[gnome-shell/wip/chergert/no-fsync-on-main-thread] global: force fsync() to worker thread when saving state

[Christian Hergert <chergert src gnome org>]

commit ff7d4bfe12d5006dc029a4417268c6be7692e946
Author: Christian Hergert <chergert redhat com>
Date:   Wed Feb 26 14:46:20 2020 -0800

    global: force fsync() to worker thread when saving state
    
    The g_file_replace_contents_async() API can potentially call fsync() from
    the thread calling into it upon completion. This can have disasterous
    effects when run from the compositor main thread such as complete stalls.
    
    This is a followup to 86a00b6872375a266449beee1ea6d5e94f1ebbcb which
    assumed (like the rest of us) that the fsync() would be performed on the
    thread that was doing the I/O operations.
    
    You can verify this with an strace -e fsync and cause terminal to display
    a command completed notification (eg: from a backdrop window).
    
    This also fixes a lifecycle bug for the variant, as
    g_file_replace_contents_async() does not copy the data during the operation
    as that is the responsibility of the caller. Instead, we just use a GBytes
    variant and reference the variant there.
    
    https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/1050

 src/shell-global.c | 71 ++++++++++++++++++++++++++++++++++++++++++++++++------
 1 file changed, 64 insertions(+), 7 deletions(-)
---
diff --git a/src/shell-global.c b/src/shell-global.c
index 2be2051f14..7c0fe23707 100644
--- a/src/shell-global.c
+++ b/src/shell-global.c
@@ -1659,6 +1659,57 @@ delete_variant_cb (GObject      *object,
   g_hash_table_remove (global->save_ops, object);
 }
 
+static void
+replace_contents_worker (GTask        *task,
+                         gpointer      source_object,
+                         gpointer      task_data,
+                         GCancellable *cancellable)
+{
+  GFile *file = source_object;
+  GBytes *bytes = task_data;
+  GError *error = NULL;
+  const gchar *data;
+  gsize len;
+
+  data = g_bytes_get_data (bytes, &len);
+
+  if (!g_file_replace_contents (file, data, len, NULL, FALSE,
+                                G_FILE_CREATE_REPLACE_DESTINATION,
+                                NULL, cancellable, &error))
+    g_task_return_error (task, g_steal_pointer (&error));
+  else
+    g_task_return_boolean (task, TRUE);
+}
+
+static void
+replace_contents_async (GFile               *path,
+                        GBytes              *bytes,
+                        GCancellable        *cancellable,
+                        GAsyncReadyCallback  callback,
+                        gpointer             user_data)
+{
+  GTask *task;
+
+  g_assert (G_IS_FILE (path));
+  g_assert (bytes != NULL);
+  g_assert (!cancellable || G_IS_CANCELLABLE (cancellable));
+
+  task = g_task_new (path, cancellable, callback, user_data);
+  g_task_set_source_tag (task, replace_contents_async);
+  g_task_set_task_data (task, g_bytes_ref (bytes), (GDestroyNotify)g_bytes_unref);
+  g_task_run_in_thread (task, replace_contents_worker);
+
+  g_object_unref (task);
+}
+
+static gboolean
+replace_contents_finish (GFile         *file,
+                         GAsyncResult  *result,
+                         GError       **error)
+{
+  return g_task_propagate_boolean (G_TASK (result), error);
+}
+
 static void
 replace_variant_cb (GObject      *object,
                     GAsyncResult *result,
@@ -1667,7 +1718,7 @@ replace_variant_cb (GObject      *object,
   ShellGlobal *global = user_data;
   GError *error = NULL;
 
-  if (!g_file_replace_contents_finish (G_FILE (object), result, NULL, &error))
+  if (!replace_contents_finish (G_FILE (object), result, &error))
     {
       if (!g_error_matches (error, G_IO_ERROR, G_IO_ERROR_CANCELLED))
         {
@@ -1703,12 +1754,18 @@ save_variant (ShellGlobal *global,
     }
   else
     {
-      g_file_replace_contents_async (path,
-                                     g_variant_get_data (variant),
-                                     g_variant_get_size (variant),
-                                     NULL, FALSE,
-                                     G_FILE_CREATE_REPLACE_DESTINATION,
-                                     cancellable, replace_variant_cb, global);
+      GBytes *bytes = g_bytes_new_with_free_func (g_variant_get_data (variant),
+                                                  g_variant_get_size (variant),
+                                                  (GDestroyNotify)g_variant_unref,
+                                                  g_variant_ref (variant));
+      /* g_file_replace_contents_async() can potentially fsync() from the
+       * calling thread when completing the asynchronous task. Instead, we
+       * want to force that fsync() to a thread to avoid blocking the
+       * compository main loop. Using our own replace_contents_async()
+       * simply executes the operation synchronously from a thread.
+       */
+      replace_contents_async (path, bytes, cancellable, replace_variant_cb, global);
+      g_bytes_unref (bytes);
     }
 
   g_object_unref (path);