doc: sequentialize calls to sphinx-build

In certain conditions the parallel calls to sphinx-build could
collide, yielding a crash like

Exception occurred:
  File "/usr/lib/python3/dist-packages/sphinx/environment.py", line 1261, in get_doctree
    doctree = pickle.load(f)
EOFError: Ran out of input

diff --git a/doc/Makefile.local b/doc/Makefile.local
index 16459e3544c2d7d668691ad89e3f20457dcf77b3..cb0f1f6488f6d9cbe0e3e8d345c5d90053c2de9c 100644 (file)
--- a/doc/Makefile.local
+++ b/doc/Makefile.local
@@ -37,6 +37,14 @@ INFO_INFO_FILES := $(INFO_TEXI_FILES:.texi=.info)
 %.gz: %
        rm -f $@ && gzip --stdout $^ > $@
 
+# Sequentialize the calls to sphinx-build to avoid races with
+# reading/writing cached state. This uses GNU make specific
+# "order-only" prerequisites.
+
+sphinx-html: | $(DOCBUILDDIR)/.roff.stamp
+sphinx-texinfo: | sphinx-html
+sphinx-info: | sphinx-texinfo
+
 sphinx-html:
        $(SPHINXBUILD) -b html $(ALLSPHINXOPTS) $(DOCBUILDDIR)/html