Use DOCKER_* settings for config validation

Remove all the hardcoded valid options/choices from the Config objects
and get them from Django settings instead.

Author: humitos

readthedocs/config/config.py

@@ -146,6 +146,8 @@ class BuildConfigBase:
         'mkdocs',
         'submodules',
     ]
+    valid_build_images = [k.split(':')[1] for k in DOCKER_IMAGE_SETTINGS]
+    default_build_image = DOCKER_DEFAULT_VERSION
     version = None
 
     def __init__(self, env_config, raw_config, source_file):
@@ -246,6 +248,24 @@ def python_full_version(self):
             )
         return ver
 
+    def get_valid_python_versions_for_image(self, build_image):
+        """
+        Return all the valid Python versions for a Docker image.
+
+        The Docker image (``build_image``) has to be its complete name, already
+        validated: ``readthedocs/build:4.0``, not just ``4.0``.
+
+        Returns supported versions for the ``DOCKER_DEFAULT_VERSION`` if not
+        ``build_image`` found.
+        """
+
+        if build_image not in DOCKER_IMAGE_SETTINGS:
+            build_image = '{}:{}'.format(
+                DOCKER_DEFAULT_IMAGE,
+                self.default_build_image,
+            )
+        return DOCKER_IMAGE_SETTINGS[build_image]['python']['supported_versions']
+
     def as_dict(self):
         config = {}
         for name in self.PUBLIC_ATTRIBUTES:
@@ -268,18 +288,23 @@ class BuildConfigV1(BuildConfigBase):
         '"python.extra_requirements" section must be a list.'
     )
 
-    PYTHON_SUPPORTED_VERSIONS = [2, 2.7, 3, 3.5]
-    DOCKER_SUPPORTED_VERSIONS = ['1.0', '2.0', 'latest']
-
     version = '1'
 
     def get_valid_python_versions(self):
-        """Get all valid python versions."""
+        """
+        Return all valid Python versions.
+
+        .. note::
+
+            It does not take current build image used into account.
+        """
         try:
             return self.env_config['python']['supported_versions']
         except (KeyError, TypeError):
-            pass
-        return self.PYTHON_SUPPORTED_VERSIONS
+            versions = set()
+            for name, options in DOCKER_IMAGE_SETTINGS.items():
+                versions = versions.union(options['python']['supported_versions'])
+            return versions
 
     def get_valid_formats(self):  # noqa
         """Get all valid documentation formats."""
@@ -339,7 +364,7 @@ def validate_build(self):
                 with self.catch_validation_error('build'):
                     build['image'] = validate_choice(
                         str(_build['image']),
-                        self.DOCKER_SUPPORTED_VERSIONS,
+                        self.valid_build_images,
                     )
             if ':' not in build['image']:
                 # Prepend proper image name to user's image name
@@ -577,8 +602,6 @@ class BuildConfigV2(BuildConfigBase):
 
     version = '2'
     valid_formats = ['htmlzip', 'pdf', 'epub']
-    valid_build_images = ['1.0', '2.0', '3.0', 'stable', 'latest']
-    default_build_image = 'latest'
     valid_install_method = [PIP, SETUPTOOLS]
     valid_sphinx_builders = {
         'html': 'sphinx',
@@ -793,13 +816,7 @@ def get_valid_python_versions(self):
         This should be called after ``validate_build()``.
         """
         build_image = self.build.image
-        if build_image not in DOCKER_IMAGE_SETTINGS:
-            build_image = '{}:{}'.format(
-                DOCKER_DEFAULT_IMAGE,
-                self.default_build_image,
-            )
-        python = DOCKER_IMAGE_SETTINGS[build_image]['python']
-        return python['supported_versions']
+        return self.get_valid_python_versions_for_image(build_image)
 
     def validate_doc_types(self):
         """

readthedocs/settings/base.py

@@ -272,7 +272,7 @@ def USE_PROMOS(self):  # noqa
     # Docker
     DOCKER_ENABLE = False
     DOCKER_DEFAULT_IMAGE = 'readthedocs/build'
-    DOCKER_DEFAULT_VERSION = '3.0'
+    DOCKER_DEFAULT_VERSION = 'stable'
     DOCKER_IMAGE = '{}:{}'.format(DOCKER_DEFAULT_IMAGE, DOCKER_DEFAULT_VERSION)
     DOCKER_IMAGE_SETTINGS = {
         'readthedocs/build:1.0': {
@@ -288,6 +288,8 @@ def USE_PROMOS(self):  # noqa
             'python': {'supported_versions': [2, 2.7, 3, 3.5, 3.6, 3.7]},
         },
     }
+
+    # Alias tagged via ``docker tag`` on the build servers
     DOCKER_IMAGE_SETTINGS.update({
         'readthedocs/build:stable': DOCKER_IMAGE_SETTINGS.get('readthedocs/build:3.0'),
         'readthedocs/build:latest': DOCKER_IMAGE_SETTINGS.get('readthedocs/build:4.0'),