[Bug 1621340] Re: 'multipath -r' causes /dev/mapper/<wwid> being removed

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https://bugs.launchpad.net/bugs/1621340

Title:
  'multipath -r' causes /dev/mapper/<wwid> being removed

Status in multipath-tools package in Ubuntu:
  In Progress
Status in multipath-tools source package in Xenial:
  In Progress

Bug description:
  "multipath -r" causes the /dev/mapper/<wwid> to disappear momentarily,
  which leads to some issue in consumer applications as such OpenStack.
  After some investigation, I found that /dev/mapper/<wwid> was deleted
  by udev during the reload, and it was re-created soon later by
  multipathd (livdevmapper code of cause).  Detailed findings are as
  follows:

  For reload in domap (rename as well),

          case ACT_RELOAD:
                  r = dm_addmap_reload(mpp, params);
                  if (r)
                          r = dm_simplecmd_noflush(DM_DEVICE_RESUME, mpp->alias,
                                                   0, MPATH_UDEV_RELOAD_FLAG);
                  break;

  
  it passes 0 to dm_simplecmd_noflush as argument for needsync, which makes dm_task_set_cookie call being skipped in dm_simplecmd,

          if (udev_wait_flag && !dm_task_set_cookie(dmt, &cookie, ((conf->daemon)? DM_UDEV_DISABLE_LIBRARY_FALLBACK : 0) | udev_flags)) {
                  dm_udev_complete(cookie);
                  goto out;
          }

  
  because of the short-circuit evaluation. Thus _do_dm_ioctl in libdevmapper will add DM_UDEV_DISABLE_DM_RULES_FLAG flag to dmi->event_nr, and that will eventually be used in the udev rules (55-dm.rules),

  ENV{DM_UDEV_DISABLE_DM_RULES_FLAG}!="1", ENV{DM_NAME}=="?*",
  SYMLINK+="mapper/$env{DM_NAME}"

  
  Since the DM_UDEV_DISABLE_DM_RULES_FLAG is set, the rule will not match. As a result the link is removed.

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