[lvm-devel] [PATCH] config: set external_device_info_source=none if udev isn't running

[mwilck at suse.com]

From: Martin Wilck <mwilck at suse.com>

LVM2 has several configuration options related to device detection
and udev. In particular, we have obtain_device_list_from_udev=(0|1)
and external_device_info_source=("none"|"udev"). The two options are
obviously semantically related, but it's rather unclear if and how
they interact.

If udev is unavailable, e.g. in containers, obtain_device_list_from_udev
(which defaults to 1) will be automatically reset to 0. However,
if external_device_info_source="udev" is set, this setting is not
reset to "none", leading to error messages like

  Udev database has incomplete information about device /dev/vda.
  /dev/vda: Failed to get external handle [udev].

This patch changes that, treating external_device_info_source the
same way as obtain_device_list_from_udev, thereby making LVM2's
device detection more consistent.

The default for external_device_info_source is "none", but I believe
there are very good reasons to change this setting to "udev", because
LVM will get detection of multipath and md devices wrong most of the
time otherwise. LVM should follow the same logic as systemd and other
system components with respect to device detection.

Signed-off-by: Martin Wilck <mwilck at suse.com>
---
 lib/commands/toolcontext.c | 17 ++++++++++++++---
 1 file changed, 14 insertions(+), 3 deletions(-)

diff --git a/lib/commands/toolcontext.c b/lib/commands/toolcontext.c
index 63b6811..55f6806 100644
--- a/lib/commands/toolcontext.c
+++ b/lib/commands/toolcontext.c
@@ -598,9 +598,20 @@ static int _process_config(struct cmd_context *cmd)
 	dev_ext_info_src = find_config_tree_str(cmd, devices_external_device_info_source_CFG, NULL);
 	if (dev_ext_info_src && !strcmp(dev_ext_info_src, "none"))
 		init_external_device_info_source(DEV_EXT_NONE);
-	else if (dev_ext_info_src && !strcmp(dev_ext_info_src, "udev"))
-		init_external_device_info_source(DEV_EXT_UDEV);
-	else {
+	else if (dev_ext_info_src && !strcmp(dev_ext_info_src, "udev")) {
+		/*
+		 * Override existing config and hardcode external_device_info_source=none if:
+		 *   - udev is not running
+		 *   - udev is disabled using DM_DISABLE_UDEV environment variable
+		 * See also treatment of obtain_device_list_from_udev in _init_dev_cache()
+		 */
+		if (udev_is_running())
+			init_external_device_info_source(DEV_EXT_UDEV);
+		else  {
+			log_notice("udev is not running. Using external_device_info_source=none");
+			init_external_device_info_source(DEV_EXT_NONE);
+		}
+	} else {
 		log_error("Invalid external device info source specification.");
 		return 0;
 	}
-- 
2.29.2