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

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"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.

** Affects: multipath-tools (Ubuntu)
     Importance: Undecided
     Assignee: Liang Chen (cbjchen)
         Status: In Progress


** Tags: patch
-- 
'multipath -r' causes /dev/mapper/<wwid> being removed
https://bugs.launchpad.net/bugs/1621340
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