[gnome-settings-daemon/benzea/merge-mtime-code] Use atime instead of mtime in thumbnail cleaner

[Benjamin Berg <bberg src gnome org>]

commit 65050ba8002c4f907d9dd8f3ced09d79a72a9160
Author: Jing Wang <99jingw gmail com>
Date:   Mon Oct 28 19:06:48 2019 -0400

    Use atime instead of mtime in thumbnail cleaner
    
    Currently we expire thumbnails some amount of time after creation, even
    if the thumbnail is regularly accessed. This is probably not what we
    want to happen. Using atime instead fixes this behavior.
    
    If the user is running on a file system with atime disabled, this is
    still no worse than using mtime.

 plugins/housekeeping/gsd-housekeeping-manager.c | 29 +++++++++++++++++--------
 1 file changed, 20 insertions(+), 9 deletions(-)
---
diff --git a/plugins/housekeeping/gsd-housekeeping-manager.c b/plugins/housekeeping/gsd-housekeeping-manager.c
index adc41ede..40d016aa 100644
--- a/plugins/housekeeping/gsd-housekeeping-manager.c
+++ b/plugins/housekeeping/gsd-housekeeping-manager.c
@@ -78,7 +78,7 @@ typedef struct {
 
 
 typedef struct {
-        time_t  mtime;
+        gint64  atime;
         char   *path;
         glong   size;
 } ThumbData;
@@ -104,7 +104,7 @@ read_dir_for_purge (const char *path, GList *files)
         read_path = g_file_new_for_path (path);
         enum_dir = g_file_enumerate_children (read_path,
                                               G_FILE_ATTRIBUTE_STANDARD_NAME ","
-                                              G_FILE_ATTRIBUTE_TIME_MODIFIED ","
+                                              G_FILE_ATTRIBUTE_TIME_ACCESS ","
                                               G_FILE_ATTRIBUTE_STANDARD_SIZE,
                                               G_FILE_QUERY_INFO_NONE,
                                               NULL,
@@ -120,17 +120,28 @@ read_dir_for_purge (const char *path, GList *files)
                                 ThumbData *td;
                                 GFile     *entry;
                                 char      *entry_path;
-                                GTimeVal   mod_time;
+                                gint64     atime;
 
                                 entry = g_file_get_child (read_path, name);
                                 entry_path = g_file_get_path (entry);
                                 g_object_unref (entry);
 
-                                g_file_info_get_modification_time (info, &mod_time);
+                                // Note that using atime here is no worse than using mtime.
+                                // - Even if the file system is mounted with noatime, the atime and
+                                //   mtime will be set to the same value on file creation.
+                                // - Since the thumbnailer never edits thumbnails, and instead swaps
+                                //   in newly created temp files, atime will always be >= mtime.
+                                // - atime should never be absent, which would cause
+                                //   g_file_info_get_attribute_int64 to return 0. The presence of
+                                //   atime is determined by the filters we pass to
+                                //   g_file_enumerate_children, not the file system atime settings.
+
+                                g_assert (g_file_info_has_attribute (info, G_FILE_ATTRIBUTE_TIME_ACCESS));
+                                atime = g_file_info_get_attribute_uint64 (info, 
G_FILE_ATTRIBUTE_TIME_ACCESS);
 
                                 td = g_new0 (ThumbData, 1);
                                 td->path = entry_path;
-                                td->mtime = mod_time.tv_sec;
+                                td->atime = atime;
                                 td->size = g_file_info_get_size (info);
 
                                 files = g_list_prepend (files, td);
@@ -147,7 +158,7 @@ read_dir_for_purge (const char *path, GList *files)
 static void
 purge_old_thumbnails (ThumbData *info, PurgeData *purge_data)
 {
-        if ((purge_data->now - info->mtime) > purge_data->max_age) {
+        if ((purge_data->now - info->atime) > purge_data->max_age) {
                 g_unlink (info->path);
                 info->size = 0;
         } else {
@@ -156,9 +167,9 @@ purge_old_thumbnails (ThumbData *info, PurgeData *purge_data)
 }
 
 static int
-sort_file_mtime (ThumbData *file1, ThumbData *file2)
+sort_file_atime (ThumbData *file1, ThumbData *file2)
 {
-        return file1->mtime - file2->mtime;
+        return file1->atime - file2->atime;
 }
 
 static char **
@@ -249,7 +260,7 @@ purge_thumbnail_cache (GsdHousekeepingManager *manager)
 
         if ((purge_data.total_size > purge_data.max_size) && (purge_data.max_size >= 0)) {
                 GList *scan;
-                files = g_list_sort (files, (GCompareFunc) sort_file_mtime);
+                files = g_list_sort (files, (GCompareFunc) sort_file_atime);
                 for (scan = files; scan && (purge_data.total_size > purge_data.max_size); scan = scan->next) 
{
                         ThumbData *info = scan->data;
                         g_unlink (info->path);