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-fetcher-curl.c

@@ -99,8 +99,7 @@ struct FetcherRequest {
   OstreeFetcherRequestFlags flags;
   gboolean is_membuf;
   GError *caught_write_error;
-  char *out_tmpfile;
-  int out_tmpfile_fd;
+  OtTmpfile tmpf;
   GString *output_buf;
 
   CURL *easy;
@@ -269,12 +268,11 @@ request_get_uri (FetcherRequest *req, guint idx)
 static gboolean
 ensure_tmpfile (FetcherRequest *req, GError **error)
 {
-  if (req->out_tmpfile_fd == -1)
+  if (!req->tmpf.initialized)
     {
-      if (!glnx_open_tmpfile_linkable_at (req->fetcher->tmpdir_dfd, ".",
-                                          O_WRONLY | O_CLOEXEC, &req->out_tmpfile_fd,
-                                          &req->out_tmpfile,
-                                          error))
+      if (!ot_open_tmpfile_linkable_at (req->fetcher->tmpdir_dfd, ".",
+                                        O_WRONLY | O_CLOEXEC, &req->tmpf,
+                                        error))
         return FALSE;
     }
   return TRUE;
@@ -387,18 +385,14 @@ check_multi_info (OstreeFetcher *fetcher)
                 {
                   g_task_return_error (task, g_steal_pointer (&local_error));
                 }
-              else if (fchmod (req->out_tmpfile_fd, 0644) < 0)
+              else if (fchmod (req->tmpf.fd, 0644) < 0)
                 {
                   glnx_set_error_from_errno (error);
                   g_task_return_error (task, g_steal_pointer (&local_error));
                 }
-              else if (!glnx_link_tmpfile_at (fetcher->tmpdir_dfd,
-                                         GLNX_LINK_TMPFILE_REPLACE,
-                                         req->out_tmpfile_fd,
-                                         req->out_tmpfile,
-                                         fetcher->tmpdir_dfd,
-                                         tmpfile_path,
-                                         error))
+              else if (!ot_link_tmpfile_at (&req->tmpf, GLNX_LINK_TMPFILE_REPLACE,
+                                            fetcher->tmpdir_dfd, tmpfile_path,
+                                            error))
                 g_task_return_error (task, g_steal_pointer (&local_error));
               else
                 {
@@ -586,8 +580,8 @@ write_cb (void *ptr, size_t size, size_t nmemb, void *data)
     {
       if (!ensure_tmpfile (req, &req->caught_write_error))
         return -1;
-     g_assert (req->out_tmpfile_fd >= 0);
-      if (glnx_loop_write (req->out_tmpfile_fd, ptr, realsize) < 0)
+      g_assert (req->tmpf.fd >= 0);
+      if (glnx_loop_write (req->tmpf.fd, ptr, realsize) < 0)
         {
           glnx_set_error_from_errno (&req->caught_write_error);
           return -1;
@@ -622,9 +616,7 @@ request_unref (FetcherRequest *req)
   g_ptr_array_unref (req->mirrorlist);
   g_free (req->filename);
   g_clear_error (&req->caught_write_error);
-  if (req->out_tmpfile_fd != -1)
-    (void) close (req->out_tmpfile_fd);
-  g_free (req->out_tmpfile);
+  ot_tmpfile_clear (&req->tmpf);
   if (req->output_buf)
     g_string_free (req->output_buf, TRUE);
   curl_easy_cleanup (req->easy);
@@ -847,7 +839,6 @@ _ostree_fetcher_request_async (OstreeFetcher         *self,
   /* We'll allocate the tmpfile on demand, so we handle
    * file I/O errors just in the write func.
    */
-  req->out_tmpfile_fd = -1;
   if (req->is_membuf)
     req->output_buf = g_string_new ("");