Check that psd is not None before using ifo for followup

[maxtrevor]

The PyCBC Live analysis of O3 replay data recently encountered a bug we had not seen before.

In one analysis stride we see

2024-12-13T00:57:02.290-08:00 pycbc-live-test 0 V1 time has invalid data, resetting buffer
2024-12-13T00:57:02.290-08:00 pycbc-live-test 0 Insufficient data for V1 analysis

and then later in the same stride:

2024-12-13T00:57:08.820-08:00 pycbc-live-test 0 Found H1-L1 coinc with ifar 0.0008384836735184549
2024-12-13T00:57:08.821-08:00 pycbc-live-test 0 computing followup data for coinc
2024-12-13T00:57:08.822-08:00 pycbc-live-test 0 Generating SPAtmplt, duration 104.0 s, index 215458, starting from 23.1 Hz
2024-12-13T00:57:08.951-08:00 pycbc-live-test 0 Generating SPAtmplt, duration 240.0 s, index 215458, starting from 23.1 Hz
Traceback (most recent call last):
  File "/home/pycbc.live/.conda/envs/o4-test-env-2024-11-01/lib/python3.9/runpy.py", line 197, in _run_module_as_main
    return _run_code(code, main_globals, None,
  File "/home/pycbc.live/.conda/envs/o4-test-env-2024-11-01/lib/python3.9/runpy.py", line 87, in _run_code
    exec(code, run_globals)
  File "/home/pycbc.live/.conda/envs/o4-test-env-2024-11-01/lib/python3.9/site-packages/mpi4py/__main__.py", line 7, in <module>
    main()
  File "/home/pycbc.live/.conda/envs/o4-test-env-2024-11-01/lib/python3.9/site-packages/mpi4py/run.py", line 198, in main
    run_command_line(args)
  File "/home/pycbc.live/.conda/envs/o4-test-env-2024-11-01/lib/python3.9/site-packages/mpi4py/run.py", line 47, in run_command_line
    run_path(sys.argv[0], run_name='__main__')
  File "/home/pycbc.live/.conda/envs/o4-test-env-2024-11-01/lib/python3.9/runpy.py", line 288, in run_path
    return _run_module_code(code, init_globals, run_name,
  File "/home/pycbc.live/.conda/envs/o4-test-env-2024-11-01/lib/python3.9/runpy.py", line 97, in _run_module_code
    _run_code(code, mod_globals, init_globals,
  File "/home/pycbc.live/.conda/envs/o4-test-env-2024-11-01/lib/python3.9/runpy.py", line 87, in _run_code
    exec(code, run_globals)
  File "/home/pycbc.live/.conda/envs/o4-test-env-2024-11-01/bin/pycbc_live", line 1379, in <module>
    evnt.check_coincs(list(results.keys()), best_coinc, psds)
  File "/home/pycbc.live/.conda/envs/o4-test-env-2024-11-01/bin/pycbc_live", line 520, in check_coincs
    sld = self.compute_followup_data(
  File "/home/pycbc.live/.conda/envs/o4-test-env-2024-11-01/bin/pycbc_live", line 221, in compute_followup_data
    pvalue_info = followup_event_significance(
  File "/home/pycbc.live/.conda/envs/o4-test-env-2024-11-01/lib/python3.9/site-packages/pycbc/filter/matchedfilter.py", line 1955, in followup_event_significance
    stilde = data_reader.overwhitened_data(htilde.delta_f)
  File "/home/pycbc.live/.conda/envs/o4-test-env-2024-11-01/lib/python3.9/site-packages/pycbc/strain/strain.py", line 1773, in overwhitened_data
    psdt = pycbc.psd.interpolate(self.psd, fseries.delta_f)
  File "/home/pycbc.live/.conda/envs/o4-test-env-2024-11-01/lib/python3.9/site-packages/pycbc/psd/estimate.py", line 313, in interpolate
    new_n = (len(series)-1) * series.delta_f / delta_f + 1
TypeError: object of type 'NoneType' has no len()

I have diagnosed what happened as follows:

• when advancing the V1 StrainBuffer, part of the data is found to be invalid by https://github.com/gwastro/pycbc/blob/master/pycbc/strain/strain.py#L1888-L1897
• This results in the V1 psd being set to None by https://github.com/gwastro/pycbc/blob/master/pycbc/strain/strain.py#L1686
• A coinc event was found in the same stride and attempted to use the V1 data for followup
• The use of V1 data for followup somehow got past the check https://github.com/gwastro/pycbc/blob/master/pycbc/filter/matchedfilter.py#L1928
• The analysis reached https://github.com/gwastro/pycbc/blob/master/pycbc/filter/matchedfilter.py#L1955 and tried to overwhiten the data
• The function call https://github.com/gwastro/pycbc/blob/master/pycbc/strain/strain.py#L1773 produced an error because it was passed the V1 psd that had previously been set to None

While I am not sure why the check https://github.com/gwastro/pycbc/blob/master/pycbc/filter/matchedfilter.py#L1928 did not catch that the data was invalid, adding an explicit check whether the psd is None should prevent this error from happening again.