Pre-#148 behaviour: a coalesced FC03/FC04 read that crossed a write-only or
PLC-fault register marked every member tag Bad until the operator manually
flagged the offending tag with CoalesceProhibited. Healthy tags around the
hole stayed broken indefinitely.
Post-#148: two-stage recovery, no operator intervention needed.
1. Same-scan fallback: when a coalesced read fails with a Modbus exception
(IllegalDataAddress, SlaveDeviceFailure, etc.), the planner does NOT
mark members handled. The per-tag fallback in the same scan reads each
member individually — non-protected members surface Good values
immediately, and only the actual protected register stays Bad.
2. Cross-scan prohibition: the failed range (Unit, Region, Start, End) is
recorded in a per-driver `_autoProhibited` set. On subsequent scans the
planner checks each candidate merge against the set and refuses to
re-form any block that overlaps a known-bad range. Net effect: after one
scan with a failure, the protected range goes "per-tag mode" indefinitely
while ranges around it keep coalescing normally.
Communication failures (timeouts, socket drops) are NOT auto-prohibited —
they're transport-level, not structural. The same coalesced read can succeed
once the transport recovers; recording it as "permanently bad" would defeat
coalescing for the whole driver instance.
Auto-prohibition state lives for the driver lifetime and clears on
ReinitializeAsync (operator restart). A periodic re-probe is a follow-up if
deployments need it without a restart.
Implementation:
- Added `_autoProhibited` HashSet<(byte, ModbusRegion, ushort, ushort)> +
`_autoProhibitedLock` on ModbusDriver.
- `RangeIsAutoProhibited(unit, region, start, end)` overlap check called
from the planner when forming blocks.
- `RecordAutoProhibition(...)` called from the catch (ModbusException)
branch.
- The catch (Exception) branch (non-Modbus failures) keeps the pre-#148
"mark all Bad in this scan, don't auto-prohibit" behaviour.
- Internal `AutoProhibitedRangeCount` accessor for tests.
Tests (3 new ModbusCoalescingAutoRecoveryTests):
- First_Failure_Falls_Back_To_PerTag_Same_Scan — three tags around a
protected register at 102: T100 + T104 surface Good values via the
per-tag fallback in the SAME scan; T102 surfaces the exception.
- Second_Scan_Skips_Coalesced_Read_Of_Prohibited_Range — confirms scan 2
doesn't re-attempt the failed merge (no FC03 with quantity > 1 at the
prohibited start).
- Tags_Outside_Prohibited_Range_Still_Coalesce — separate cluster at HR
200..202 keeps coalescing normally even after the 100..104 cluster is
prohibited.
234/234 unit tests green.
Follow-ups intentionally NOT shipped (smaller, independent changes):
- Bisection-style range narrowing — currently the prohibition range is the
full failed block; the planner doesn't try to find the exact protected
register. Operator-visible diagnostic + prohibition stays correct.
- Periodic re-probe to clear stale prohibitions.
- Surface auto-prohibited ranges through GetHostStatuses or a new
diagnostic so the Admin UI can show what's been auto-isolated.
