lib: Add a helper for mmap->bytes with openat(), use it in repo

This kills another GSystem consumer...I think down the line I'd like
to do something like "detect whether file is > 1k if so, mmap,
otherwise just readall()" so we can use this helper in more places.

Closes: #319
Approved by: jlebon

src/libotutil/ot-fs-utils.c

@@ -230,3 +230,24 @@ ot_openat_ignore_enoent (int dfd,
  out:
   return ret;
 }
+
+GBytes *
+ot_file_mapat_bytes (int dfd,
+                     const char *path,
+                     GError **error)
+{
+  glnx_fd_close int fd = openat (dfd, path, O_RDONLY | O_CLOEXEC);
+  g_autoptr(GMappedFile) mfile = NULL;
+
+  if (fd < 0)
+    {
+      glnx_set_error_from_errno (error);
+      return FALSE;
+    }
+
+  mfile = g_mapped_file_new_from_fd (fd, FALSE, error);
+  if (!mfile)
+    return FALSE;
+
+  return g_mapped_file_get_bytes (mfile);
+}

src/libotutil/ot-fs-utils.h

@@ -66,4 +66,8 @@ gboolean ot_openat_ignore_enoent (int dfd,
                                   int *out_fd,
                                   GError **error);
 
+GBytes *ot_file_mapat_bytes (int dfd,
+                             const char *path,
+                             GError **error);
+
 G_END_DECLS