Warn when _parse_ccsds_head is given more than 10 bytes

src/pyfaradaycup/decommutator/ccsds_reader_pipeline.py

@@ -19,6 +19,7 @@
 import struct
 import sys
 import time
+import warnings
 from typing import TYPE_CHECKING
 
 import dateutil.parser
@@ -330,10 +331,10 @@ def read_file_sc(  # ruff:ignore[C901, PLR0912, PLR0915]
     # and thus which SC_HK.blk file to use
     # we'll assume the first bytes in the file are a header
     try:
-        if ptp:  # ruff:ignore[SIM108]
-            cchead = _parse_ccsds_head(bytestr[17:])
+        if ptp:
+            cchead = _parse_ccsds_head(bytestr[17:27])
         else:
-            cchead = _parse_ccsds_head(bytestr)
+            cchead = _parse_ccsds_head(bytestr[:10])
         if (
             (cchead["CCSDS_Version"] != 0)
             | (cchead["CCSDS_PacketType"] != 0)
@@ -576,6 +577,11 @@ def _parse_ccsds_head(bytestr: bytes) -> dict[str, int]:
     ValueError
         If ``bytestr`` is shorter than 10 bytes.
 
+    Warns
+    -----
+    UserWarning
+        If ``bytestr`` is longer than 10 bytes.
+
     Notes
     -----
     The first 6 bytes are the standard CCSDS primary header. The next
@@ -587,6 +593,16 @@ def _parse_ccsds_head(bytestr: bytes) -> dict[str, int]:
     exp_length = 10
     if len(bytearr) < exp_length:
         raise ValueError("CCSDS header is not as long as expected")  # ruff:ignore[EM101, TRY003]
+    # For now, issue a warning when the bytearr is too long.
+    # If bytearr being too long turns out to be normal and
+    # expected behavior, we can remove this warning later,
+    # or set it to the debug level of verbosity with a logger.
+    if len(bytearr) > exp_length:
+        warnings.warn(
+            f"CCSDS header is {len(bytearr)} bytes long; only the first "
+            f"{exp_length} bytes will be decoded",
+            stacklevel=2,
+        )
 
     head = {}
     head["CCSDS_Version"] = bytearr[0] >> 5

tests/decommutator/test_ccsds_reader_pipeline.py

@@ -119,16 +119,19 @@ def test_parse_ccsds_head_field_isolation(name: str) -> None:
     assert _parse_ccsds_head(_make_ccsds_head(fields)) == fields
 
 
-def test_parse_ccsds_head_ignores_extra_bytes() -> None:
+def test_parse_ccsds_head_warns_about_extra_bytes() -> None:
     """
-    Test that only the first 10 bytes are decoded.
+    Test that extra bytes give a warning and are not decoded.
 
-    Callers do not always pass exactly 10 bytes: ``read_file_sc`` passes
-    the contents of the whole file to read the first header. The bytes
-    after the header must not change the result.
+    Callers are expected to pass exactly 10 bytes, so a longer input
+    probably means a slicing mistake in the caller, such as passing the
+    rest of the file. The warning makes such a mistake visible, and the
+    bytes after the header must still not change the result.
     """
     header = _make_ccsds_head(_typical_fields)
-    assert _parse_ccsds_head(header + b"\xff" * 20) == _typical_fields
+    with pytest.warns(UserWarning, match="only the first 10 bytes"):
+        result = _parse_ccsds_head(header + b"\xff" * 20)
+    assert result == _typical_fields
 
 
 @pytest.mark.parametrize("length", range(10))