timothy
fba5233caf
168 records -> docs/decisions/records/<area>/<topic>.md (163 active, 23 dirs) and docs/decisions/archive/<area>/<topic>.md (5 archived). The filename IS the key, so one-active-record-per-key becomes a filesystem property rather than a validator check, and supersession becomes a `git mv`. WHY: the monolith was a concurrency problem before an aesthetic one. A 3,900-line append target made parallel sessions collide -- PR #605 and PR #614 both hit append-vs-append conflicts during routine rebases, and hand-resolving those inside the corpus is exactly the operation the rationale-rewrite guard exists to police. HOW IT IS VERIFIED: a ~170-file diff cannot be meaningfully read, so correctness does not rest on reading it. The parser was taught BOTH formats first, so the body-diff guard parses the old form at the merge-base and the new form at head -- the migration validates itself, no bypass. The proof is a field-level equivalence harness: 168 records before and after, zero lost, zero gained, zero field mismatches, zero rationale bodies differing. Reviewers should scrutinise the harness; it is the actual evidence. What measuring caught that reading would not have: - ~500 lines sit OUTSIDE any record -- decisions.md's lifecycle schema and each topic file's preamble, mostly the only copy. Source files are kept and stripped, never deleted. They also cannot be filed per-area: topic files hold several areas and 4 of 23 areas span several files. - Archive discovery was a non-recursive glob; after the split it found ZERO archived records, surfacing as four bogus "supersedes points to unknown key" errors rather than an obvious failure. - ~32 live docs point into the corpus BY DATE, which the split dangles. Each stripped file now ends with a generated "Records formerly in this file" index, which also rescues the identical breadcrumbs in old issue comments. - decisions.md's "In this file:" list was 97 same-file anchor bullets that the split makes WRONG, not merely stale. Dropped; the generated index replaces them with links that resolve. The equivalence harness now runs against a checked-in FIXTURE, not the live corpus. The earlier version migrated the real tree, which made it a one-shot: the moment the migration landed there was nothing left to move and the tests failed for reasons unrelated to the code. A fixture keeps them testing the SCRIPT rather than the repo's current state. Keys preserved verbatim, warts included: `sched` (12) and `scheduling` (1) remain two directories for one concept. Renaming a key is not a move -- it changes identity, breaks the equivalence proof, and invalidates MemPalace's per-key drawers. Taxonomy normalisation is separate work. refs #610
3.3 KiB
key, title, status, since, supersedes, superseded-by, rule, signals, mechanics
| key | title | status | since | supersedes | superseded-by | rule | signals | mechanics |
|---|---|---|---|---|---|---|---|---|
| ffmpeg.work-ahead-slot-atomic | 2026-07-21 — Work-ahead slots are claimed atomically by the caller, released by the transcode it hands them to (#536) | active | 2026-07-21 | none | none | `workAheadSegmenterLimit` is enforced by a single compare-exchange claim on a shared `WorkAheadSlots` pool taken by the *caller* of `Transcode`, which then passes ownership in and gets the release in `Transcode`'s `finally` — never a `Volatile.Read` compare in one place and an `Interlocked.Increment` in another. | work-ahead slot, workAheadSegmenterLimit, check-then-act across an await, unthrottled tune-in, TOCTOU · paths: `ErsatzTV.Application/Streaming/WorkAheadSlots.cs`, `HlsSessionWorker.Run`/`Transcode`, `ErsatzTV.Core.Tests/Streaming/WorkAheadSlotsTests.cs` · issues: #536, #350, #529, #231, #250 | `WorkAheadSlots.TryAcquire(limit)` CAS loop; `Transcode(bool ownsWorkAheadSlot, …)` |
The defect was structural, not a missing Interlocked. The write side already used
Interlocked.Increment, which is why the code read as thread-safe. The read side was a separate,
earlier Volatile.Read(ref _workAheadCount) < await GetWorkAheadLimit(...) in Run, and the
increment happened later inside Transcode — with at least one await (a DB-backed config read) in
between. Interlocked on one half of a check-then-act buys nothing. Three simultaneous tune-ins on
prod with workAheadSegmenterLimit: 1 all observed 0 < 1 and all ran with no -readrate.
Why the caller claims and the callee releases. Moving the whole acquire/release pair inside
Transcode would be more symmetric, but Run needs the outcome before the call: it sets
_state = SeekAndWorkAhead | SeekAndRealtime, and Transcode reads that state on entry
(wasSeekAndWorkAhead) to decide whether the item starts at DateTimeOffset.Now or at
_transcodedUntil. Folding acquisition inward would have silently changed that branch. Instead the
parameter was inverted — Transcode(bool ownsWorkAheadSlot, …) with realtime = !ownsWorkAheadSlot
derived on the first line — so the contract is stated in the signature rather than implied by a
double negative, and there is exactly one release site guarded by the same flag.
Compare-exchange rather than increment-then-back-off. The issue proposed
Interlocked.Increment followed by a decrement when the post-increment value exceeds the limit.
That is correct on holder count, but the counter transiently overshoots, so a concurrent reader of
Count can observe a value above the limit. The CAS loop never publishes a state that violates the
invariant, which matters because the QSV hardware-frame pool sizing (ffmpeg.qsv-extra-hw-frames-floor)
is derived from that bound.
Testing shape is inherited from #231/#250. A single Barrier(N) + Task.WhenAll round does not
reliably collide on this hardware; the tests hammer 8 threads × 20 000 rounds with a per-round
barrier that validates the winner count and resets the pool. The negative control is documented in
the test file: reinstate the check-then-act body (not if (true), which trips CS0219 under
warnings-as-errors and leaves --no-build running a stale, still-fixed dll). Verified: with the
pre-fix shape, 15 912 of 20 000 rounds over-claimed.