[gimp-help/gimp-help-2-10] m4macros: improve Python 3 detection.

[Jehan <jehanp src gnome org>]

commit 9c27a4674044b5ba67ea15f6452fac47242589fd
Author: Jehan <jehan girinstud io>
Date:   Fri Jul 23 16:59:20 2021 +0200

    m4macros: improve Python 3 detection.
    
    Do all proper checks when no minimum Python version is set. In
    particular, even in such case, we must actually test against version
    3.0.0 at least and we must still do the module check.
    
    Also output a python version before doing version checks. This helps to
    diagnose and debug failing configuration (especially when done on remote
    servers with no shell access).
    
    Moreover in the loop checking various well-known python executable
    names, we must break after the version check if no module check is set.
    Right now we would have continued instead of stopping as soon as we get
    a suitable executable.
    
    When printing the Python version, print the full one (don't stop at 3
    characters).
    
    Finally improve the logging of various tests to determine the right
    Python interpreter for easier diagnostic.

 m4macros/python3dev.m4 | 78 +++++++++++++++++++++++++++++---------------------
 1 file changed, 45 insertions(+), 33 deletions(-)
---
diff --git a/m4macros/python3dev.m4 b/m4macros/python3dev.m4
index e0614bcf0..9a14e8b02 100644
--- a/m4macros/python3dev.m4
+++ b/m4macros/python3dev.m4
@@ -43,40 +43,48 @@ AC_DEFUN([AM_PATH_PYTHON3],
   AC_ARG_VAR([PYTHON], [the Python interpreter])
 
   m4_if([$1],[],[
-    dnl No version check is needed.
-    # Find any Python interpreter.
-    if test -z "$PYTHON"; then
-      AC_PATH_PROGS([PYTHON], _AM_PYTHON_INTERPRETER_LIST, :)
-    fi
-    am_display_PYTHON=python
+    dnl We still require a version check for Python >= 3.0.
+    am_display_PYTHON_MINIMUM=3.0.0
   ], [
-    dnl A version check is needed.
-    if test -n "$PYTHON"; then
-      # If the user set $PYTHON, use it and don't search something else.
-      AC_MSG_CHECKING([whether $PYTHON version >= $1])
-      AM_PYTHON3_CHECK_VERSION([$PYTHON], [$1],
-          [AC_MSG_RESULT(yes)],
-          [AC_MSG_ERROR(too old)])
-      am_display_PYTHON=$PYTHON
+    am_display_PYTHON_MINIMUM=$1
+  ])
+
+  if test -n "$PYTHON"; then
+    # If the user set $PYTHON, use it and don't search something else.
+    AC_CACHE_CHECK([$PYTHON (from \$PYTHON env variable) version], [am_cv_python_version],
+        [am_cv_python_version=`$PYTHON -c "import sys; sys.stdout.write(sys.version.split()[[0]])"`])
+    AC_MSG_CHECKING([whether $PYTHON version >= $am_display_PYTHON_MINIMUM])
+    AM_PYTHON3_CHECK_VERSION([$PYTHON], [$am_display_PYTHON_MINIMUM],
+        [AC_MSG_RESULT(yes)],
+        [AC_MSG_ERROR(too old)])
+    m4_if([$2],[],[], [AM_PYTHON_CHECK_MODULE([$PYTHON], [$2],
+                            [am_display_PYTHON=$PYTHON],
+                            [PYTHON=:])])
+  else
+    # Otherwise, try each interpreter until we find one that satisfies
+    # VERSION.
+    AC_MSG_NOTICE([checking for a Python 3 interpreter with version >= $am_display_PYTHON_MINIMUM among: 
_AM_PYTHON_INTERPRETER_LIST])
+    for am_cv_pathless_PYTHON in _AM_PYTHON_INTERPRETER_LIST none; do
+      test "$am_cv_pathless_PYTHON" = none && break
+      AC_MSG_CHECKING([$am_cv_pathless_PYTHON version])
+      am_cv_pathless_PYTHON_version=`$am_cv_pathless_PYTHON -c "import sys; 
sys.stdout.write(sys.version.split()[[0]])"`
+      AC_MSG_RESULT([$am_cv_pathless_PYTHON_version])
+      AC_MSG_CHECKING([whether $am_cv_pathless_PYTHON version >= $am_display_PYTHON_MINIMUM])
+      AM_PYTHON3_CHECK_VERSION([$am_cv_pathless_PYTHON], [$am_display_PYTHON_MINIMUM],
+                               [AC_MSG_RESULT(yes)],
+                               [AC_MSG_RESULT(too old)
+                                continue])
+      m4_if([$2],[], [break],
+            [AM_PYTHON_CHECK_MODULE([$am_cv_pathless_PYTHON], [$2], [break])])
+    done
+    # Set $PYTHON to the absolute path of $am_cv_pathless_PYTHON.
+    if test "$am_cv_pathless_PYTHON" = none; then
+      PYTHON=:
     else
-      # Otherwise, try each interpreter until we find one that satisfies
-      # VERSION.
-      AC_CACHE_CHECK([for a Python 3 interpreter with version >= $1],
-          [am_cv_pathless_PYTHON],[
-          for am_cv_pathless_PYTHON in _AM_PYTHON_INTERPRETER_LIST none; do
-          test "$am_cv_pathless_PYTHON" = none && break
-          AM_PYTHON3_CHECK_VERSION([$am_cv_pathless_PYTHON], [$1], [], [continue])
-          m4_if([$2],[],[], [AM_PYTHON_CHECK_MODULE([$am_cv_pathless_PYTHON], [$2], [break])])
-          done])
-      # Set $PYTHON to the absolute path of $am_cv_pathless_PYTHON.
-      if test "$am_cv_pathless_PYTHON" = none; then
-        PYTHON=:
-      else
-        AC_PATH_PROG([PYTHON], [$am_cv_pathless_PYTHON])
-      fi
-      am_display_PYTHON=$am_cv_pathless_PYTHON
+      AC_PATH_PROG([PYTHON], [$am_cv_pathless_PYTHON])
     fi
-  ])
+    am_display_PYTHON=$am_cv_pathless_PYTHON
+  fi
 
   if test "$PYTHON" = :; then
   dnl Run any user-specified action, or abort.
@@ -88,7 +96,7 @@ AC_DEFUN([AM_PATH_PYTHON3],
   dnl library.
 
   AC_CACHE_CHECK([for $am_display_PYTHON version], [am_cv_python_version],
-    [am_cv_python_version=`$PYTHON -c "import sys; sys.stdout.write(sys.version[[:3]])"`])
+    [am_cv_python_version=`$PYTHON -c "import sys; sys.stdout.write(sys.version.split()[[0]])"`])
   AC_SUBST([PYTHON_VERSION], [$am_cv_python_version])
 
   dnl Use the values of $prefix and $exec_prefix for the corresponding
@@ -207,7 +215,11 @@ sys.exit(gi.check_version(version))"
 AC_DEFUN([AM_PYTHON_CHECK_MODULE],
  [prog="import $2"
   AC_MSG_CHECKING(if $1 can import module $2)
-  AS_IF([AM_RUN_LOG([$1 -c "$prog"])], [$3], [$4])])
+  AS_IF([AM_RUN_LOG([$1 -c "$prog"])],
+        [AC_MSG_RESULT(yes)
+         $3],
+        [AC_MSG_RESULT(no)
+         $4])])
 
 # AM_PYTHON3_CHECK_VERSION(PROG, VERSION, [ACTION-IF-TRUE], [ACTION-IF-FALSE])
 # ---------------------------------------------------------------------------