Use a dedicated rebootstrap directory only if buildout:rebootstrap-directory is set.

README.txt

@@ -34,6 +34,13 @@ Example profile and invocation
   parts =
     realrun
 
+  # Uncomment the following to use a dedicated buildout directory to
+  # build rebootstrap python # that is mandatory if the major version
+  # of python is different # between buildout's python and rebootstrap
+  # python.
+  #
+  # rebootstrap-directory = rebootstrap
+  
   [slapospython]
   recipe = plone.recipe.command
   stop-on-error = true

slapos/rebootstrap/__init__.py

@@ -25,17 +25,23 @@ class Rebootstrap:
     self.logger = logging.getLogger(__name__)
     self.buildout = buildout
     buildout_directory = buildout['buildout']['directory']
+    # fetch section to build python, obligatory
+    self.python_section = buildout['buildout'].get('python','').strip()
+    if not self.python_section:
+      raise zc.buildout.UserError('buildout:python is not defined.')
+    if self.python_section not in buildout:
+      raise zc.buildout.UserError('[%s] is not defined.' % self.python_section)
+    self.wanted_python = buildout[self.python_section]['executable']
+    rebootstrap_directory = buildout['buildout'].get('rebootstrap-directory')
+    if rebootstrap_directory:
       self.rebootstrap_directory = os.path.join(
         buildout_directory, 'rebootstrap'
       )
-    # fetch section to build python, obligatory
-    self.python_section = (
-      buildout['buildout'].get('python') or \
-      buildout['rebootstrap']['section']
-    ).strip()
-    self.wanted_python = buildout[self.python_section]['executable'].replace(
-      buildout_directory, self.rebootstrap_directory
+      self.wanted_python = self.wanted_python.replace(
+        buildout_directory, self.rebootstrap_directory, 1
       )
+    else:
+      self.rebootstrap_directory = buildout_directory
     # query for currently running python
     self.running_python = sys.executable
 
@@ -43,6 +49,10 @@ class Rebootstrap:
     if self.running_python != self.wanted_python:
       self.install_section()
       self.reboot()
+    elif self.python_section:
+      buildout = self.buildout['buildout']
+      if self.python_section not in buildout['parts'].split():
+        buildout['parts'] = self.python_section + '\n' + buildout['parts']
 
   def reboot(self):
     message = """
@@ -58,12 +68,13 @@ Buildout will be restarted automatically to have this change applied.
     self.logger.info(message)
     args = map(zc.buildout.easy_install._safe_arg, sys.argv)
     env = os.environ
+    if 'ORIG_PYTHON' not in env:
+      env['ORIG_PYTHON'] = sys.executable
     os.execve(self.wanted_python, [self.wanted_python] + args, env)
 
   def install_section(self):
-    # For safety, we always try to install/update instead of the condition
-    # if not os.path.exists(self.wanted_python):
-    if True:
+    if not os.path.exists(self.wanted_python) or \
+        self.rebootstrap_directory != self.buildout['buildout']['directory']:
       self.logger.info('Installing section %r to provide %r' % (
         self.python_section, self.wanted_python))
       args = map(zc.buildout.easy_install._safe_arg, sys.argv)
@@ -118,3 +129,26 @@ Buildout will be restarted automatically to have this change applied.
           '%r\nUnfortunately even after installing this section executable was'
           ' not found.\nThis is section responsibility to provide python (eg. '
           'by compiling it).' % (self.python_section, self.wanted_python))
+
+_uninstall_part_orig = zc.buildout.buildout.Buildout._uninstall_part
+def _uninstall_part(self, part, installed_part_options):
+  _uninstall_part_orig(self, part, installed_part_options)
+  try:
+    location = self[part].get('location')
+  except zc.buildout.buildout.MissingSection:
+    return
+  if location and sys.executable.startswith(location):
+    message = """
+************ REBOOTSTRAP: IMPORTANT NOTICE ************
+%r part that provides the running Python is uninstalled.
+Buildout will be restarted automatically with the original Python.
+************ REBOOTSTRAP: IMPORTANT NOTICE ************
+""" % part
+    self._logger.info(message)
+    if getattr(self, 'dry_run', False):
+      sys.exit()
+    args = map(zc.buildout.easy_install._safe_arg, sys.argv)
+    env = os.environ
+    orig_python = env['ORIG_PYTHON']
+    os.execve(orig_python, [orig_python] + args, env)
+zc.buildout.buildout.Buildout._uninstall_part = _uninstall_part