python-r1.eclass: Support python_check_deps() in python_setup

Provide an alternate mode for python_setup() that behaves similarly to
python-any-r1 eclass. If python_check_deps() function is declared
by the ebuild, the python_setup logic switches to accepting any
implementation that is in PYTHON_COMPAT, installed and satisfies
python_check_deps() independently of USE flags.

This new logic makes it possible to replace some of the existing
REQUIRED_USE constraints for build-time dependencies with more friendly
any-of dependencies. For example, if a package supports both Python 2 &
Python 3 but has a purely Python 2 build-time dependency (e.g. for
building documentation) we had to force Python 2 being enabled via
REQUIRED_USE. Using python_check_deps() with appropriate any-of
dependency, we can use Python 2 for this task without actually forcing
the user to change USE flags or install the package for Python 2.

eclass/python-r1.eclass

@@ -231,7 +231,7 @@ _python_set_globals() {
  PYTHON_DEPS=${deps}
  PYTHON_REQUIRED_USE=${requse}
  PYTHON_USEDEP=${usedep}
- readonly PYTHON_DEPS PYTHON_REQUIRED_USE PYTHON_USEDEP
+ readonly PYTHON_DEPS PYTHON_REQUIRED_USE
  fi
 }
 _python_set_globals
@@ -563,9 +563,27 @@ python_foreach_impl() {
 # @FUNCTION: python_setup
 # @USAGE: [<impl-pattern>...]
 # @DESCRIPTION:
-# Find the best (most preferred) Python implementation that is enabled
-# and matches at least one of the patterns passed (or '*' if no patterns
-# passed). Set the Python build environment up for that implementation.
+# Find the best (most preferred) Python implementation that is suitable
+# for running common Python code. Set the Python build environment up
+# for that implementation. This function has two modes of operation:
+# pure and any-of dep.
+#
+# The pure mode is used if python_check_deps() function is not declared.
+# In this case, an implementation is considered suitable if it is
+# supported (in PYTHON_COMPAT), enabled (via USE flags) and matches
+# at least one of the patterns passed (or '*' if no patterns passed).
+#
+# Implementation restrictions in the pure mode need to be accompanied
+# by appropriate REQUIRED_USE constraints. Otherwise, the eclass may
+# fail at build time due to unsatisfied dependencies.
+#
+# The any-of dep mode is used if python_check_deps() is declared.
+# In this mode, an implementation is considered suitable if it is
+# supported, matches at least one of the patterns and python_check_deps()
+# has successful return code. USE flags are not considered.
+#
+# The python_check_deps() function in the any-of mode needs to be
+# accompanied by appropriate any-of dependencies.
 #
 # The patterns can be either fnmatch-style patterns (matched via bash
 # == operator against PYTHON_COMPAT values) or '-2' / '-3' to indicate
@@ -577,11 +595,7 @@ python_foreach_impl() {
 # of python_foreach_impl calls (e.g. for shared processes like doc
 # building). python_foreach_impl sets up the build environment itself.
 #
-# If the specific commands support only a subset of Python
-# implementations, patterns need to be passed to restrict the allowed
-# implementations.
-#
-# Example:
+# Pure mode example:
 # @CODE
 # DEPEND="doc? ( dev-python/epydoc[$(python_gen_usedep 'python2*')] )"
 # REQUIRED_USE="doc? ( $(python_gen_useflags 'python2*') )"
@@ -603,6 +617,9 @@ python_setup() {
  pycompat=( ${PYTHON_COMPAT_OVERRIDE} )
  fi
 
+ local has_check_deps
+ declare -f python_check_deps >/dev/null && has_check_deps=1
+
  # (reverse iteration -- newest impl first)
  local found
  for (( i = ${#_PYTHON_SUPPORTED_IMPLS[@]} - 1; i >= 0; i-- )); do
@@ -612,14 +629,25 @@ python_setup() {
  has "${impl}" "${pycompat[@]}" || continue
 
  # match USE flags only if override is not in effect
- if [[ ! ${PYTHON_COMPAT_OVERRIDE} ]]; then
+ # and python_check_deps() is not defined
+ if [[ ! ${PYTHON_COMPAT_OVERRIDE} && ! ${has_check_deps} ]]; then
  use "python_targets_${impl}" || continue
  fi
 
  # check patterns
  _python_impl_matches "${impl}" "${@-*}" || continue
 
  python_export "${impl}" EPYTHON PYTHON
+
+ # if python_check_deps() is declared, switch into any-of mode
+ if [[ ${has_check_deps} ]]; then
+ # first check if the interpreter is installed
+ python_is_installed "${impl}" || continue
+ # then run python_check_deps
+ local PYTHON_USEDEP="python_targets_${impl}(-),python_single_target_${impl}(+)"
+ python_check_deps || continue
+ fi
+
  found=1
  break
  done