Three review findings against SqlPollReader (967e5140).
1. WideRow's where-pair form emits no ORDER BY and no row limit, so a
selector that is not actually unique made the reader publish an
arbitrary, storage-order-dependent row — silently, unlike IndexByKey's
duplicate-key warning. Row selection moves out of MapMember into a new
SelectWideRow, which is called once per group and emits the same
rate-limited contract warning (naming table, selector and row count).
The row-limit half of the suggested fix was deliberately NOT taken: a
TOP 1 with no ORDER BY is not deterministic either, and it would
destroy the Rows.Count > 1 evidence the warning depends on, turning a
loud misconfiguration into a permanently silent one. Rationale is in
SelectWideRow's docs. No SQL changed, so no planner golden string moved.
2. The "a frozen database can never accumulate more than
maxConcurrentGroups connections" claim was asserted in the docs and
untested. ReadAsync_aTimedOutGroupKeepsItsConcurrencySlotUntilTheQuery-
TrulyFinishes wedges a group behind an EXCLUSIVE transaction, times it
out, and proves the slot is still held (a second poll on a cap of 1
times out on the gate without ever reaching the factory) and is handed
back only once the abandoned query truly finishes. Moving the release
from the work to the waiter fails this test and no other.
3. Task.Run thread-pool starvation: documented, not changed.
Microsoft.Data.SqlClient's async path parks no pool thread on a frozen
server, so the driver cannot starve itself; LongRunning would cost a
dedicated OS thread per group per poll forever to insure against a
provider this driver does not ship. RunGroupAsync now records the
decision and what a BadTimeout does and does not mean, for the
blackhole gate.
Also: the timeout warning now names the group's table, so a BadTimeout
storm identifies which source is frozen.
Claude-Session: https://claude.ai/code/session_01GASWkNEi68FSCtvr6rLoEW
Executes a poll pass: resolve refs -> SqlGroupPlanner.Plan -> one command per
group -> slice the result set back to per-tag DataValueSnapshots, positionally
aligned with the caller's reference list (design §3.2).
The frozen-peer contract (design §8.3, the R2-01 S7 lesson) is the core of it.
CommandTimeout is only the server-side backstop; the client-side bound is a
linked CancelAfter PLUS Task.WaitAsync, because a linked token bounds only a
provider that honours it and ADO.NET providers vary. The whole per-group
operation — waiting for a concurrency slot, OpenAsync, and the query — runs off
one operationTimeout budget, so ReadAsync returns on time regardless of what the
database is doing. A breach Bad-codes that group (BadTimeout); caller
cancellation propagates instead, since nobody consumes a torn-down poll.
Also: absent row (BadNoData) stays distinct from a NULL cell (Uncertain, or Bad
under nullIsBad); duplicate plan members are all fed (two tags on one keyValue
bind one parameter but stay two members); the concurrency slot is released by the
work, not the waiter, so a frozen database can never hold more than
maxConcurrentGroups connections; the whole call throws only when the connection
itself cannot open, which is what earns PollGroupEngine's backoff.
SqlStatusCodes is driver-local, matching every sibling driver's own table — there
is no shared helper in Core.Abstractions and drivers do not reference the OPC UA
SDK. Values were read off Opc.Ua.StatusCodes rather than copied from a sibling,
because two of those tables carry transcription errors.
Claude-Session: https://claude.ai/code/session_01GASWkNEi68FSCtvr6rLoEW
Joseph Doherty