autotest: check a short FTP reply carries nothing past its size

Lists a directory to leave entries in the reply buffer, then asks past the
end of that listing, and checks the one byte NAK which comes back has
nothing but zeros behind the error code.

Tools/autotest/arduplane.py

@@ -9434,6 +9434,7 @@ class AutoTestPlane(vehicle_test_suite.TestSuite):
             self.MAVFTPBurstEOFOffset,
             self.MAVFTPBurstMissionDat,
             self.MAVFTPParamPck,
+            self.MAVFTPShortReplyPadding,
             self.MAVFTPListDirectoryEdgeCases,
             self.MAVFTPListDirectoryLongNames,
             self.MAVFTPDuplicateRequest,

Tools/autotest/vehicle_test_suite.py

@@ -16897,6 +16897,48 @@ switch value'''
         raise NotAchievedException(
             "%s did not %s" % (path, "appear" if present else "go away"))
 
+    def MAVFTPShortReplyPadding(self):
+        '''test a short FTP reply carries no stale bytes past its size'''
+
+        dirname = "ftp_padding_test"
+        self.create_ftp_listing_pages(dirname, 20)
+
+        try:
+            seq = self.ftp_reset_sessions()
+
+            # list first, so the reply buffer is left holding entries
+            reply = self.ftp_op(seq, mavftp_op.OP_ListDirectory, self.ftp_path_bytes(dirname))
+            self.assert_ftp_ack(reply, "listing to fill the reply buffer")
+
+            # then ask past the end of the listing, which is answered with a
+            # one-byte EndOfFile NAK
+            path_bytes = self.ftp_path_bytes(dirname)
+            self.ftp_send(FTP_OP(
+                seq=reply.seq, session=0, opcode=mavftp_op.OP_ListDirectory,
+                size=len(path_bytes), req_opcode=0, burst_complete=0,
+                offset=1000, payload=path_bytes,
+            ))
+            m = self.mav.recv_match(type='FILE_TRANSFER_PROTOCOL', blocking=True, timeout=5)
+            if m is None:
+                raise NotAchievedException("No reply listing past the end")
+
+            raw = bytearray(m.payload)
+            size = raw[4]
+            opcode = raw[3]
+            if opcode != mavftp_op.OP_Nack:
+                raise NotAchievedException(f"Expected a Nack past the end, got opcode {opcode}")
+            if size != 1 or raw[12] != FtpError.EndOfFile:
+                raise NotAchievedException(
+                    f"Expected a one-byte EndOfFile Nack, got size={size} error={raw[12]}")
+
+            # everything after the error byte belongs to no reply at all
+            stale = bytes(raw[12 + size:]).rstrip(b"\0")
+            if stale:
+                raise NotAchievedException(
+                    f"Reply carried {len(stale)} bytes past its size: {stale[:32]!r}")
+        finally:
+            shutil.rmtree(dirname)
+
     def MAVFTPListDirectoryEdgeCases(self):
         '''test how FTP directory listing rejects and terminates'''