ubuntu_performance_multipass failed with "ZeroDivisionError: float division by zero"

[Po-Hsu Lin]

Test log:
21:37:38 INFO | START ubuntu_performance_multipass.multipassboot ubuntu_performance_multipass.multipassboot timestamp=1717105058 timeout=1800 localtime=May 30 21:37:38
21:37:38 DEBUG| Persistent state client._record_indent now set to 2
21:37:38 DEBUG| Persistent state client.unexpected_reboot now set to ('ubuntu_performance_multipass.multipassboot', 'ubuntu_performance_multipass.multipassboot')
21:37:38 DEBUG| Waiting for pid 5606 for 1800 seconds
21:37:38 WARNI| System python is too old, crash handling disabled
21:37:39 INFO | failed to launch
21:37:39 INFO | failed to launch
21:37:39 INFO | failed to launch
21:37:40 INFO | failed to launch
21:38:57 DEBUG| Running 'multipass list'
21:38:58 DEBUG| [stdout] Name State IPv4 Image
21:38:58 DEBUG| [stdout] test4xenial Running 10.250.98.194 Ubuntu 16.04 LTS
21:38:58 DEBUG| Running 'multipass exec test4xenial -- uname -r'
21:38:58 DEBUG| [stdout] 4.4.0-210-generic
21:38:58 DEBUG| Running 'multipass exec test4xenial -- systemd-analyze'
21:38:59 DEBUG| [stdout] Startup finished in 3.839s (kernel) + 29.210s (userspace) = 33.050s
21:39:02 ERROR| Exception escaping from test:
Traceback (most recent call last):
  File "/home/ubuntu/autotest/client/shared/test.py", line 411, in _exec
    _call_test_function(self.execute, *p_args, **p_dargs)
  File "/home/ubuntu/autotest/client/shared/test.py", line 830, in _call_test_function
    raise error.UnhandledTestFail(e)
UnhandledTestFail: Unhandled ZeroDivisionError: float division by zero
Traceback (most recent call last):
  File "/home/ubuntu/autotest/client/shared/test.py", line 823, in _call_test_function
    return func(*args, **dargs)
  File "/home/ubuntu/autotest/client/shared/test.py", line 291, in execute
    postprocess_profiled_run, args, dargs)
  File "/home/ubuntu/autotest/client/shared/test.py", line 212, in _call_run_once
    self.run_once(*args, **dargs)
  File "/home/ubuntu/autotest/client/tests/ubuntu_performance_multipass/ubuntu_performance_multipass.py", line 242, in run_once
    stddev = sqrt(float(reduce(lambda x, y: x + y, map(lambda x: (x - average) ** 2, results))) / (len(results) - 1))
ZeroDivisionError: float division by zero

[Po-Hsu Lin (cypressyew)]

This happens when calculating the sample standard deviation. When there is just one result, formula "(len(results) - 1)" will be zero.