Set default mirror via command-line or user config

In user config (buildstream.conf), it is set with the "default-mirror"
field.

On the command-line, it is set with "--default-mirror"

buildstream/_context.py

@@ -259,7 +259,7 @@ class Context():
         # Shallow validation of overrides, parts of buildstream which rely
         # on the overrides are expected to validate elsewhere.
         for _, overrides in _yaml.node_items(self._project_overrides):
-            _yaml.node_validate(overrides, ['artifacts', 'options', 'strict'])
+            _yaml.node_validate(overrides, ['artifacts', 'options', 'strict', 'default-mirror'])
 
         profile_end(Topics.LOAD_CONTEXT, 'load')
 

buildstream/_frontend/app.py

@@ -203,7 +203,8 @@ class App():
         # Load the Project
         #
         try:
-            self.project = Project(directory, self.context, cli_options=self._main_options['option'])
+            self.project = Project(directory, self.context, cli_options=self._main_options['option'],
+                                   default_mirror=self._main_options.get('default_mirror'))
         except LoadError as e:
 
             # Let's automatically start a `bst init` session in this case

buildstream/_frontend/cli.py

@@ -217,6 +217,8 @@ def print_version(ctx, param, value):
               help="Elements must be rebuilt when their dependencies have changed")
 @click.option('--option', '-o', type=click.Tuple([str, str]), multiple=True, metavar='OPTION VALUE',
               help="Specify a project option")
+@click.option('--default-mirror', default=None,
+              help="The mirror to fetch from first, before attempting other mirrors")
 @click.pass_context
 def cli(context, **kwargs):
     """Build and manipulate BuildStream projects

buildstream/_project.py

@@ -67,7 +67,7 @@ class HostMount():
 #
 class Project():
 
-    def __init__(self, directory, context, *, junction=None, cli_options=None):
+    def __init__(self, directory, context, *, junction=None, cli_options=None, default_mirror=None):
 
         # The project name
         self.name = None
@@ -92,7 +92,7 @@ class Project():
         self.element_overrides = {}              # Element specific configurations
         self.source_overrides = {}               # Source specific configurations
         self.mirrors = OrderedDict()             # contains dicts of alias-mappings to URIs.
-        self.default_mirror = None               # The name of the preferred mirror.
+        self.default_mirror = default_mirror     # The name of the preferred mirror.
 
         #
         # Private Members
@@ -319,6 +319,10 @@ class Project():
         #
         self.options.process_node(config)
 
+        # Override default_mirror if not set by command-line
+        if not self.default_mirror:
+            self.default_mirror = _yaml.node_get(overrides, str, 'default-mirror', default_value=None)
+
         #
         # Now all YAML composition is done, from here on we just load
         # the values from our loaded configuration dictionary.

tests/completions/completions.py

@@ -27,6 +27,7 @@ MAIN_OPTIONS = [
     "--colors ",
     "--config ",
     "--debug ",
+    "--default-mirror ",
     "--directory ",
     "--error-lines ",
     "--fetchers ",