devicemapper: add --storage-opt dm.libdm_log_level=X option

Because we use our own logging callbacks in order to use libdm
effectively, it is quite difficult to debug complicated devicemapper
issues (because any warnings or notices from libdm are muted by our own
callback function). e07d3cd9a ("devmapper: Fix libdm logging") further
reduced the ability of this debugging by only allowing _LOG_FATAL errors
to be passed to the output.

Unfortunately libdm is very chatty, so in order to avoid making the logs
even more crowded, add a dm.libdm_log_level storage option that allows
people who are debugging the lovely world of libdm to be able to dive in
without recompiling binaries.

The valid values of dm.libdm_log_level map directly to the libdm logging
levels, and are in the range [2,7] as of the time of writing with 7
being _LOG_DEBUG and 2 being _LOG_FATAL. The default is _LOG_FATAL.

Signed-off-by: Aleksa Sarai <asarai@suse.de>

daemon/graphdriver/devmapper/deviceset.go

@@ -2761,6 +2761,18 @@ func NewDeviceSet(root string, doInit bool, options []string, uidMaps, gidMaps [
 				return nil, errors.New("dm.thinp_autoextend_threshold must be greater than 0 and less than 100")
 			}
 			lvmSetupConfig.AutoExtendThreshold = per
+		case "dm.libdm_log_level":
+			level, err := strconv.ParseInt(val, 10, 32)
+			if err != nil {
+				return nil, errors.Wrapf(err, "could not parse `dm.libdm_log_level=%s`", val)
+			}
+			if level < devicemapper.LogLevelFatal || level > devicemapper.LogLevelDebug {
+				return nil, errors.Errorf("dm.libdm_log_level must be in range [%d,%d]", devicemapper.LogLevelFatal, devicemapper.LogLevelDebug)
+			}
+			// Register a new logging callback with the specified level.
+			devicemapper.LogInit(devicemapper.DefaultLogger{
+				Level: int(level),
+			})
 		default:
 			return nil, fmt.Errorf("devmapper: Unknown option %s\n", key)
 		}