element.py: Switch to SandboxRemote if config option is set

buildstream/element.py

@@ -2126,7 +2126,24 @@ class Element(Plugin):
         project = self._get_project()
         platform = Platform.get_platform()
 
-        if directory is not None and os.path.exists(directory):
+        if self.__remote_execution_url is not None and self.BST_VIRTUAL_DIRECTORY:
+            if not self.__artifacts.has_push_remotes(element=self):
+                # Give an early warning if remote execution will not work
+                raise ElementError("Artifact {} is configured to use remote execution but has no push remotes. "
+                                   .format(self.name) +
+                                   "The remote artifact server(s) may not be correctly configured or contactable.")
+
+            self.info("Using a remote 'sandbox' for artifact {}".format(self.name))
+            sandbox = SandboxRemote(context, project,
+                                              directory,
+                                              stdout=stdout,
+                                              stderr=stderr,
+                                              config=config,
+                                              server_url=self.__remote_execution_url,
+                                              allow_real_directory=False)
+            yield sandbox
+        elif directory is not None and os.path.exists(directory):
+            self.info("Using a local sandbox for artifact {}".format(self.name))
             sandbox = platform.create_sandbox(context, project,
                                               directory,
                                               stdout=stdout,