#3357: preserve concurrent session log identity

crates/engine/src/system/logging.rs

@@ -214,10 +214,16 @@ fn rotate(dir: &Path) -> std::io::Result<(PathBuf, File)> {
         }
     }
     let path = numbered(dir, 0);
-    let file = File::create(&path)?;
+    let file = create_session_file(&path)?;
     Ok((path, file))
 }
 
+// Another process may create the current log after our rotation. Never truncate its
+// file: its writer and crash reporter may already hold the same inode open.
+fn create_session_file(path: &Path) -> std::io::Result<File> {
+    std::fs::OpenOptions::new().write(true).create_new(true).open(path)
+}
+
 /// Rotate the session logs in `dir` (the `Logs` folder, see [`log_dir`]) and write records to
 /// a fresh `effectcraft.log` there, first the ones logged so far; returns its path.
 pub fn attach_session_log(dir: &Path) -> Result<PathBuf, String> {
@@ -428,6 +434,20 @@ mod tests {
         assert_eq!(timestamp(u64::MAX), "584556019-04-03T14:25:51.615Z");
     }
 
+    #[test]
+    fn concurrent_session_creation_does_not_truncate_the_winner() {
+        let dir = temp_dir("concurrent-create");
+        std::fs::create_dir_all(&dir).unwrap();
+        let path = dir.join(SESSION_LOG);
+        let mut winner = create_session_file(&path).unwrap();
+        winner.write_all(b"crash-report session 1-2\nwinner content\n").unwrap();
+        assert_eq!(create_session_file(&path).unwrap_err().kind(), std::io::ErrorKind::AlreadyExists);
+        winner.write_all(b"still the same session\n").unwrap();
+        assert_eq!(read(&path), "crash-report session 1-2\nwinner content\nstill the same session\n");
+        drop(winner);
+        std::fs::remove_dir_all(dir).unwrap();
+    }
+
     /// Each launch keeps the previous runs' logs, so the log of a run that crashed survives.
     #[test]
     fn rotation_keeps_the_previous_logs_and_starts_an_empty_file() {