bug#34677: error building python-sphinx@1.7.7

[Marco van Hulten <Marco.Hulten@uib.no>]

Hello—

python-sphinx@1.7.7 fails building with the following:

...
[20/23] Compiling C object 'mpc@exe/src_charset.c.o'.
[21/23] Linking target mpc.
[22/23] Generating HTML documentation with a custom command.
FAILED: doc/html
/gnu/store/q6qyrgr41lqzbray6944l1rs7i6b83kp-python-sphinx-1.7.7/bin/sphinx-build -q -b html -d doc/doctrees /tmp/guix-build-mpd-mpc-0.31.drv-0/mpc-0.31/doc doc/html

Exception occurred:
  File "/gnu/store/q6qyrgr41lqzbray6944l1rs7i6b83kp-python-sphinx-1.7.7/lib/python3.7/site-packages/sphinx/environment/__init__.py", line 787, in get_doctree
    doctree = pickle.load(f)
EOFError: Ran out of input
The full traceback has been saved in /tmp/guix-build-mpd-mpc-0.31.drv-0/sphinx-err-1gyuv1e6.log, if you want to report the issue to the developers.
Please also report this if it was a user error, so that a better error message can be provided next time.
A bug report can be filed in the tracker at <https://github.com/sphinx-doc/sphinx/issues>. Thanks!
[23/23] Generating Manpage documentation with a custom command.
ninja: build stopped: subcommand failed.
Backtrace: 
           4 (primitive-load "/gnu/store/rxn07yzqdchsa6rf8aknm1vnszq�")
In ice-9/eval.scm:
   191:35  3 (_ #f)
In srfi/srfi-1.scm:
   863:16  2 (every1 #<procedure 71fb40 at /gnu/store/073micbrwvihh�> �)
In /gnu/store/073micbrwvihh46kj1b607xjgjyfxv2f-module-import/guix/build/gnu-build-system.scm:
   799:28  1 (_ _)
In /gnu/store/073micbrwvihh46kj1b607xjgjyfxv2f-module-import/guix/build/utils.scm:
    616:6  0 (invoke _ . _)

/gnu/store/073micbrwvihh46kj1b607xjgjyfxv2f-module-import/guix/build/utils.scm:616:6: In procedure invoke:
Throw to key `srfi-34' with args `(#<condition &invoke-error [program: "ninja" arguments: ("-j" "2") exit-status: 1 term-signal: #f stop-signal: #f] 8f0f00>)'.


Where to find the *.log file (which is probably on LSB distributions on the named place)?

—Marco