cli: Interactively warn if the user is trying to close the workspace they're...

cli: Interactively warn if the user is trying to close the workspace they're using to load the project

This involves changes in:
* _stream.py:
  * Add the helper Stream.workspace_is_required()
* userconfig.yaml:
  * Add a default value for prompt.really-workspace-close-project-inaccessible
* _context.py:
  * Load the prompt 'really-workspace-close-project-inaccessible' from
    user config.
* cli.py:
  * If buildstream is invoked interactively, prompt the user to confirm
    that they want to close the workspace they're using to load this
    project.

buildstream/_context.py

@@ -121,6 +121,10 @@ class Context():
         # remove a workspace directory.
         self.prompt_workspace_close_remove_dir = None
 
+        # Boolean, whether we double-check with the user that they meant to
+        # close the workspace when they're using it to access the project.
+        self.prompt_workspace_close_project_inaccessible = None
+
         # Boolean, whether we double-check with the user that they meant to do
         # a hard reset of a workspace, potentially losing changes.
         self.prompt_workspace_reset_hard = None
@@ -249,12 +253,15 @@ class Context():
             defaults, Mapping, 'prompt')
         _yaml.node_validate(prompt, [
             'auto-init', 'really-workspace-close-remove-dir',
+            'really-workspace-close-project-inaccessible',
             'really-workspace-reset-hard',
         ])
         self.prompt_auto_init = _node_get_option_str(
             prompt, 'auto-init', ['ask', 'no']) == 'ask'
         self.prompt_workspace_close_remove_dir = _node_get_option_str(
             prompt, 'really-workspace-close-remove-dir', ['ask', 'yes']) == 'ask'
+        self.prompt_workspace_close_project_inaccessible = _node_get_option_str(
+            prompt, 'really-workspace-close-project-inaccessible', ['ask', 'yes']) == 'ask'
         self.prompt_workspace_reset_hard = _node_get_option_str(
             prompt, 'really-workspace-reset-hard', ['ask', 'yes']) == 'ask'
 

buildstream/_frontend/cli.py

@@ -747,11 +747,18 @@ def workspace_close(app, remove_dir, all_, elements):
 
         elements = app.stream.redirect_element_names(elements)
 
-        # Check that the workspaces in question exist
+        # Check that the workspaces in question exist, and that it's safe to
+        # remove them.
         nonexisting = []
         for element_name in elements:
             if not app.stream.workspace_exists(element_name):
                 nonexisting.append(element_name)
+            if (app.stream.workspace_is_required(element_name) and app.interactive and
+                    app.context.prompt_workspace_close_project_inaccessible):
+                click.echo("Removing '{}' will prevent you from running buildstream commands".format(element_name))
+                if not click.confirm('Are you sure you want to close this workspace?'):
+                    click.echo('Aborting', err=True)
+                    sys.exit(-1)
         if nonexisting:
             raise AppError("Workspace does not exist", detail="\n".join(nonexisting))
 

buildstream/_stream.py

@@ -714,6 +714,20 @@ class Stream():
 
         return False
 
+    # workspace_is_required()
+    #
+    # Checks whether the workspace belonging to element_name is required to
+    # load the project
+    #
+    # Args:
+    #    element_name (str): The element whose workspace may be required
+    #
+    # Returns:
+    #    (bool): True if the workspace is required
+    def workspace_is_required(self, element_name):
+        required_elm = self._project.required_workspace_element()
+        return required_elm == element_name
+
     # workspace_list
     #
     # Serializes the workspaces and dumps them in YAML to stdout.

buildstream/data/userconfig.yaml

@@ -128,6 +128,14 @@ prompt:
   #
   really-workspace-close-remove-dir: ask
 
+  # Whether to really proceed with 'bst workspace close' when doing so would
+  # stop them from running bst commands in this workspace.
+  #
+  #  ask - Ask the user if they are sure.
+  #  yes - Always close, without asking.
+  #
+  really-workspace-close-project-inaccessible: ask
+
   # Whether to really proceed with 'bst workspace reset' doing a hard reset of
   # a workspace, potentially losing changes.
   #