devices: rework libudev usage

related to config settings:
  obtain_device_info_from_udev (controls if lvm gets
    a list of devices from readdir /dev or from libudev)
  external_device_info_source (controls if lvm asks
    libudev for device information)

. Make the obtain_device_list_from_udev setting
  affect only the choice of readdir /dev vs libudev.
  The setting no longer controls if udev is used for
  device type checks.

. Change obtain_device_list_from_udev default to 0.
  This helps avoid boot timeouts due to slow libudev
  queries, avoids reported failures from
  udev_enumerate_scan_devices, and avoids delays from
  "device not initialized in udev database" errors.
  Even without errors, for a system booting with 1024 PVs,
  lvm2-pvscan times improve from about 100 sec to 15 sec,
  and the pvscan command from about 64 sec to about 4 sec.

. For external_device_info_source="none", remove all
  libudev device info queries, and use only lvm
  native device info.

. For external_device_info_source="udev", first check
  lvm native device info, then check libudev info.

. Remove sleep/retry loop when attempting libudev
  queries for device info.  udev info will simply
  be skipped if it's not immediately available.

. Only set up a libdev connection if it will be used by
  obtain_device_list_from_udev/external_device_info_source.

. For native multipath component detection, use
  /etc/multipath/wwids.  If a device has a wwid
  matching an entry in the wwids file, then it's
  considered a multipath component.  This is
  necessary to natively detect multipath
  components when the mpath device is not set up.

1 files changed, 12 insertions, 2 deletions

diff --git a/lib/misc/lvm-wrappers.c b/lib/misc/lvm-wrappers.c
index c36e2cfc7..2e0cfd514 100644
--- a/lib/misc/lvm-wrappers.c
+++ b/lib/misc/lvm-wrappers.c
@@ -25,19 +25,29 @@ struct udev *_udev;
 int udev_init_library_context(void)
 {
 	if (_udev)
-		udev_unref(_udev);
+		return 1;
+
+	if (getenv("DM_DISABLE_UDEV"))
+		return 0;
 
 	if (!(_udev = udev_new())) {
 		log_error("Failed to create udev library context.");
 		return 0;
 	}
 
+	if (!udev_is_running()) {
+		udev_unref(_udev);
+		_udev = NULL;
+		return 0;
+	}
+
 	return 1;
 }
 
 void udev_fin_library_context(void)
 {
-	udev_unref(_udev);
+	if (_udev)
+		udev_unref(_udev);
 	_udev = NULL;
 }