Implemented alternatives the same way as they're in python36 module

RHEL-23019

python36.spec

@@ -25,7 +25,7 @@
 
 Name:       python36
 Version:    %{pybasever}.6
-Release:    13.buildroot%{?dist}
+Release:    14.buildroot%{?dist}
 Summary:    Interpreter of the Python programming language
 
 License:    Python
@@ -66,6 +66,11 @@ Obsoletes: python3 < 3.6.6-13
 
 Requires:   %{_libexecdir}/platform-python
 
+# Require alternatives version that implements the --keep-foreign flag
+Requires:         alternatives >= 1.19.2-1
+Requires(post):   alternatives >= 1.19.2-1
+Requires(postun): alternatives >= 1.19.2-1
+
 %global _description \
 Python is an accessible, high-level, dynamically typed, interpreted programming\
 language, designed with an emphasis on code readibility.\
@@ -90,6 +95,12 @@ Summary:    Libraries and header files needed for Python development
 Requires:   python36
 Requires:   platform-python-devel
 
+# Require alternatives version that implements the --keep-foreign flag
+Requires(postun): alternatives >= 1.19.2-1
+# python36 installs the alternatives master symlink to which we attach a slave
+Requires(post): python36
+Requires(postun): python36
+
 %description devel
 This package contains the header files and configuration needed to compile
 Python extension modules (typically written in C or C++), to embed Python
@@ -108,6 +119,12 @@ Requires:   python36
 Requires:   python36-devel
 Requires:   platform-python-debug
 
+# Require alternatives version that implements the --keep-foreign flag
+Requires(postun): alternatives >= 1.19.2-1
+# python36 installs the alternatives master symlink to which we attach a slave
+Requires(post): python36
+Requires(postun): python36
+
 %description debug
 python3-debug provides a version of the Python runtime with numerous debugging
 features enabled, aimed at advanced Python users such as developers of Python
@@ -140,13 +157,9 @@ mkdir -p %{buildroot}%{_bindir}
 
 # Symlink the executable to libexec
 ln -s %{_libexecdir}/platform-python%{pybasever} %{buildroot}%{_bindir}/python%{pybasever}
-# Additional symlink for buildroot
-ln -s %{_bindir}/python%{pybasever} %{buildroot}%{_bindir}/python3
 
 # Symlink the config executables
 ln -s python%{LDVERSION_optimized}-config %{buildroot}%{_bindir}/python%{pybasever}-config
-# Additional symlink for buildroot
-ln -s %{_bindir}/python%{pybasever}-config %{buildroot}%{_bindir}/python3-config
 
 # LDVERSION specific section
 InstallPython() {
@@ -165,20 +178,85 @@ InstallPython %{LDVERSION_debug}
 # General debug symlinks
 ln -s python%{LDVERSION_debug}-config %{buildroot}%{_bindir}/python%{pybasever}-debug-config
 ln -s python%{LDVERSION_debug} %{buildroot}%{_bindir}/python%{pybasever}-debug
-# Additional symlinks for buildroot
-ln -s %{_bindir}/python%{pybasever}-debug-config %{buildroot}%{_bindir}/python3-debug-config
-ln -s %{_bindir}/python%{pybasever}-debug %{buildroot}%{_bindir}/python3-debug
 %endif # with debug_build
 
 InstallPython %{LDVERSION_optimized}
 
+# All ghost files controlled by alternatives need to exist for the files
+# section check to succeed
+# - Don't list /usr/bin/python as a ghost file so `yum install /usr/bin/python`
+#   doesn't install this package
+touch %{buildroot}%{_bindir}/python3
+touch %{buildroot}%{_bindir}/python3-config
+touch %{buildroot}%{_bindir}/python3-debug
+touch %{buildroot}%{_bindir}/python3-debug-config
+
+%post
+# Alternative for /usr/bin/python3 -> /usr/bin/python3.6
+# Create only if it doesn't exist already
+EXISTS=`alternatives --display python3 | \
+        grep -c "^/usr/bin/python3.6 - priority [0-9]*"`
+
+if [ $EXISTS -eq 0 ]; then
+    alternatives --install %{_bindir}/python3 \
+                           python3 \
+                           %{_bindir}/python3.6 \
+                           1000000
+fi
+
+%postun
+# Do this only during uninstall process (not during update)
+if [ $1 -eq 0 ]; then
+    alternatives --keep-foreign --remove python3 \
+                        %{_bindir}/python3.6
+
+    # Remove link python → python3 if no other python3.* exists
+    if ! alternatives --display python3 > /dev/null; then
+        alternatives --keep-foreign --remove python \
+                            %{_bindir}/python3
+    fi
+fi
+
+
+%post -n python36-devel
+alternatives --add-slave python3 %{_bindir}/python3.6 \
+    %{_bindir}/python3-config \
+    python3-config \
+    %{_bindir}/python3.6-config
+
+%postun -n python36-devel
+# Do this only during uninstall process (not during update)
+if [ $1 -eq 0 ]; then
+    alternatives --keep-foreign --remove-slave python3 %{_bindir}/python3.6 \
+        python3-config
+fi
+
+
+%post -n python36-debug
+alternatives --add-slave python3 %{_bindir}/python3.6 \
+    %{_bindir}/python3-debug \
+    python3-debug \
+    %{_bindir}/python3.6-debug
+alternatives --add-slave python3 %{_bindir}/python3.6 \
+    %{_bindir}/python3-debug-config \
+    python3-debug-config \
+    %{_bindir}/python3.6-debug-config
+
+%postun -n python36-debug
+# Do this only during uninstall process (not during update)
+if [ $1 -eq 0 ]; then
+    alternatives --keep-foreign --remove-slave python3 %{_bindir}/python3.6 \
+        python3-debug
+    alternatives --keep-foreign --remove-slave python3 %{_bindir}/python3.6 \
+        python3-debug-config
+fi
 
 %files
 %license LICENSE
 %doc README.rst
 %{_bindir}/python%{pybasever}
 %{_bindir}/python%{LDVERSION_optimized}
-%{_bindir}/python3
+%ghost %{_bindir}/python3
 
 %files devel
 %license LICENSE
@@ -186,7 +264,7 @@ InstallPython %{LDVERSION_optimized}
 %{_bindir}/python%{pybasever}-config
 %{_bindir}/python%{LDVERSION_optimized}-config
 %{_bindir}/python%{LDVERSION_optimized}-*-config
-%{_bindir}/python3-config
+%ghost %{_bindir}/python3-config
 
 %files debug
 %license LICENSE
@@ -196,13 +274,17 @@ InstallPython %{LDVERSION_optimized}
 %{_bindir}/python%{LDVERSION_debug}-*-config
 %{_bindir}/python%{pybasever}-debug-config
 %{_bindir}/python%{pybasever}-debug
-%{_bindir}/python3-debug
+%ghost %{_bindir}/python3-debug
-%{_bindir}/python3-debug-config
+%ghost %{_bindir}/python3-debug-config
 
 
 #               IMPORTANT              #
 # See note at the top of the spec file #
 %changelog
+* Mon Jan 29 2024 Lumír Balhar <lbalhar@redhat.com> - 3.6.6-14.buildroot
+- Implemented alternatives the same way as they're in python36 module
+Resolves: RHEL-23019
+
 * Sat Sep 29 2018 Tomas Orsava <torsava@redhat.com> - 3.6.6-13.buildroot
 - Merged `stream-3.6` branch into the `rhel-8.0` branch for the buildroot
 - Differences: