bug#64358: “guix refresh” chokes on cran.scm

[Ricardo Wurmus <rekado@elephly.net>]

The new refresh behavior to update inputs automatically seems to
sometimes cause the updater to lose track of the position in the file,
leading to errors like this:

--8<---------------cut here---------------start------------->8---
Backtrace:
In ice-9/boot-9.scm:
  1752:10 18 (with-exception-handler _ _ #:unwind? _ #:unwind-for-type _)
In unknown file:
          17 (apply-smob/0 #<thunk 7f4fe56ba2a0>)
In ice-9/boot-9.scm:
    724:2 16 (call-with-prompt _ _ #<procedure default-prompt-handler (k proc)>)
In ice-9/eval.scm:
    619:8 15 (_ #(#(#<directory (guile-user) 7f4fe56bfc80>)))
In guix/ui.scm:
   2309:7 14 (run-guix . _)
  2272:10 13 (run-guix-command _ . _)
In ice-9/boot-9.scm:
  1752:10 12 (with-exception-handler _ _ #:unwind? _ #:unwind-for-type _)
  1752:10 11 (with-exception-handler _ _ #:unwind? _ #:unwind-for-type _)
In guix/store.scm:
   659:37 10 (thunk)
  2168:25  9 (run-with-store #<store-connection 256.99 7f4fc7ad02d0> _ #:guile-for-build _ #:system _ #:target _)
In guix/scripts/refresh.scm:
   592:16  8 (_ _)
In srfi/srfi-1.scm:
    634:9  7 (for-each #<procedure 7f4fc6860080 at guix/scripts/refresh.scm:593:17 (update)> _)
In guix/scripts/refresh.scm:
   363:21  6 (update-package _ #<package r-dygraphs@1.1.1.6 /home/rekado/dev/gx/branches/master/gnu/packages/cran.scm:30341 7f4fc8e040b0> _ …)
In ice-9/boot-9.scm:
  1747:15  5 (with-exception-handler #<procedure 7f4fc0fe54e0 at ice-9/boot-9.scm:1831:7 (exn)> _ #:unwind? _ #:unwind-for-type _)
In ice-9/ports.scm:
   433:17  4 (call-with-input-file _ _ #:binary _ #:encoding _ #:guess-encoding _)
In guix/packages.scm:
   762:17  3 (_ #<input: /home/rekado/dev/gx/branches/master/gnu/packages/cran.scm 16>)
In guix/utils.scm:
   423:23  2 (go-to-location #<input: /home/rekado/dev/gx/branches/master/gnu/packages/cran.scm 16> 30341 2)
In ice-9/boot-9.scm:
  1685:16  1 (raise-exception _ #:continuable? _)
  1685:16  0 (raise-exception _ #:continuable? _)

ice-9/boot-9.scm:1685:16: In procedure raise-exception:
unexpected end of line #<input: /home/rekado/dev/gx/branches/master/gnu/packages/cran.scm 16>
--8<---------------cut here---------------end--------------->8---

Prior to that we see warnings like this:

--8<---------------cut here---------------start------------->8---
/home/rekado/dev/gx/branches/master/gnu/packages/cran.scm:36350:2: r-readtext: updating from version 0.82 to version 0.90...
/home/rekado/dev/gx/branches/master/gnu/packages/cran.scm:36350:2: warning: r-readtext: no `version' field in source; skipping
--8<---------------cut here---------------end--------------->8---

It’s as if the position in the file has been lost and it tries to update
the definition of r-readtext that is no longer where the current port
position is.

I use “./pre-inst-env guix refresh -t cran -u”, which updates dozens of
packages.  Perhaps the changes result in untracked position shifts in
the file?

-- 
Ricardo