feat(maven): delivery-gate and buffer the hosted downloads_count bump

Moaz Khalifarequested to merge
mkhalifa/maven-downloads-count into main

What

Split from the Step 11 cache-hit serve path (!1439 (merged)) at that MR's review: the hosted repositories.downloads_count activation and the shared delivery-gate machinery ship here, first, so the serve-path MR's remote last_downloaded_at gate lands on top of these delivery verdicts. Merge order: this MR merges first; !1439 (merged) rebases onto it.

In scope:

  • buffered.go — the interim bufferedUpdate fire-and-forget dispatch (Maven twin of internal/format/npm/buffered.go): 5s per-worker deadline, 64-deep in-flight semaphore that sheds as result=dropped, panic recovery, Warn-not-Error failure logging, test-only wait hook. Carries TODO(buffered).
  • metrics.go — bufferedCounterUpdates + RegisterMetrics, registered at the composition root beside npm's and oci's.
  • download.go delivery gate — servePrimaryGet / serveStream / serveFull / serveRedirect report delivery (full 200 body copy or written 302); bumpArchiveDownloadCount fires for archive-class GETs under that verdict. A 206/416 partial, CAS miss, or abandoned transfer counts nothing (mirroring npm's deliveryModeNone gate). The synchronous pre-response bump is removed.
  • BumpRepoDownloadCount activation (datastore) — executes its UPDATE directly with the soft-delete leg, instead of the no-op stub; unit + integration pins flip to the positive increment.
  • S10 amendment — the counting rule retires the pre-response bump and states the delivery gate and the 206 case.
  • Marker alignment (TODO(buffered)), observability catalog row, plan split record (Status row 11a).

The last_downloaded_at label of the shared metric, the remote parents' bump, and the serve-path tests live in !1439 (merged).

Merge order

This MR lands first (the plan records the split and the order). The delivery verdicts it introduces are what !1439 (merged)'s remote last_downloaded_at gate consumes; until this merges, those verdicts do not exist on main.

Tests

  • TestBufferedUpdate_* (buffered_test.go): happy path, fn error, panic safety, shedding, deadline, cancellation detach, metric outcomes.
  • TestPrimaryGET_ArchiveBumpDispatchesOffRequestPath + TestPrimaryGET_CounterBumpRules zero arm (download_internal_test.go / download_test.go): one off-request-path dispatch per delivered archive; non-archive never dispatches.
  • Usage-data flips (download_usagedata_test.go): abandoned transfer / over-long object no longer bump downloads_count.
  • TestMavenRepositoryStore_BumpRepoDownloadCount_*: argument guards + execute pin (unit, recordingDB) and the +1 increment (integration).
  • TestRegisterMetrics_ExposedViaGatherer with the mavenCollectors length guard.

Run: go test ./internal/format/maven/... ./internal/datastore/... -tags=integration and the unit suites.

Related to #286 (closed)

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