RTPS: Handle SequenceNumber_t correctly

As per DDS-RTPS specification, `firstSN` (named `firstAvailableSeqNum` in scapy) and `lastSN` (named `lastSeqNum` in scapy) fields of a Heartbeat submessage are of type `SequenceNumber_t`, which consists of two subfields, namely, `int32_t high` and `uint32_t low`. This change allows us to correctly assign values to fields.

For example, currently, `firstAvailableSeqNum = 0x01` is packed as `\x00\x00\x00\x01\x00\x00\x00\x00`, which is semantically wrong.
With the change, it is packed as `\x00\x00\x00\x00\x01\x00\x00\x00`.

scapy/contrib/rtps/rtps.py

@@ -25,7 +25,6 @@
     X3BytesField,
     XByteField,
     XIntField,
-    XLongField,
     XNBytesField,
     XShortField,
     XStrLenField,
@@ -396,8 +395,14 @@ class RTPSSubMessage_HEARTBEAT(EPacket):
             fmt="4s",
             enum=_rtps_reserved_entity_ids,
         ),
-        XLongField("firstAvailableSeqNum", 0),
-        XLongField("lastSeqNum", 0),
+        EField(IntField("firstAvailableSeqNumHi", 0),
+               endianness_from=e_flags),
+        EField(IntField("firstAvailableSeqNumLow", 0),
+               endianness_from=e_flags),
+        EField(IntField("lastSeqNumHi", 0),
+               endianness_from=e_flags),
+        EField(IntField("lastSeqNumLow", 0),
+               endianness_from=e_flags),
         EField(IntField("count", 0),
                endianness_from=e_flags),
     ]

test/contrib/rtps.uts

@@ -405,3 +405,41 @@ p1 = RTPS(
 assert p0.build() == d
 assert p1.build() == d
 assert p1 == p0
+
++ Test for pr #3914
+= RTPS Heartbeat SequenceNumber_t packing and dissection
+
+d = b"\x52\x54\x50\x53\x02\x02\x01\x0f\x01\x0f\x45\xd2\xb3\xf5\x58\xb9" \
+    b"\x01\x00\x00\x00\x07\x01\x1c\x00\x00\x00\x03\xc7\x00\x00\x03\xc2" \
+    b"\x00\x00\x00\x00\x01\x00\x00\x00\x00\x00\x00\x00\x01\x00\x00\x00" \
+    b"\x01\x00\x00\x00"
+
+p0 = RTPS(d)
+
+p1 = RTPS(
+    protocolVersion=ProtocolVersionPacket(major=2, minor=2),
+    vendorId=VendorIdPacket(vendor_id=0x010f),
+    guidPrefix=GUIDPrefixPacket(
+        hostId=0x010f45d2, appId=0xb3f558b9, instanceId=0x01000000
+    ),
+    magic=b"RTPS",
+) / RTPSMessage(
+    submessages=[
+        RTPSSubMessage_HEARTBEAT(
+            submessageId=0x07,
+            submessageFlags=0x01,
+            octetsToNextHeader=28,
+            reader_id=b"\x00\x00\x03\xc7",
+            writer_id=b"\x00\x00\x03\xc2",
+            firstAvailableSeqNumHi=0,
+            firstAvailableSeqNumLow=1,
+            lastSeqNumHi=0,
+            lastSeqNumLow=1,
+            count=1
+        )
+    ]
+)
+
+assert p0.build() == d
+assert p1.build() == d
+assert p0 == p1