[PATCH RFC] configure: prefer sphinx-build to sphinx-build-3

[John Snow <jsnow@redhat.com>]

sphinx-build is the name of the script entry point from the sphinx
package itself. sphinx-build-3 is a pacakging convention by Linux
distributions. Prefer, where possible, the canonical package name.

In the event that this resolves to a python2 version, test the
suitability of the binary early in the configuration process, and
continue looking for sphinx-build-3 if necessary.

This prioritizes a virtual environment version of sphinx above any
distribution versions, if attempting to build of a virtual python
environment.

Signed-off-by: John Snow <jsnow@redhat.com>
---
 configure | 50 ++++++++++++++++++++++++++++++++------------------
 1 file changed, 32 insertions(+), 18 deletions(-)

diff --git a/configure b/configure
index 233c671aaa..82143e8a41 100755
--- a/configure
+++ b/configure
@@ -928,13 +928,34 @@ do
     fi
 done
 
+# Check we have a new enough version of sphinx-build
+test_sphinx_build() {
+    sphinx=$1
+    # This is a bit awkward but works: create a trivial document and
+    # try to run it with our configuration file (which enforces a
+    # version requirement). This will fail if either
+    # sphinx-build doesn't exist at all or if it is too old.
+    mkdir -p "$TMPDIR1/sphinx"
+    touch "$TMPDIR1/sphinx/index.rst"
+    "$sphinx" -c "$source_path/docs" -b html "$TMPDIR1/sphinx" "$TMPDIR1/sphinx/out" >/dev/null 2>&1
+}
+
+# We require the python3 version of sphinx, but sphinx-build-3 is a
+# distro package. prefer 'sphinx-build' to find the venv version, if
+# any, but ensure it is a suitable version.
 sphinx_build=
-for binary in sphinx-build-3 sphinx-build
+sphinx_ok=
+for binary in sphinx-build sphinx-build-3
 do
     if has "$binary"
     then
-        sphinx_build=$(command -v "$binary")
-        break
+        sphinx_candidate=$(command -v "$binary")
+        if test_sphinx_build "$sphinx_candidate"
+        then
+            sphinx_build=$sphinx_candidate
+            sphinx_ok=yes
+            break
+        fi
     fi
 done
 
@@ -4928,24 +4949,17 @@ if check_include sys/kcov.h ; then
     kcov=yes
 fi
 
-# Check we have a new enough version of sphinx-build
-has_sphinx_build() {
-    # This is a bit awkward but works: create a trivial document and
-    # try to run it with our configuration file (which enforces a
-    # version requirement). This will fail if either
-    # sphinx-build doesn't exist at all or if it is too old.
-    mkdir -p "$TMPDIR1/sphinx"
-    touch "$TMPDIR1/sphinx/index.rst"
-    "$sphinx_build" -c "$source_path/docs" -b html "$TMPDIR1/sphinx" "$TMPDIR1/sphinx/out" >/dev/null 2>&1
-}
-
 # Check if tools are available to build documentation.
 if test "$docs" != "no" ; then
-  if has_sphinx_build; then
-    sphinx_ok=yes
-  else
-    sphinx_ok=no
+
+  if [ "$sphinx_ok" != "yes" ]; then
+    if test_sphinx_build "$sphinx_build"; then
+      sphinx_ok=yes
+    else
+      sphinx_ok=no
+    fi
   fi
+
   if has makeinfo && has pod2man && test "$sphinx_ok" = "yes"; then
     docs=yes
   else
-- 
2.21.1