncompressed_objects_dir_fd, loose_path,
+                           error))
     return FALSE;
 
   return TRUE;
@@ -185,7 +182,6 @@ create_file_copy_from_input_at (OstreeRepo     *repo,
                                 GCancellable   *cancellable,
                                 GError        **error)
 {
-  g_autofree char *temp_filename = NULL;
   const gboolean union_mode = options->overwrite_mode == OSTREE_REPO_CHECKOUT_OVERWRITE_UNION_FILES;
   const gboolean add_mode = options->overwrite_mode == OSTREE_REPO_CHECKOUT_OVERWRITE_ADD_FILES;
   const gboolean sepolicy_enabled = options->sepolicy && !repo->disable_xattrs;
@@ -252,7 +248,7 @@ create_file_copy_from_input_at (OstreeRepo     *repo,
     }
   else if (g_file_info_get_file_type (file_info) == G_FILE_TYPE_REGULAR)
     {
-      glnx_fd_close int temp_fd = -1;
+      g_auto(OtTmpfile) tmpf = { 0, };
       guint32 file_mode;
       GLnxLinkTmpfileReplaceMode replace_mode;
 
@@ -261,9 +257,8 @@ create_file_copy_from_input_at (OstreeRepo     *repo,
       if (options->mode == OSTREE_REPO_CHECKOUT_MODE_USER)
         file_mode &= ~(S_ISUID|S_ISGID);
 
-      if (!glnx_open_tmpfile_linkable_at (destination_dfd, ".", O_WRONLY | O_CLOEXEC,
-                                          &temp_fd, &temp_filename,
-                                          error))
+      if (!ot_open_tmpfile_linkable_at (destination_dfd, ".", O_WRONLY | O_CLOEXEC,
+                                        &tmpf, error))
         return FALSE;
 
       if (sepolicy_enabled)
@@ -274,11 +269,11 @@ create_file_copy_from_input_at (OstreeRepo     *repo,
                                           g_file_info_get_attribute_uint32 (file_info, "unix::mode"),
                                           &label, cancellable, error))
             return FALSE;
-          if (fsetxattr (temp_fd, "security.selinux", label, strlen (label), 0) < 0)
+          if (fsetxattr (tmpf.fd, "security.selinux", label, strlen (label), 0) < 0)
             return glnx_throw_errno_prefix (error, "Setting security.selinux");
         }
 
-      if (!write_regular_file_content (repo, options, temp_fd, file_info, xattrs, input,
+      if (!write_regular_file_content (repo, options, tmpf.fd, file_info, xattrs, input,
                                        cancellable, error))
         return FALSE;
 
@@ -290,10 +285,9 @@ create_file_copy_from_input_at (OstreeRepo     *repo,
       else
         replace_mode = GLNX_LINK_TMPFILE_NOREPLACE;
 
-      if (!glnx_link_tmpfile_at (destination_dfd, replace_mode,
-                                 temp_fd, temp_filename, destination_dfd,
-                                 destination_name,
-                                 error))
+      if (!ot_link_tmpfile_at (&tmpf, replace_mode,
+                               destination_dfd, destination_name,
+                               error))
         return FALSE;
     }
   else