From: =?utf-8?b?Ik1hcmNvIFRyZXZpc2FuIChUcmV2acOxbyki?= <mail@3v1n0.net>
Date: Tue, 16 Oct 2018 20:59:23 +0200
Subject: gdm3.service: wait for drm device before trying to start it

Bug-Ubuntu: https://bugs.launchpad.net/ubuntu/+source/gdm3/+bug/1794280
Forwarded: not-needed
---
 data/gdm.service.in      |   1 +
 data/meson.build         |   1 +
 utils/gdm-wait-for-drm.c | 101 +++++++++++++++++++++++++++++++++++++++++++++++
 utils/meson.build        |   9 +++++
 4 files changed, 112 insertions(+)
 create mode 100644 utils/gdm-wait-for-drm.c

diff --git a/data/gdm.service.in b/data/gdm.service.in
index 066d763..ea7f219 100644
--- a/data/gdm.service.in
+++ b/data/gdm.service.in
@@ -28,6 +28,7 @@ BusName=org.gnome.DisplayManager
 EnvironmentFile=-${LANG_CONFIG_FILE}
 ExecReload=/bin/kill -SIGHUP $MAINPID
 KeyringMode=shared
+ExecStartPre=${libexecdir}/gdm-wait-for-drm
 
 [Install]
 Alias=display-manager.service
diff --git a/data/meson.build b/data/meson.build
index c8e9797..6d253e6 100644
--- a/data/meson.build
+++ b/data/meson.build
@@ -162,6 +162,7 @@ gdm_rules = configure_file(
 # DBus service files
 service_config = configuration_data()
 service_config.set('sbindir', gdm_prefix / get_option('sbindir'))
+service_config.set('libexecdir', gdm_prefix / get_option('libexecdir'))
 service_config.set('GDM_INITIAL_VT', get_option('initial-vt'))
 service_config.set('LANG_CONFIG_FILE', lang_config_file)
 if plymouth_dep.found()
diff --git a/utils/gdm-wait-for-drm.c b/utils/gdm-wait-for-drm.c
new file mode 100644
index 0000000..aeffb3c
--- /dev/null
+++ b/utils/gdm-wait-for-drm.c
@@ -0,0 +1,101 @@
+#include <glib.h>
+#include <gudev/gudev.h>
+
+/*
+ * Workaround for LP: #1794280.
+ *
+ * That bug is because the DRM device isn't ready by the time GDM tries to
+ * start wayland/X.
+ * This is a script to add to ExecStartPre of gdm.service. It does the
+ * following:
+ *
+ * 1. Enumerate drm devices from udev, looking for a DRM master. If found,
+ *    exit.
+ * 2. Connect to the 'uevent' signal of gudev, watching for the same to be
+ *    added. Again exit if any are found.
+ * 3. If, after 10 seconds, we haven't seen anything, try to proceed anyway as
+ *    a failsafe.
+ */
+
+static gboolean
+handle_device (GUdevDevice *device)
+{
+        const gchar * const * tags;
+        tags = g_udev_device_get_tags (device);
+        g_debug ("%s\n", g_udev_device_get_name (device));
+        if (g_strv_contains (tags, "master-of-seat"))
+        {
+                g_debug ("    is seat master\n");
+                return TRUE;
+        }
+
+        return FALSE;
+}
+
+static void
+uevent_cb (GUdevClient *client G_GNUC_UNUSED,
+           gchar       *action,
+           GUdevDevice *device,
+           gpointer     user_data)
+{
+        GMainLoop *loop;
+
+        g_debug ("uevent action: %s\n", action);
+
+        loop = (GMainLoop *) user_data;
+
+        if (g_strcmp0 (action, "add") == 0)
+        {
+                if (handle_device (device))
+                {
+                        g_debug ("        this is good\n");
+                        g_main_loop_quit (loop);
+                }
+                else
+                {
+                        g_debug ("        this is bad\n");
+                }
+        }
+}
+
+int
+main()
+{
+        const gchar * const subsystems[] = { "drm", NULL };
+
+        g_autoptr(GList) devices = NULL;
+        g_autoptr(GMainLoop) loop = NULL;
+        g_autoptr(GUdevClient) udev_client = NULL;
+        g_autoptr(GUdevEnumerator) enumerator = NULL;
+
+        loop = g_main_loop_new (NULL, FALSE);
+
+        udev_client = g_udev_client_new (subsystems);
+        enumerator = g_udev_enumerator_new (udev_client);
+        g_udev_enumerator_add_match_is_initialized (enumerator);
+        g_udev_enumerator_add_match_subsystem (enumerator, "drm");
+
+        devices = g_udev_enumerator_execute (enumerator);
+
+        g_debug ("enumerating devices...\n");
+
+        for (GList *l = devices; l != NULL; l = l->next)
+        {
+                g_autoptr(GUdevDevice) device = G_UDEV_DEVICE (l->data);
+
+                if (handle_device (device))
+                {
+                        g_debug ("        good enough for gdm\n");
+                        return EXIT_SUCCESS;
+                }
+        }
+
+        g_signal_connect (udev_client, "uevent", G_CALLBACK (uevent_cb), loop);
+
+        /* after 10 seconds we try anyway */
+        g_timeout_add_seconds (10, (GSourceFunc) g_main_loop_quit, loop);
+
+        g_main_loop_run (loop);
+
+        return EXIT_SUCCESS;
+}
diff --git a/utils/meson.build b/utils/meson.build
index f719c49..d7f991b 100644
--- a/utils/meson.build
+++ b/utils/meson.build
@@ -39,3 +39,12 @@ gdm_disable_wayland = executable('gdm-disable-wayland',
   install_dir: get_option('libexecdir'),
 )
 
+gdm_wait_for_drm = executable('gdm-wait-for-drm',
+  'gdm-wait-for-drm.c',
+  dependencies: [
+    glib_dep,
+    dependency('gudev-1.0'),
+  ],
+  install: true,
+  install_dir: get_option('libexecdir'),
+)