[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 afab160e253fee457543335ba95298f5391a3502
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).

 src/shell-global.c | 106 ++++++++++++++++++++++++++++++++++++++++++++++++++---
 1 file changed, 100 insertions(+), 6 deletions(-)
---
diff --git a/src/shell-global.c b/src/shell-global.c
index 2be2051f14..88c8a13735 100644
--- a/src/shell-global.c
+++ b/src/shell-global.c
@@ -1681,6 +1681,92 @@ replace_variant_cb (GObject      *object,
   g_hash_table_remove (global->save_ops, object);
 }
 
+typedef struct
+{
+  GBytes *bytes;
+  gchar *etag;
+  gchar *new_etag;
+  GFileCreateFlags flags;
+  guint make_backup : 1;
+} ReplaceContents;
+
+static void
+replace_contents_free (ReplaceContents *rc)
+{
+  g_clear_pointer (&rc->bytes, g_bytes_unref);
+  g_clear_pointer (&rc->etag, g_free);
+  g_clear_pointer (&rc->new_etag, g_free);
+  g_slice_free (ReplaceContents, rc);
+}
+
+static void
+replace_contents_worker (GTask        *task,
+                         gpointer      source_object,
+                         gpointer      task_data,
+                         GCancellable *cancellable)
+{
+  GFile *file = source_object;
+  ReplaceContents *rc = task_data;
+  GError *error = NULL;
+  const gchar *data;
+  gsize len;
+
+  data = g_bytes_get_data (rc->bytes, &len);
+
+  if (!g_file_replace_contents (file, data, len,
+                                rc->etag, rc->make_backup, rc->flags,
+                                &rc->new_etag, 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,
+                        const gchar         *etag,
+                        gboolean             make_backup,
+                        GFileCreateFlags     flags,
+                        GCancellable        *cancellable,
+                        GAsyncReadyCallback  callback,
+                        gpointer             user_data)
+{
+  ReplaceContents *rc;
+  GTask *task;
+
+  g_assert (G_IS_FILE (path));
+  g_assert (bytes != NULL);
+  g_assert (!cancellable || G_IS_CANCELLABLE (cancellable));
+
+  rc = g_slice_new0 (ReplaceContents);
+  rc->bytes = g_bytes_ref (bytes);
+  rc->etag = g_strdup (etag);
+  rc->make_backup = !!make_backup;
+  rc->flags = flags;
+
+  task = g_task_new (path, cancellable, callback, user_data);
+  g_task_set_source_tag (task, replace_contents_async);
+  g_task_set_task_data (task, rc, (GDestroyNotify)replace_contents_free);
+  g_task_run_in_thread (task, replace_contents_worker);
+
+  g_object_unref (task);
+}
+
+static gboolean
+replace_contents_finish (GFile         *file,
+                         GAsyncResult  *result,
+                         gchar        **new_etag,
+                         GError       **error)
+{
+  ReplaceContents *rc = g_task_get_task_data (G_TASK (result));
+  gboolean ret = g_task_propagate_boolean (G_TASK (result), error);
+
+  if (new_etag != NULL)
+    *new_etag = ret ? g_steal_pointer (&rc->new_etag) : NULL;
+
+  return ret;
+}
+
 static void
 save_variant (ShellGlobal *global,
               GFile       *dir,
@@ -1703,12 +1789,20 @@ 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, NULL, FALSE,
+                              G_FILE_CREATE_REPLACE_DESTINATION,
+                              cancellable, replace_variant_cb, global);
+      g_bytes_unref (bytes);
     }
 
   g_object_unref (path);