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[PATCH 2/2] configure: Check that sphinx-build is using Python 3

From: Peter Maydell
Subject: [PATCH 2/2] configure: Check that sphinx-build is using Python 3
Date: Mon, 3 Feb 2020 13:25:33 +0000

Currently configure's has_sphinx_build() check simply runs a dummy
sphinx-build and either passes or fails.  This means that "no
sphinx-build at all" and "sphinx-build exists but is too old" are
both reported the same way.

Further, we want to assume that all the Python we write is running
with at least Python 3.5; configure checks that for our scripts, but
Sphinx extensions run with whatever Python version sphinx-build
itself is using.

Add a check to our conf.py which makes sphinx-build fail if it would
be running our extensions with an old Python, and handle this
in configure so we can report failure helpfully to the user.
This will mean that configure --enable-docs will fail like this
if the sphinx-build provided is not suitable:

Warning: sphinx-build exists but it is either too old or uses too old a Python 

ERROR: User requested feature docs
       configure was not able to find it.
       Install texinfo, Perl/perl-podlators and a Python 3 version of 

(As usual, the default is to simply not build the docs, as we would
if sphinx-build wasn't present at all.)

Signed-off-by: Peter Maydell <address@hidden>
At the moment our Sphinx extensions all work under Python 2;
but the one for handling parsing QAPI docs out of the JSON is going
to want to include some of the scripts/qapi Python which is more
complicated and definitely now 3-only.  In any case it's nicer to
fail cleanly rather than let users stumble into corner cases we don't
test and don't want to support even if they happen to work today.
 configure    | 12 ++++++++++--
 docs/conf.py | 10 ++++++++++
 2 files changed, 20 insertions(+), 2 deletions(-)

diff --git a/configure b/configure
index 830f325822a..95055f2e9dd 100755
--- a/configure
+++ b/configure
@@ -4808,11 +4808,19 @@ has_sphinx_build() {
 # Check if tools are available to build documentation.
 if test "$docs" != "no" ; then
-  if has makeinfo && has pod2man && has_sphinx_build; then
+  if has_sphinx_build; then
+    sphinx_ok=yes
+  else
+    sphinx_ok=no
+  fi
+  if has makeinfo && has pod2man && test "$sphinx_ok" = "yes"; then
     if test "$docs" = "yes" ; then
-      feature_not_found "docs" "Install texinfo, Perl/perl-podlators and 
+      if has $sphinx_build && test "$sphinx_ok" != "yes"; then
+        echo "Warning: $sphinx_build exists but it is either too old or uses 
too old a Python version" >&2
+      fi
+      feature_not_found "docs" "Install texinfo, Perl/perl-podlators and a 
Python 3 version of python-sphinx"
diff --git a/docs/conf.py b/docs/conf.py
index ee7faa6b4e7..7588bf192ee 100644
--- a/docs/conf.py
+++ b/docs/conf.py
@@ -28,6 +28,16 @@
 import os
 import sys
+import sphinx
+from sphinx.errors import VersionRequirementError
+# Make Sphinx fail cleanly if using an old Python, rather than obscurely
+# failing because some code in one of our extensions doesn't work there.
+# Unfortunately this doesn't display very neatly (there's an unavoidable
+# Python backtrace) but at least the information gets printed...
+if sys.version_info < (3,5):
+    raise VersionRequirementError(
+        "QEMU requires a Sphinx that uses Python 3.5 or better\n")
 # The per-manual conf.py will set qemu_docdir for a single-manual build;
 # otherwise set it here if this is an entire-manual-set build.

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