bug#17591: Offload hook fails to send files

[ludo@gnu.org (Ludovic Courtès)]

We’ve been observing things like this (as of v0.6-257-g2508358):

--8<---------------cut here---------------start------------->8---
load on machine 'hydra.gnunet.org' is 0.52 (normalized: 0.17333333333333334)
@ build-started /gnu/store/wcgr1ddm3d6c166pmf5r17yfw2566w0d-libstdc++-4.8.3.drv - x86_64-linux /var/log/guix/drvs/wc//gr1ddm3d6c166pmf5r17yfw2566w0d-libstdc++-4.8.3.drv.bz2
sending 2 store files to 'hydra.gnunet.org'...
Backtrace:
In ice-9/boot-9.scm:
 157: 15 [catch #t #<catch-closure 1e3e2e0> ...]
In unknown file:
   ?: 14 [apply-smob/1 #<catch-closure 1e3e2e0>]
In ice-9/boot-9.scm:
  63: 13 [call-with-prompt prompt0 ...]
In ice-9/eval.scm:
 432: 12 [eval # #]
In ice-9/boot-9.scm:
2320: 11 [save-module-excursion #<procedure 1e70c80 at ice-9/boot-9.scm:3961:3 ()>]
3966: 10 [#<procedure 1e70c80 at ice-9/boot-9.scm:3961:3 ()>]
1645: 9 [%start-stack load-stack ...]
1650: 8 [#<procedure 1e6f000 ()>]
In unknown file:
   ?: 7 [primitive-load "/usr/local/bin/guix"]
In guix/ui.scm:
 630: 6 [run-guix-command archive "--import"]
In ice-9/boot-9.scm:
 157: 5 [catch srfi-34 #<procedure 2c9f920 at guix/ui.scm:198:2 ()> ...]
 157: 4 [catch system-error ...]
In guix/scripts/archive.scm:
 327: 3 [#<procedure 2ca0cf0 at guix/scripts/archive.scm:312:2 ()>]
In guix/store.scm:
 768: 2 [import-paths #<build-daemon 256.14 2c9b780> #<input: file 0>]
 404: 1 [process-stderr #<build-daemon 256.14 2c9b780> #<input: file 0>]
In unknown file:
   ?: 0 [bytevector-length #<eof>]

ERROR: In procedure bytevector-length:
ERROR: In procedure scm_c_bytevector_length: Wrong type argument in position 1 (expecting bytevector): #<eof>
builder for `/gnu/store/qbvyn4yq7m92mpp7wvwpx8gnk32863vw-binutils-2.24.drv' failed to produce output path `/gnu/store/k1hdhfx3b2lkcwkch70fcj998jg7qs7y-binutils-2.24'
--8<---------------cut here---------------end--------------->8---

If we try later the very same thing, it typically succeeds:

--8<---------------cut here---------------start------------->8---
@ build-started /gnu/store/wcgr1ddm3d6c166pmf5r17yfw2566w0d-libstdc++-4.8.3.drv - x86_64-linux /var/log/guix/drvs/wc//gr1ddm3d6c166pmf5r17yfw2566w0d-libstdc++-4.8.3.drv.bz2
sending 2 store files to 'hydra.gnunet.org'...
importing path `/gnu/store/5wxd03bpxbw6g4ik9nvq5242p2h1f9zw-binutils-2.24-guile-builder'
importing path `/gnu/store/qbvyn4yq7m92mpp7wvwpx8gnk32863vw-binutils-2.24.drv'
offloading '/gnu/store/qbvyn4yq7m92mpp7wvwpx8gnk32863vw-binutils-2.24.drv' to 'hydra.gnunet.org'...
@ build-remote /gnu/store/qbvyn4yq7m92mpp7wvwpx8gnk32863vw-binutils-2.24.drv hydra.gnunet.org
--8<---------------cut here---------------end--------------->8---

In the first case, ‘send-files’ appears to be failing early, before the
first store item has been transmitted, because the corresponding
“importing path” message hasn’t been printed yet (it’s printed once the
contents of the item being imported have been read–see local-store.cc.)

Ludo’.