MAG L1C: harden previous-day handling per independent review

- Ignore (with a warning) a previous day dataset that has no epoch
  variable instead of raising KeyError.
- Log skipped inheritance (too few pre-midnight samples, unknown
  cadence) at warning level to match the other unusable-neighbor paths.
- Select the previous-day file by the job descriptor (norm + sensor)
  rather than "norm" alone, so a wrong-sensor file can never shadow the
  right one.
- Drop the "exactly as before" claim from the mag_l1c docstring: the
  generate_missing_timestamps precision fix means gap fills at 8+ vec/s
  can differ from previously generated products by up to 128 ns even
  without a previous-day file.

Author: sapols

imap_processing/cli.py

@@ -1360,7 +1360,7 @@ def do_processing(  # noqa: PLR0912
                 for path in dependencies.get_valid_inputs_for_start_date(
                     start_datetime - timedelta(days=1)
                 ).get_file_paths(source="mag", data_type="l1b")
-                if "norm" in path.name
+                if self.descriptor in path.name
             ]
             previous_day_dataset = (
                 load_cdf(previous_day_files[0]) if previous_day_files else None

imap_processing/mag/l1c/mag_l1c.py

@@ -46,7 +46,7 @@ def mag_l1c(
         current day opens with a gap, timestamps generated for that gap continue the
         previous day's cadence and phase so the L1C timeline stays regular across the
         day boundary. If not provided (or not usable), gaps at the start of the day
-        are filled on the current day's own grid, exactly as before.
+        are filled on the current day's own grid.
 
     Returns
     -------
@@ -321,7 +321,10 @@ def _validated_previous_day(
             f"expected normal mode L1B data for sensor mag{sensor}."
         )
         return None
-    if previous_day_dataset["epoch"].data.size == 0:
+    if (
+        "epoch" not in previous_day_dataset
+        or previous_day_dataset["epoch"].data.size == 0
+    ):
         logger.warning("Ignoring previous day dataset with no epochs.")
         return None
     return previous_day_dataset
@@ -390,7 +393,7 @@ def _previous_day_grid(
     previous_epochs = previous_day_dataset["epoch"].data
     anchor_index = int(np.searchsorted(previous_epochs, midnight_ns, side="left")) - 1
     if anchor_index < 1:
-        logger.info(
+        logger.warning(
             "Previous day dataset has fewer than two samples before the current day; "
             "not inheriting its timeline."
         )
@@ -407,7 +410,7 @@ def _previous_day_grid(
             rate = vecsec.value
             break
     if rate is None:
-        logger.info(
+        logger.warning(
             f"Previous day dataset ends with sample spacing {anchor_spacing} ns, "
             f"which matches no known MAG rate; not inheriting its timeline."
         )

imap_processing/tests/mag/test_mag_l1c.py

@@ -1377,3 +1377,16 @@ def test_process_mag_l1c_previous_day_anchor_ignores_buffer_samples():
     assert leading.size > 0
     assert leading[0] == anchor + cadence
     assert np.all((leading - anchor) % cadence == 0)
+
+
+def test_mag_l1c_ignores_previous_day_without_epochs():
+    """A previous day dataset with no epoch variable is ignored, not raised."""
+    day1 = np.datetime64("2025-01-01")
+    day2 = np.datetime64("2025-01-02")
+    _, norm_day2, burst_day2, _ = _build_cross_day_l1b(day1, day2)
+    no_epochs = xr.Dataset(attrs={"Logical_source": "imap_mag_l1b_norm-mago"})
+
+    baseline = mag_l1c(norm_day2, day2, burst_day2)
+    output = mag_l1c(norm_day2, day2, burst_day2, previous_day_dataset=no_epochs)
+
+    assert np.array_equal(output["epoch"].data, baseline["epoch"].data)