aggregators: Don't crash interpolating across gaps in the data

[ooonea]

Problem

The time series aggregator builds the synthetic "totals"/"avg" series by linearly interpolating each component series onto the common step grid. When a series contains a None value — a gap, as produced by sparse netperf output at low rates combined with latency — and that None ends up as an interpolation anchor, it computes dv_dt = (v_next - v_prev) / (t_next - t_prev) and raises:

TypeError: unsupported operand type(s) for -: 'NoneType' and 'float'

aborting the whole run in postprocess(aggregate(...)) before any data file is written, so the result is lost.

This is the low-throughput-plus-latency scenario from #265 (which reports the resulting gaps); here a gap used as an interpolation anchor crashes the run outright. I hit it reliably running RRUL from a router through an SQM-shaped Starlink uplink (~15 Mbit/s shaped upload + ~60 ms RTT → sparse TCP_STREAM samples). It does not reproduce on a clean loopback run (no gaps), which is why it can go unnoticed.

Fix

The code already guards t_prev is None (start/end of a series). This guards the anchor values too: when v_prev or v_next is None, leave a gap (None) — the same thing already done when the interpolation distance is too long — instead of crashing. The valid-data path is unchanged.

Test

unittests/test_aggregators.py drives TimeseriesAggregator.aggregate() with a minimal series containing a None gap: it raises the TypeError on current main and passes with this change.

Note

This fixes the crash; the gaps discussed in #265 remain (interpolating across very sparse data is not well-defined, as you noted there). Happy to adjust the approach.

[tohojo]

Thank you for the patch!

The fix looks good, but the test is not actually run as part of the test suite. Please add it to the top-level tests; something like this (on top of your patch):

diff --git i/unittests/__init__.py w/unittests/__init__.py
index 043569ccca37..5b2c97052d14 100644
--- i/unittests/__init__.py
+++ w/unittests/__init__.py
@@ -24,6 +24,7 @@ from __future__ import absolute_import, division, print_function, unicode_litera
 import os
 import unittest
 from . import test_util
+from . import test_aggregators
 from . import test_formatters
 from . import test_metadata
 from . import test_parsers
@@ -33,6 +34,7 @@ from . import test_gui
 
 
 test_suite = unittest.TestSuite([test_util.test_suite,
+                                 test_aggregators.test_suite,
                                  test_formatters.test_suite,
                                  test_metadata.test_suite,
                                  test_parsers.test_suite,
diff --git i/unittests/test_aggregators.py w/unittests/test_aggregators.py
index 8d21c75c4804..84cf927595b9 100644
--- i/unittests/test_aggregators.py
+++ w/unittests/test_aggregators.py
@@ -66,6 +66,7 @@ class TestTimeseriesAggregator(unittest.TestCase):
         self.assertTrue(any(v is not None for v in data),
                         "valid data points should still be interpolated")
 
+test_suite = unittest.TestLoader().loadTestsFromTestCase(TestTimeseriesAggregator)
 
 if __name__ == "__main__":
     unittest.main()