bug#30644: Cuirass runs out of build users

[Andreas Enge]

Hello,

in trying out the new bayfront machine, I raised the number of concurrent
build jobs to 10; the number of build accounts is programmatically set to
twice this number.

Looking at the build directories in /tmp, 9 build jobs were started on
February 26 at around 21:50, a 10th one at 22:24.

Later, I have been getting messages such as these:
2018-02-27T18:31:13 building 14491 derivations in batches of 200
2018-02-27T18:31:13 building batch of 200 jobs (0/14491)
2018-02-27T18:31:13 build failed: 
'/gnu/store/3qy3s1dy3nnb108ifz9yiscs687g0xkh-gfortran-5.5.0.drv'
2018-02-27T18:31:13 build failed: 
'/gnu/store/1zdxbjmcw1qnmzh2qcbafsrbr8i2j1x3-glib-2.54.2.drv'
2018-02-27T18:31:14 build failed: 
'/gnu/store/w56wpx4jar88hfabipvf6gd6hz81p6wk-python2-gevent-1.2.2.drv'
2018-02-27T18:31:14 build failed: 
'/gnu/store/i2zmgbsi599pxskqh15s38fpms359jp7-gcc-cross-sans-libc-arm-linux-gnueabihf-5.5.0.drv'
2018-02-27T18:31:14 build failed: 
'/gnu/store/qd59gqza3ly4rk2xjf40q2539rjgzh5r-gcc-cross-sans-libc-powerpc-linux-gnu-5.5.0.drv'
2018-02-27T18:31:14 build failed: 
'/gnu/store/3qy3s1dy3nnb108ifz9yiscs687g0xkh-gfortran-5.5.0.drv'
2018-02-27T18:31:14 build failed: 
'/gnu/store/1zdxbjmcw1qnmzh2qcbafsrbr8i2j1x3-glib-2.54.2.drv'
2018-02-27T18:31:15 build started: 
'/gnu/store/08pjxws1pd04syqf28d3hhsfrb946bpc-dzen-0.9.5-1.488ab66.drv'
2018-02-27T18:31:15 build failed: 
'/gnu/store/3qy3s1dy3nnb108ifz9yiscs687g0xkh-gfortran-5.5.0.drv'
2018-02-27T18:31:15 build failed: 
'/gnu/store/1zdxbjmcw1qnmzh2qcbafsrbr8i2j1x3-glib-2.54.2.drv'
2018-02-27T18:31:15 build started: 
'/gnu/store/01r4xm1nqixcxjbbn6xakpg3m5wkr12v-po4a-0.47.drv'
2018-02-27T18:31:15 build failed: 
'/gnu/store/d545d4jzw0mfa61d9ml3vygchr93xp51-python-gevent-1.2.2.drv'
2018-02-27T18:31:15 build started: 
'/gnu/store/02chd3k5qc8p8fc46j4w955cfld77cw1-spinsim-0.75-1.66915a7ad.drv'
2018-02-27T18:31:15 build started: 
'/gnu/store/81b2340psschvvk7ybhnz6nx9jkpdlwa-Net-SMTP-SSL-1.04.tar.gz.drv'
2018-02-27T18:31:15 build started: 
'/gnu/store/rkkacywdj7yya3wf51i141x5khzld4zl-clx-0.0.0-1.tar.xz.drv'
2018-02-27T18:31:16 build started: 
'/gnu/store/286gkv0z6g9fb4frcx6yp2fg9v6iwk3b-ecl-alexandria-0.0.0-1.926a066.drv'
2018-02-27T18:31:16 build started: 
'/gnu/store/hmy3ds71gbv1mspc9p5yvvhz5pibs8wl-elixir-1.5.3.tar.xz.drv'
2018-02-27T18:31:16 build failed: 
'/gnu/store/1zdxbjmcw1qnmzh2qcbafsrbr8i2j1x3-glib-2.54.2.drv'
2018-02-27T18:31:16 build started: 
'/gnu/store/y0p2myl0al8x3rlbxphgyjk83zl3kmb5-idris-mode-0.9.19.tar.drv'
2018-02-27T18:31:16 batch of builds (partially) failed:all build users are 
currently in use; consider creating additional users and adding them to the 
`guixbuild' group (status: 1)

So it looks as if build jobs get stalled, but somehow do not free their
build users.

Hm, the directories in /tmp are created by guixbuilder01 to guixbuilder10.
10 is the default value, but it should have been overwritten.

And in /etc/passwd, I see 20 users. But apparently, they are not used!

Andreas