Cannot cache bootstrap stage of live-build - cp error reading chroot/proc/1/task/1/* - invalid argument

[dbgr <dbgr@systemli.org>]

Hello.

I am using the live-build version 20191221 (the one in testing) on a 
debian stable/buster system.

Whenever I try to build an image (even the most basic with all the 
default options) the live-build hangs trying to save the bootstrap stage 
to cache with the following messages:

  I: Base system installed successfully.
  [2020-04-11 11:17:29] lb bootstrap_cache save
  P: Saving bootstrap stage to cache...
  cp: error reading 'chroot/proc/1/task/1/attr/prev': Ivalid argument
  cp: error reading 'chroot/proc/1/task/1/attr/exec': Ivalid argument
  cp: error reading 'chroot/proc/1/task/1/attr/fscreate': Ivalid 
argument
  cp: error reading 'chroot/proc/1/task/1/attr/keycreate': Ivalid 
argument
  cp: error reading 'chroot/proc/1/task/1/attr/sockcreate': Ivalid 
argument
  cp: error reading 'chroot/proc/1/task/1/mem': I/O error
  cp: error reading 'chroot/proc/1/task/1/clear_refs': Ivalid argument

When this happens the ../cache/bootstrap/proc/1/task/1/pagemap file 
starts to build up with garbage data until it runs my storage out of 
space or until I stop the process with crtl+c.

I believe this is related to the saving of the bootstrap stage to cache 
(besides the message saying so =P) because as soon I set the flag 
--cache-stage deliberately stating to exclude this stage from caching 
(with an empty value or defining other stages) the issue do not occur 
and the build finishes without a problem.

This also happens in other machines running debian stable/buster with 
this version of live-build and with the one in stable (20190311).

Can anyone help me diagnose this problem? The possibility to cache the 
bootstrap stage would be very helpful for me :)

Thanks for your attention.

--

dbgr