Validate platform settings.

cylc/flow/cfgspec/globalcfg.py

@@ -1559,6 +1559,7 @@ def load(self) -> None:
 
         self._set_default_editors()
         self._no_platform_group_name_overlap()
+        self._validate_bg_at_platforms()
 
     def _validate_source_dirs(self) -> None:
         """Check source dirs are absolute paths."""
@@ -1615,6 +1616,23 @@ def _expand_platforms(self):
                 self.sparse['platforms']
             )
 
+    def _validate_bg_at_platforms(self):
+        """The background and at job runners must have a single host.
+
+        (The Job ID is only valid on the exact submission host.)
+        """
+        bad_platforms = []
+        for name, deets in self.sparse.get('platforms').items():
+            runner = deets.get('job runner', 'background')
+            hosts = deets.get('hosts', [])
+            if runner in ('at', 'background') and len(hosts) > 1:
+                bad_platforms.append((name, runner, hosts))
+        if bad_platforms:
+            msg = '"background" and "at" are single-host job runners:'
+            for name, runner, hosts in bad_platforms:
+                msg += f'\n * {name} {runner} hosts: {", ".join(hosts)}'
+            raise GlobalConfigError(msg)
+
     def platform_dump(
         self,
         print_platform_names: bool = True,