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setup-storage: wait for LV device node before using it #139

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40 changes: 39 additions & 1 deletion lib/setup-storage/Commands.pm
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Original file line number Diff line number Diff line change
Expand Up @@ -646,6 +646,38 @@ sub create_volume_group {
}
}

################################################################################
#
# @brief Build a shell command that waits for a device node to appear,
# bounded by a fixed number of retries
#
# This works around a well-known device-mapper/udev race condition: right
# after e.g. lvcreate returns, the kernel has already created the
# underlying /dev/dm-N device, but the "friendly name" symlinks
# /dev/<vg>/<lv> (and /dev/mapper/<vg>-<lv>) are only created
# asynchronously, once udev gets around to processing the corresponding
# uevent. "udevadm settle" -- which setup-storage already calls before
# every command -- normally takes care of this, but it relies on a
# reachable udev daemon and can return prematurely or be a no-op in
# environments where udev is slow or unreachable, e.g. privileged
# containers sharing the host's /dev (see
# https://github.com/rancher/os/issues/838). Polling for the device node
# itself is environment-agnostic and catches that case too.
#
# @param $device Device path that is expected to appear
#
# @return Shell command string that succeeds as soon as $device exists,
# and fails if it is still missing after the bounded number of retries
#
################################################################################
sub wait_for_device {

my ($device) = @_;

return 'i=0; while [ $i -lt 30 ] && [ ! -e ' . $device .
' ]; do sleep 1; i=$((i+1)); done; [ -e ' . $device . ' ]';
}

################################################################################
#
# @brief Create the volume group $config, unless it exists already; if the
Expand Down Expand Up @@ -730,7 +762,13 @@ sub setup_logical_volumes {
# create a new volume
&FAI::push_command( "lvcreate $create_options -n $lv -L " .
&FAI::convert_unit($lv_size->{eff_size} . "B") . " $vg", "vg_enabled_$vg",
"exist_/dev/$vg/$lv" );
"lv_dev_created_$vg/$lv" );

# the /dev/$vg/$lv symlink may not exist immediately after lvcreate
# returns -- see wait_for_device() for details on why this is needed
# in addition to the generic udevadm settle calls
&FAI::push_command( &FAI::wait_for_device("/dev/$vg/$lv"),
"lv_dev_created_$vg/$lv", "exist_/dev/$vg/$lv" );

# create the filesystem on the volume
&FAI::build_mkfs_commands("/dev/$vg/$lv",
Expand Down