Fix dropped change flags + FIDI parsing

- capture.py: DirectSniffer/GsmtapListener yield (type, data, flags),
  CaptureManager passes flags to insert_message (change flags were
  previously dropped, leaving every change row with flags=0)
- protocol.py: add _parse_fidi (1-byte value) — FIDI was mis-mapped to
  _parse_change (4-byte flags), so FIDI messages were silently dropped
- database.py: pass flags into decode_sniff_msg
- decode.py: decode_change(flags) reads flags directly; decode_sniff_msg
  gains flags parameter; change messages no longer require non-empty data
- tests: fix FIDI test, add change/fidi decode tests (43 total)

simtrace2_pysniff/server/capture.py

@@ -24,7 +24,7 @@ class GsmtapListener:
             if sub_type is None:
                 continue
             msg_type = 'atr' if sub_type == GSMTAP_SIM_ATR else 'tpdu'
-            yield msg_type, data
+            yield msg_type, data, 0
 
 
 class DirectSniffer:
@@ -49,7 +49,7 @@ class DirectSniffer:
         for msg in self._session.iter_messages():
             if not self._running:
                 break
-            yield msg.type, msg.data
+            yield msg.type, msg.data, msg.flags
 
 
 class CaptureManager:
@@ -103,9 +103,9 @@ class CaptureManager:
         return sid
 
     def _capture_loop(self):
-        for msg_type, data in self._backend.iter_messages():
+        for msg_type, data, flags in self._backend.iter_messages():
             if self._session_id is None:
                 break
             elapsed = round(time.monotonic() - self._start_time, 3)
-            mid = self._db.insert_message(self._session_id, elapsed, msg_type, data)
+            mid = self._db.insert_message(self._session_id, elapsed, msg_type, data, flags)
             self._latest_msg_id = mid

simtrace2_pysniff/server/database.py

@@ -168,18 +168,19 @@ class Database:
 
     def _row_to_message(self, row):
         data_blob = row[4]
+        flags = row[5]
         msg = {
             'id': row[0],
             'session_id': row[1],
             'elapsed': row[2],
             'type': row[3],
             'data': data_blob.hex(),
-            'flags': row[5],
+            'flags': flags,
         }
-        if row[3] in ('tpdu', 'change', 'fidi', 'atr', 'pps') and data_blob:
+        if row[3] in ('tpdu', 'change', 'fidi', 'atr', 'pps'):
             try:
                 from .decode import decode_sniff_msg
-                msg['decoded'] = decode_sniff_msg(data_blob, row[3])
+                msg['decoded'] = decode_sniff_msg(data_blob, row[3], flags)
             except Exception as e:
                 import sys
                 print(f'Decode error for msg {row[0]} ({row[3]}, {len(data_blob)} bytes): {e}',

simtrace2_pysniff/server/decode.py

@@ -590,12 +590,8 @@ CHANGE_FLAGS = {
 }
 
 
-def decode_change(raw_data):
-    """Decode a SIMtrace2 sniff_change payload (4-byte flags)."""
-    if len(raw_data) < 4:
-        return None
-    import struct
-    flags = struct.unpack('<I', raw_data[:4])[0]
+def decode_change(flags):
+    """Decode SIMtrace2 sniff_change flags into a list of human-readable names."""
     bits = []
     for mask, name in sorted(CHANGE_FLAGS.items()):
         if flags & mask:
@@ -628,7 +624,7 @@ def decode_fidi(raw_data):
     }
 
 
-def decode_sniff_msg(raw_data, msg_type):
+def decode_sniff_msg(raw_data, msg_type, flags=0):
     """Decode a raw sniff message of the given type.
 
     Returns a structured dict with at least a 'type' key, or None if
@@ -636,8 +632,8 @@ def decode_sniff_msg(raw_data, msg_type):
     """
     if msg_type == 'tpdu' and raw_data:
         return decode_message(raw_data)
-    if msg_type == 'change' and raw_data:
-        return decode_change(raw_data)
+    if msg_type == 'change':
+        return decode_change(flags)
     if msg_type == 'fidi' and raw_data:
         return decode_fidi(raw_data)
     if msg_type == 'atr':