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From 6155789a63258865d65df4bf6abb4c83a22e58a8 Mon Sep 17 00:00:00 2001
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From: Christian Hergert <chergert@redhat.com>
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Date: Wed, 26 Feb 2020 14:46:20 -0800
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Subject: [PATCH 4/6] global: force fsync() to worker thread when saving state
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The g_file_replace_contents_async() API can potentially call fsync() from
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the thread calling into it upon completion. This can have disasterous
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effects when run from the compositor main thread such as complete stalls.
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This is a followup to 86a00b6872375a266449beee1ea6d5e94f1ebbcb which
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assumed (like the rest of us) that the fsync() would be performed on the
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thread that was doing the I/O operations.
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You can verify this with an strace -e fsync and cause terminal to display
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a command completed notification (eg: from a backdrop window).
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This also fixes a lifecycle bug for the variant, as
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g_file_replace_contents_async() does not copy the data during the operation
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as that is the responsibility of the caller. Instead, we just use a GBytes
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variant and reference the variant there.
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https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/1050
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---
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src/shell-global.c | 70 +++++++++++++++++++++++++++++++++++++++++-----
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1 file changed, 63 insertions(+), 7 deletions(-)
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diff --git a/src/shell-global.c b/src/shell-global.c
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index 8f5418677..3e4864555 100644
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--- a/src/shell-global.c
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+++ b/src/shell-global.c
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@@ -1804,6 +1804,55 @@ delete_variant_cb (GObject *object,
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g_hash_table_remove (global->save_ops, object);
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}
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+static void
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+replace_contents_worker (GTask *task,
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+ gpointer source_object,
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+ gpointer task_data,
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+ GCancellable *cancellable)
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+{
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+ GFile *file = source_object;
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+ GBytes *bytes = task_data;
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+ GError *error = NULL;
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+ const gchar *data;
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+ gsize len;
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+
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+ data = g_bytes_get_data (bytes, &len;;
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+
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+ if (!g_file_replace_contents (file, data, len, NULL, FALSE,
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+ G_FILE_CREATE_REPLACE_DESTINATION,
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+ NULL, cancellable, &error))
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+ g_task_return_error (task, g_steal_pointer (&error));
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+ else
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+ g_task_return_boolean (task, TRUE);
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+}
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+
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+static void
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+replace_contents_async (GFile *path,
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+ GBytes *bytes,
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+ GCancellable *cancellable,
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+ GAsyncReadyCallback callback,
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+ gpointer user_data)
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+{
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+ g_autoptr(GTask) task = NULL;
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+
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+ g_assert (G_IS_FILE (path));
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+ g_assert (bytes != NULL);
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+ g_assert (!cancellable || G_IS_CANCELLABLE (cancellable));
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+
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+ task = g_task_new (path, cancellable, callback, user_data);
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+ g_task_set_source_tag (task, replace_contents_async);
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+ g_task_set_task_data (task, g_bytes_ref (bytes), (GDestroyNotify)g_bytes_unref);
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+ g_task_run_in_thread (task, replace_contents_worker);
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+}
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+
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+static gboolean
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+replace_contents_finish (GFile *file,
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+ GAsyncResult *result,
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+ GError **error)
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+{
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+ return g_task_propagate_boolean (G_TASK (result), error);
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+}
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+
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static void
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replace_variant_cb (GObject *object,
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GAsyncResult *result,
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@@ -1812,7 +1861,7 @@ replace_variant_cb (GObject *object,
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ShellGlobal *global = user_data;
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GError *error = NULL;
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- if (!g_file_replace_contents_finish (G_FILE (object), result, NULL, &error))
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+ if (!replace_contents_finish (G_FILE (object), result, &error))
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{
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if (!g_error_matches (error, G_IO_ERROR, G_IO_ERROR_CANCELLED))
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{
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@@ -1848,12 +1897,19 @@ save_variant (ShellGlobal *global,
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}
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else
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{
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- g_file_replace_contents_async (path,
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- g_variant_get_data (variant),
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- g_variant_get_size (variant),
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- NULL, FALSE,
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- G_FILE_CREATE_REPLACE_DESTINATION,
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- cancellable, replace_variant_cb, global);
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+ g_autoptr(GBytes) bytes = NULL;
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+
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+ bytes = g_bytes_new_with_free_func (g_variant_get_data (variant),
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+ g_variant_get_size (variant),
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+ (GDestroyNotify)g_variant_unref,
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+ g_variant_ref (variant));
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+ /* g_file_replace_contents_async() can potentially fsync() from the
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+ * calling thread when completing the asynchronous task. Instead, we
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+ * want to force that fsync() to a thread to avoid blocking the
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+ * compository main loop. Using our own replace_contents_async()
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+ * simply executes the operation synchronously from a thread.
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+ */
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+ replace_contents_async (path, bytes, cancellable, replace_variant_cb, global);
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}
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g_object_unref (path);
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--
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2.26.0
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