[epiphany] adblock: Clarify the need for G_FILE_CREATE_REPLACE_DESTINATION

[Michael Catanzaro <mcatanzaro src gnome org>]

commit 5adf08469767f8b34345021c3f272d6b33255706
Author: Adrian Perez de Castro <aperez igalia com>
Date:   Sun Sep 8 13:35:48 2019 +0300

    adblock: Clarify the need for G_FILE_CREATE_REPLACE_DESTINATION
    
    Because it does not hurt to have the explanation in the code for
    future reference.

 embed/ephy-filters-manager.c | 8 ++++++++
 1 file changed, 8 insertions(+)
---
diff --git a/embed/ephy-filters-manager.c b/embed/ephy-filters-manager.c
index 3e9213eaf..f44425402 100644
--- a/embed/ephy-filters-manager.c
+++ b/embed/ephy-filters-manager.c
@@ -335,6 +335,14 @@ filter_info_save_sidecar (FilterInfo          *self,
        self->checksum,
        self->last_update);
 
+  /* Using G_FILE_CREATE_REPLACE_DESTINATION is needed to ensure that
+   * different processes trying to write the same file replace its
+   * contents atomically so there is no partially written data. If two
+   * processes write the same file, the one which finishes writing later
+   * "wins", but that is fine because both would have downloaded the
+   * same rule set and their metadata files will only have a slightly
+   * different (but very close) timestamp.
+   */
   g_file_replace_contents_bytes_async (sidecar_file,
                                        data,
                                        NULL,   /* etag */