Remediate the 2026-09-03 feature-test findings

shopdb/utils/datevalues.py

@@ -0,0 +1,82 @@
+"""Parsing a date out of a request body, strictly.
+
+WHY THIS EXISTS
+
+Two plugins carried an identical private `_parse_date` that swallowed the
+error:
+
+    try:
+        return datetime.strptime(str(value)[:10], '%Y-%m-%d').date()
+    except (ValueError, TypeError):
+        return None
+
+So an unparseable date and a deliberately cleared one produced the same
+result, and the write went ahead. `PUT /api/warranty/<id>` with enddate
+'not-a-date' answered 200 "Warranty updated" and stored NULL - the warranty
+then vanished from every expiry surface, silently, on the strength of a typo.
+The measuring-tool calibration date did the same thing and dropped the tool out
+of the calibration report.
+
+The rule here: an EXPLICIT empty value clears the field, and anything that is
+not a date is refused. Those are different intents and they get different
+answers. Callers turn `DateValueError` into a 400 naming the field, so the
+person who mistyped it finds out at the point of the mistake rather than at the
+next audit.
+
+Accepted forms are deliberately narrow - 'YYYY-MM-DD', and the ISO-8601
+datetime forms whose leading ten characters are a date, since that is what the
+frontend's date inputs and every vendor API in the tree emit. Anything looser
+is a guess about what somebody meant.
+"""
+
+from datetime import date, datetime
+
+
+class DateValueError(ValueError):
+    """A supplied value was neither empty nor a date this accepts.
+
+    Carries the field name so the error message can say which one, which is the
+    whole point of refusing rather than nulling.
+    """
+
+    def __init__(self, field, value):
+        self.field = field
+        self.value = value
+        super().__init__(
+            '%s must be a date as YYYY-MM-DD (got %r)' % (field, value))
+
+
+def parse_date(value, field='date'):
+    """Return a date, or None for an explicitly empty value.
+
+    Raises DateValueError for anything else. None and '' mean "clear it";
+    they are the only values that produce None.
+    """
+    if value is None:
+        return None
+    if isinstance(value, datetime):
+        return value.date()
+    if isinstance(value, date):
+        return value
+    if not isinstance(value, str):
+        # A number or a list is not a mistyped date, it is the wrong type.
+        raise DateValueError(field, value)
+    text = value.strip()
+    if not text:
+        return None
+    try:
+        return datetime.strptime(text[:10], '%Y-%m-%d').date()
+    except ValueError:
+        raise DateValueError(field, value)
+
+
+def parse_date_fields(data, fields):
+    """Parse several date fields out of a payload at once.
+
+    Only keys PRESENT in the payload are returned, so a caller can tell "not
+    mentioned" (leave it alone) from "sent as empty" (clear it) - a distinction
+    a plain `data.get(...)` throws away, and the reason the alias-clearing bug
+    existed on the PC form.
+    """
+    return {field: parse_date(data[field], field)
+            for field in fields if field in data}