The pending-baseline test renames its baseline into place, as SessionStart does

tests/hook_cli.rs

@@ -1339,7 +1339,13 @@ fn stop_waits_for_a_baseline_that_is_still_being_written() {
         std::thread::spawn(move || {
             std::thread::sleep(std::time::Duration::from_secs(1));
             let baseline = r#"{"ran":[["test","!","-e","red"]],"red":[["test","!","-e","red"]]}"#;
-            std::fs::write(directory.join("s-wait.json"), baseline).unwrap();
+            // Renamed into place, as `SessionStart` writes it. Written in
+            // place instead, the file exists empty for an instant, and a Stop
+            // polling for it can read that instant as an unreadable baseline.
+            // The partial file's name is not the baseline's, so no poll opens it.
+            let partial = directory.join("s-wait.json.partial");
+            std::fs::write(&partial, baseline).unwrap();
+            std::fs::rename(&partial, directory.join("s-wait.json")).unwrap();
             std::fs::remove_file(directory.join("s-wait.pending")).unwrap();
         })
     };