bug#22129: python-matplotlib: Non-deterministic build failures on armhf

[Mark H Weaver]

Over the last month, the 'python-matplotlib' build on armhf has failed
non-deterministically 4 times due to timeout (1 hour of silence) in the
same place:

--8<---------------cut here---------------start------------->8---
phase `install' succeeded after 71.4 seconds
starting phase `install-doc'
Running Sphinx v1.2.3
Initializing GitHub plugin
loading pickled environment... failed: [Errno 2] No such file or directory: 
'/tmp/nix-build-python-matplotlib-1.4.3.drv-0/matplotlib-1.4.3/doc/build/doctrees/environment.pickle'
[autosummary] generating autosummary for: api/afm_api.rst, 
api/animation_api.rst, api/api_changes.rst, api/artist_api.rst, 
api/axes_api.rst, api/axis_api.rst, api/backend_bases_api.rst, 
api/backend_gtkagg_api.rst, api/backend_pdf_api.rst, 
api/backend_qt4agg_api.rst, ..., users/recipes.rst, users/screenshots.rst, 
users/shell.rst, users/style_sheets.rst, users/text_intro.rst, 
users/text_props.rst, users/tight_layout_guide.rst, 
users/transforms_tutorial.rst, users/usetex.rst, users/whats_new.rst
Failed to import 'matplotlib.backends.backend_wxagg': no module named 
matplotlib.backends.backend_wxagg
Failed to import 'matplotlib.backends.backend_qt4agg': no module named 
matplotlib.backends.backend_qt4agg
animation, api, axes_grid, color, event_handling, images_contours_and_fields, 
lines_bars_and_markers, misc, mplot3d, pie_and_polar_charts, pylab_examples, 
shapes_and_collections, showcase, specialty_plots, statistics, style_sheets, 
subplots_axes_and_figures, tests, text_labels_and_annotations, 
ticks_and_spines, units, user_interfaces, widgets, 
building [html]: targets for 127 source files that are out of date
updating environment: 607 added, 0 changed, 0 removed
reading sources... [  0%] api/afm_api
reading sources... [  0%] api/animation_api
reading sources... [  0%] api/api_changes
reading sources... [  0%] api/artist_api
reading sources... [  0%] api/axes_api
reading sources... [  0%] api/axis_api
reading sources... [  1%] api/backend_bases_api
reading sources... [  1%] api/backend_gtkagg_api
reading sources... [  1%] api/backend_pdf_api
reading sources... [  1%] api/backend_qt4agg_api
reading sources... [  1%] api/backend_wxagg_api
reading sources... [  1%] api/cbook_api
reading sources... [  2%] api/cm_api
reading sources... [  2%] api/collections_api
building of 
`/gnu/store/0hm09h6a4469z3j2r3p7cm6jiwidm3ff-python-matplotlib-1.4.3.drv' timed 
out after 3600 seconds of silence
@ build-failed 
/gnu/store/0hm09h6a4469z3j2r3p7cm6jiwidm3ff-python-matplotlib-1.4.3.drv - 
timeout
guix build: error: build failed: build of 
`/gnu/store/0hm09h6a4469z3j2r3p7cm6jiwidm3ff-python-matplotlib-1.4.3.drv' failed
--8<---------------cut here---------------end--------------->8---

In all four cases, the timeout occurred in the same place, after
printing "reading sources... [ 2%] api/collections_api".

The same builds succeeded on later attempts, e.g.:

  http://hydra.gnu.org:3000/build/864760#tabs-buildsteps

      Mark