From 19e084036a1dc3c5da010c3e7a42ce67df317797 Mon Sep 17 00:00:00 2001
From: Christian Hergert <chergert@redhat.com>
Date: Wed, 26 Feb 2020 14:46:20 -0800
Subject: [PATCH] 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 | 70 +++++++++++++++++++++++++++++++++++++++++-----
 1 file changed, 63 insertions(+), 7 deletions(-)

diff --git a/src/shell-global.c b/src/shell-global.c
index 2be2051f1..f14a39cf4 100644
--- a/src/shell-global.c
+++ b/src/shell-global.c
@@ -1659,6 +1659,55 @@ 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)
+{
+  g_autoptr(GTask) task = NULL;
+
+  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);
+}
+
+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 +1716,7 @@ 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 +1752,19 @@ 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);
+      g_autoptr(GBytes) bytes = NULL;
+
+      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_object_unref (path);