Before that, there was a small time window during which the GC could
wipe the .drv (before 'guix build' has been called), or the build
result (before 'retrieve-files' has started.)
* guix/scripts/offload.scm (remote-pipe): Add #:quote? parameter and
honor it.
(%gc-root-file): New variable.
(register-gc-root, remove-gc-root): New procedures.
(offload): Adjust comment. Run 'guix build' with '-r %GC-ROOT-FILE'.
(transfer-and-offload): Call 'register-gc-root' before
sending (derivation-file-name DRV). Call 'remove-gc-root' after the
call to 'offload' or 'retrieve-files'.
(send-files): Call 'remote-pipe' with #:quote? #f.
(retrieve-files): Likewise.
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