bug#33722: While offloading, 'length' was applied to #<unspecified>

[Mark H Weaver]

The following build on Hydra aborted due to an error while offloading.
The 'length' procedure was applied to #<unspecified>.  See below for the
Nix error output.

      Mark


https://hydra.gnu.org/build/3278275

Nix error output

these derivations will be built:
  /gnu/store/ndbvyl6pf7xnr2xfl9a93bzw8vmrrzfg-texlive-latex-cyrillic-44591.drv
process 22883 acquired build slot '/var/guix/offload/guix.sjd.se/1'
load on machine 'guix.sjd.se' is 1.05 (normalized: 0.525)
@ build-started 
/gnu/store/ndbvyl6pf7xnr2xfl9a93bzw8vmrrzfg-texlive-latex-cyrillic-44591.drv - 
i686-linux 
/var/log/guix/drvs/nd//bvyl6pf7xnr2xfl9a93bzw8vmrrzfg-texlive-latex-cyrillic-44591.drv
Backtrace:
           8 (primitive-load "/gnu/store/fkkjhida23k612naa9d4q6avqj5…")
In guix/ui.scm:
  1384:12  7 (run-guix-command _ . _)
In guix/scripts/offload.scm:
   669:22  6 (guix-offload . _)
In ice-9/boot-9.scm:
    837:9  5 (catch _ _ #<procedure 7f325af45ac0 at guix/ui.scm:460…> …)
    837:9  4 (catch _ _ #<procedure 7f325af45ad8 at guix/ui.scm:548…> …)
In guix/scripts/offload.scm:
   529:19  3 (process-request _ _ #<derivation /gnu/store/ndbvyl6pf…> …)
    341:4  2 (transfer-and-offload #<derivation /gnu/store/ndbvyl6p…> …)
In guix/ssh.scm:
   235:18  1 (send-files #<build-daemon 256.97 2a75f00> _ _ # _ # _)
In unknown file:
           0 (length #<unspecified>)

ERROR: In procedure length:
ERROR: In procedure length: Wrong type argument in position 1: #<unspecified>
@ hook-failed 
/gnu/store/ndbvyl6pf7xnr2xfl9a93bzw8vmrrzfg-texlive-latex-cyrillic-44591.drv - 
256 builder for 
`/gnu/store/ndbvyl6pf7xnr2xfl9a93bzw8vmrrzfg-texlive-latex-cyrillic-44591.drv' 
failed with exit code 1
error: build of 
`/gnu/store/ndbvyl6pf7xnr2xfl9a93bzw8vmrrzfg-texlive-latex-cyrillic-44591.drv' 
failed