add new clear_logfile method:

virttest/utils_logfile.py

@@ -182,3 +182,36 @@ def hook(self):
         close_log_file(log_file)
 
     return hook
+
+
+def clear_log_file(log_file_name, log_dir=None):
+    """
+    Clears the (e.g. VM) log file before a test run.
+    Closes the file if in open files.
+
+    :param log_file_name: name of file to be cleared.
+    :param log_dir: directory with logfile; default is the _log_file_dir
+    :raise LogLockError: If the lock is unavailable.
+    """
+
+    global _open_log_files, _log_file_dir, _log_lock
+
+    if log_file_name in _open_log_files:
+        close_log_file(log_file_name)  # close_log_file will acquire lock for closure
+
+    if not _acquire_lock(_log_lock):
+        raise LogLockError(
+            "Could not acquire exclusive lock to access" " _open_log_files"
+        )
+
+    try:
+        if log_dir:
+            log_file = os.path.join(log_dir, log_file_name)
+        else:
+            log_file = os.path.join(_log_file_dir, log_file_name)
+
+        if os.path.exists(log_file):
+            # Clear the log file
+            open(log_file, "w").close()
+    finally:
+        _log_lock.release()

virttest/utils_test/libvirt.py

@@ -4115,7 +4115,7 @@ def check_logfile(
 
     :param search_str: the string to be searched
     :param log_file: the given file
-    :param str_in_log: True if the file should include the given string,
+    :param str_in_log: bool, True if the file should include the given string,
                         otherwise, False
     :param cmd_parms: The parms for remote executing
     :param runner_on_target:  Remote runner
@@ -4132,6 +4132,7 @@ def check_logfile(
         cmdRes = process.run(cmd, shell=True, ignore_status=True)
     else:
         cmdRes = remote_old.run_remote_cmd(cmd, cmd_parms, runner_on_target)
+
     if str_in_log == bool(int(cmdRes.exit_status)):
         error_msg = "The string '{}' {} included in {}".format(
             search_str, "is not" if str_in_log else "is", log_file