sphinx: Use separate doctree directories for different builders

sphinx-build is buggy when multiple processes are using the same
doctree directory in parallel.  See the 3-year-old Sphinx bug
report at: https://github.com/sphinx-doc/sphinx/issues/2946

Instead of avoiding parallel builds or adding some kind of
locking, I'm using the simplest solution: just using a different
doctree cache for each builder.
Reviewed-by: NPeter Maydell <peter.maydell@linaro.org>
Signed-off-by: NEduardo Habkost <ehabkost@redhat.com>
Reviewed-by: NJohn Snow <jsnow@redhat.com>
Message-id: 20191014150133.14318-1-ehabkost@redhat.com
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>

Makefile

@@ -983,7 +983,10 @@ sphinxdocs: $(MANUAL_BUILDDIR)/devel/index.html $(MANUAL_BUILDDIR)/interop/index
 
 # Canned command to build a single manual
 # Arguments: $1 = manual name, $2 = Sphinx builder ('html' or 'man')
-build-manual = $(call quiet-command,CONFDIR="$(qemu_confdir)" sphinx-build $(if $(V),,-q) -W -n -b $2 -D version=$(VERSION) -D release="$(FULL_VERSION)" -d .doctrees/$1 $(SRC_PATH)/docs/$1 $(MANUAL_BUILDDIR)/$1 ,"SPHINX","$(MANUAL_BUILDDIR)/$1")
+# Note the use of different doctree for each (manual, builder) tuple;
+# this works around Sphinx not handling parallel invocation on
+# a single doctree: https://github.com/sphinx-doc/sphinx/issues/2946
+build-manual = $(call quiet-command,CONFDIR="$(qemu_confdir)" sphinx-build $(if $(V),,-q) -W -n -b $2 -D version=$(VERSION) -D release="$(FULL_VERSION)" -d .doctrees/$1-$2 $(SRC_PATH)/docs/$1 $(MANUAL_BUILDDIR)/$1 ,"SPHINX","$(MANUAL_BUILDDIR)/$1")
 # We assume all RST files in the manual's directory are used in it
 manual-deps = $(wildcard $(SRC_PATH)/docs/$1/*.rst) $(SRC_PATH)/docs/$1/conf.py $(SRC_PATH)/docs/conf.py