From dcf7fca2d9f87fc4d412dbbba8e441d9ff204ffc Mon Sep 17 00:00:00 2001
From: David Bremner <david@tethera.net>
Date: Sat, 2 Mar 2019 23:30:59 -0400
Subject: [PATCH] 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
---
 doc/Makefile.local | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/doc/Makefile.local b/doc/Makefile.local
index 16459e35..cb0f1f64 100644
--- 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