Require conda.file when using conda in v1

Some people are migrating from pip to conda,
but they were using a v1 config,
then when migrating they are using the conda option from v2.

We are not validating this, but using conda.file is mandatory
when using the conda option (or rtd will fail without any reason).

Close #5322
Close #5228

readthedocs/config/config.py

@@ -489,15 +489,14 @@ class BuildConfigV1(BuildConfigBase):
             raw_conda = self.raw_config['conda']
             with self.catch_validation_error('conda'):
                 validate_dict(raw_conda)
-            conda_environment = None
-            if 'file' in raw_conda:
-                with self.catch_validation_error('conda.file'):
-                    conda_environment = validate_file(
-                        raw_conda['file'],
-                        self.base_path,
-                    )
-            conda['environment'] = conda_environment
-
+            with self.catch_validation_error('conda.file'):
+                if 'file' not in raw_conda:
+                    raise ValidationError('file', VALUE_NOT_FOUND)
+                conda_environment = validate_file(
+                    raw_conda['file'],
+                    self.base_path,
+                )
+                conda['environment'] = conda_environment
             return conda
         return None
 

readthedocs/config/tests/test_config.py

@@ -582,6 +582,18 @@ def test_validates_conda_file(tmpdir):
     assert build.conda.environment == str(tmpdir.join('environment.yml'))
 
 
+def test_file_is_required_when_using_conda(tmpdir):
+    apply_fs(tmpdir, {'environment.yml': ''})
+    build = get_build_config(
+        {'conda': {'foo': 'environment.yml'}},
+        source_file=str(tmpdir.join('readthedocs.yml')),
+    )
+    with raises(InvalidConfig) as excinfo:
+        build.validate()
+    assert excinfo.value.key == 'conda.file'
+    assert excinfo.value.code == VALUE_NOT_FOUND
+
+
 def test_requirements_file_empty():
     build = get_build_config({})
     build.validate()