[PATCH v3 1/6] configure: Look for auxiliary Python installations

[John Snow <jsnow@redhat.com>]

At the moment, we look for just "python3" and "python", which is good
enough almost all of the time. But ... if you are on a platform that
uses an older Python by default and only offers a newer Python as an
option, you'll have to specify --python=/usr/bin/foo every time.

As a courtesy, we can make a cursory attempt to locate a suitable Python
binary ourselves, looking for the remaining well-known binaries. This
also has the added benefit of making configure "just work" more often
on various BSD distributions that do not have the concept of a
"platform default python".

This configure loop will prefer, in order:

1. Whatever is specified in $PYTHON
2. python3
3. python (Which is usually 2.x, but might be 3.x on some platforms.)
4. python3.11 down through python3.6

Notes:

- Python virtual environments provide binaries for "python3", "python",
  and whichever version you used to create the venv,
  e.g. "python3.8". If configure is invoked from inside of a venv, this
  configure loop will not "break out" of that venv unless that venv is
  created using an explicitly non-suitable version of Python that we
  cannot use.

- In the event that no suitable python is found, the first python found
  is the version used to generate the human-readable error message.

- The error message isn't printed right away to allow later
  configuration code to pick up an explicitly configured python.

Signed-off-by: John Snow <jsnow@redhat.com>
---
 configure | 34 ++++++++++++++++++++++++++--------
 1 file changed, 26 insertions(+), 8 deletions(-)

diff --git a/configure b/configure
index cf6db3d5518..6abf5a72078 100755
--- a/configure
+++ b/configure
@@ -592,20 +592,40 @@ esac
 
 : ${make=${MAKE-make}}
 
-# We prefer python 3.x. A bare 'python' is traditionally
-# python 2.x, but some distros have it as python 3.x, so
-# we check that too
+
+check_py_version() {
+    # We require python >= 3.6.
+    # NB: a True python conditional creates a non-zero return code (Failure)
+    "$1" -c 'import sys; sys.exit(sys.version_info < (3,6))'
+}
+
 python=
+first_python=
 explicit_python=no
-for binary in "${PYTHON-python3}" python
+# Check for $PYTHON, python3, python, then explicitly-versioned interpreters.
+for binary in "${PYTHON-python3}" ${PYTHON:+python3} python \
+                                  python3.11 python3.10 python3.9 \
+                                  python3.8 python3.7 python3.6
 do
     if has "$binary"
     then
         python=$(command -v "$binary")
-        break
+        if test -z "$first_python"; then
+           first_python=$python
+        fi
+        if check_py_version "$python"; then
+            # This one is good.
+            first_python=
+            break
+        fi
     fi
 done
 
+# If first_python is set, we didn't find a suitable binary.
+# Use this one for possible future error messages.
+if test -n "$first_python"; then
+    python="$first_python"
+fi
 
 # Check for ancillary tools used in testing
 genisoimage=
@@ -1037,9 +1057,7 @@ then
     error_exit "GNU make ($make) not found"
 fi
 
-# Note that if the Python conditional here evaluates True we will exit
-# with status 1 which is a shell 'false' value.
-if ! $python -c 'import sys; sys.exit(sys.version_info < (3,6))'; then
+if ! check_py_version "$python"; then
   error_exit "Cannot use '$python', Python >= 3.6 is required." \
       "Use --python=/path/to/python to specify a supported Python."
 fi
-- 
2.39.0