Hosted run 2243 (dc0a24f2, job 3882) failed in
ModbusWritePduFramingTests.FC16_pdu_is_exactly_header_plus_byte_count with
"Sequence contains no matching element" at transport.Sent.Single(fc == 0x10),
one line after the write had already returned status Good: the driver sent the
FC16 PDU, but the record lost it.
The fixture left Probe at its default (Enabled = true), so InitializeAsync
started ProbeLoopAsync on a pool thread, which sends an FC03 at once. Both that
send and the write's send did List<byte[]>.Add on the capturing transport
concurrently; List is not thread-safe and one Add can overwrite the other.
- Disable the probe in this fixture, as the sibling capturing-transport
fixtures (ModbusProtocolOptionsTests, ModbusCoalescingDefaultsTests) do.
- Record PDUs in a ConcurrentQueue so a background send can never drop one.
- Dispose the driver (await using) so no loop outlives the test.
Test-only; not related to the #729/#727 changes (2243 predates them, and the
run that includes them, 2260, passed Driver.Modbus.Tests 1055/1055).
Claude-Session: https://claude.ai/code/session_01GT67AEHd2Acv1YPwGfXEoV
The FC15, FC16, BitInRegister[] RMW FC16 and FC23 builders allocated one byte
more than they filled (6 + 1 + n / 10 + 1 + n, data copied at offset 6 / 10), so
every multi-write request ended in a stray zero byte beyond the length its
byte-count field declares. A strict device or gateway that checks the frame
length against the byte count refuses it. Each PDU is now exactly 6 + n
(FC15/FC16) or 10 + n (FC23).
Tests: ModbusWritePduFramingTests asserts the exact length and byte-count field
for FC15 (1/8/9 coils), FC16 (array, Float64 array, scalar multi-register), the
BitInRegister RMW FC16 and FC23 (all red before the fix); the parity suite's
conformant device now refuses a frame-length mismatch on FC15/FC16/FC23.
Live: Driver.Modbus.IntegrationTests against 10.100.0.35:5020 (standard
profile) passes, including the FC16 and FC23 multi-register round trips.
Refs #741
Claude-Session: https://claude.ai/code/session_01Ub9hGmYLQvn9XETzw7CMsT
Joseph Doherty