Implement explicit environment assignments for `bst shell`

This introduces a new `environment` section of the project `shell`
configuration to set explicit env vars when running a non-isolated shell.

This supports host environment variable expansion.

This is a part of addressing #223

2 files changed, 13 insertions, 2 deletions

diff --git a/buildstream/_project.py b/buildstream/_project.py
index 360bf7faa..253d5210d 100644
--- a/buildstream/_project.py
+++ b/buildstream/_project.py
@@ -39,7 +39,7 @@ from ._sourcefactory import SourceFactory
 # This version is bumped whenever enhancements are made
 # to the `project.conf` format or the core element format.
 #
-BST_FORMAT_VERSION = 3
+BST_FORMAT_VERSION = 4
 
 # The separator we use for user specified aliases
 _ALIAS_SEPARATOR = ':'
@@ -104,6 +104,7 @@ class Project():
         # Shell options
         self._shell_command = []      # The default interactive shell command
         self._shell_env_inherit = []  # Environment vars to inherit when non-isolated
+        self._shell_environment = {}  # Statically set environment vars
         self._shell_host_files = []   # A list of HostMount objects
 
         profile_start(Topics.LOAD_PROJECT, self.directory.replace(os.sep, '-'))
@@ -298,12 +299,18 @@ class Project():
 
         # Parse shell options
         shell_options = _yaml.node_get(config, Mapping, 'shell', default_value={})
-        _yaml.node_validate(shell_options, ['command', 'environment-inherit', 'host-files'])
+        _yaml.node_validate(shell_options, ['command', 'environment-inherit', 'environment', 'host-files'])
         self._shell_command = _yaml.node_get(shell_options, list, 'command',
                                              default_value=['sh', '-i'])
         self._shell_env_inherit = _yaml.node_get(shell_options, list, 'environment-inherit',
                                                  default_value=[])
 
+        # Perform environment expansion right away
+        shell_environment = _yaml.node_get(shell_options, Mapping, 'environment', default_value={})
+        for key, _ in _yaml.node_items(shell_environment):
+            value = _yaml.node_get(shell_environment, str, key)
+            self._shell_environment[key] = os.path.expandvars(value)
+
         # Host files is parsed as a list for convenience
         host_files = _yaml.node_get(shell_options, list, 'host-files', default_value=[])
         for host_file in host_files:
diff --git a/buildstream/element.py b/buildstream/element.py
index bf36ee6ab..ad0e87bcf 100644
--- a/buildstream/element.py
+++ b/buildstream/element.py
@@ -1439,6 +1439,10 @@ class Element(Plugin):
                     if os.environ.get(inherit) is not None:
                         environment[inherit] = os.environ.get(inherit)
 
+                # Now add in the explicitly set environment variables
+                for key, value in _yaml.node_items(project._shell_environment):
+                    environment[key] = value
+
                 # Setup any requested bind mounts
                 if mounts is None:
                     mounts = []