Flaky: TestRebuildPackumentCache_WaitsOutItsBudgetForARenderingSlot reads its wait as shorter than its own budget
## Summary
`TestRebuildPackumentCache_WaitsOutItsBudgetForARenderingSlot`, in `internal/format/npm/packument_cache_internal_test.go`, fails intermittently.
The test saturates the rendering semaphore and gives the rebuild a 20 ms budget.
It then asserts that the call parked for at least 10 ms before it returned.
The failing run measured 6.546518 ms.
The assertion is a wall-clock one, and the branch it failed on touches no file under `internal/format`.
Measured on 2026-08-31, in job [`build-current-artifact-registry-amd64`](https://gitlab.com/gitlab-org/ops/artifact-registry/-/jobs/16214366417), child pipeline 2806779408 under parent pipeline 2806778727, on [!2040](https://gitlab.com/gitlab-org/ops/artifact-registry/-/merge_requests/2040) at head `2ef5e3508`.
## Verbatim failure
Package `gitlab.com/gitlab-org/ops/artifact-registry/internal/format/npm`, at `internal/format/npm/packument_cache_internal_test.go:3998`.
```plaintext
--- FAIL: TestRebuildPackumentCache_WaitsOutItsBudgetForARenderingSlot (0.02s)
packument_cache_internal_test.go:3998: gave up after 6.546518ms: the rebuild must wait for a slot, not refuse on a full semaphore, or a synchronous caller inherits a refusal it has no document to answer
FAIL
FAIL gitlab.com/gitlab-org/ops/artifact-registry/internal/format/npm 92.483s
```
The duration in the message changes per run.
The rest of the text is stable.
This is the only `--- FAIL` in the job.
## Why this is a flake and not a regression
Five grounds, each one measured for this report:
1. `git diff --stat origin/main...2ef5e3508 -- internal/format/` is empty. The branch touches no file under `internal/format`.
2. `GOFLAGS= go test ./internal/format/npm/ -run 'TestRebuildPackumentCache_WaitsOutItsBudgetForARenderingSlot' -count=50` passes 50 of 50 on a developer machine.
3. The previous head `a955d8a5` passed the same job, in child pipeline 2806661313. The delta from `a955d8a5` to `2ef5e3508` is comment text in two files under `internal/datastore` (4 insertions, 2 deletions). `git rev-parse <head>:internal/format/npm` gives the same tree hash `315e72406` on both heads, so the npm package is byte-identical across them.
4. The job reached `step_script`, so this is a test failure and not a `get_sources` failure under a job name.
5. The failure is a floor on measured wall-clock time. That shape fails under load without any change in the code it covers.
## Why it is worth fixing rather than retrying
The whole untagged Go suite runs in the `checkPhase` of the Nix derivation that builds the service image.
One red test there fails the derivation, then the image build, then the job, and the merge request gets no container image.
The failing run shows 67 packages green and this one test red.
`internal/format/npm` alone took 92.483 s under that load.
No branch has to touch npm code to run this test, so any merge request can lose its image build to it.
A person who hits it without the grounds above in hand has to establish them before a retry is defensible.
## Where to look
`internal/format/npm/packument_cache_internal_test.go`, in the setup of this test:
```go
ctx, cancel := context.WithTimeout(context.Background(), 20*time.Millisecond)
defer cancel()
start := time.Now()
err := RebuildPackumentCache(ctx, uuid.New(), uuid.New())
```
Two clocks run here, and they do not start together.
The deadline starts at `context.WithTimeout`.
The measurement starts at `time.Now()`, two statements later.
`reserveRenderingSlot` returns through its `case <-ctx.Done()` arm, so the call returns about 20 ms after the first clock, not after the second.
The measured wait is therefore the budget minus whatever delay separates the two statements.
A delay of about 13.5 ms brings a full 20 ms park down to the 6.546518 ms the failure reports.
The log agrees that the rebuild did park.
The line before the failure is the warning that the `case <-ctx.Done()` arm writes:
```plaintext
{"time":"2026-08-31T19:25:43.389789371Z","level":"WARN","msg":"npm packument cache rebuild dropped: waited out its budget for a rendering slot","namespace_id":"f8f2db9b-545a-4209-ae35-b8d2da125bb8","npm_package_id":"37cef7d3-f3e4-48d7-a9a3-6ef527cfcfda","rendering_cap":4}
```
The property the test defends is real and worth a test.
The measurement of it is what breaks.
Two candidate corrections: take `start` before `context.WithTimeout`, or derive the floor from `ctx.Deadline()` rather than from a literal.
A reader who knows the fixture picks between them.
## Prior art searched
Four issue searches over open and closed issues, on 2026-08-31:
| Search | Result |
|---|---|
| `WaitsOutItsBudgetForARenderingSlot` | no match |
| `packument_cache_internal_test` | no match |
| `rendering slot` | [#688](https://gitlab.com/gitlab-org/ops/artifact-registry/-/work_items/688) and [#499](https://gitlab.com/gitlab-org/ops/artifact-registry/-/work_items/499), neither a flake report |
| `flaky npm packument` | [#788](https://gitlab.com/gitlab-org/ops/artifact-registry/-/work_items/788) and [#460](https://gitlab.com/gitlab-org/ops/artifact-registry/-/work_items/460), neither about this test |
[#460](https://gitlab.com/gitlab-org/ops/artifact-registry/-/work_items/460) is a closed flake in `conformance:npm:s3-garage` on the npm deprecate packument read-after-write, and it carries a functional bug behind it.
[#499](https://gitlab.com/gitlab-org/ops/artifact-registry/-/work_items/499) asks for per-package coalescing of write-path rebuilds, which is a design change and not a timing assertion.
Open merge requests searched on the same terms.
[!2116](https://gitlab.com/gitlab-org/ops/artifact-registry/-/merge_requests/2116) and [!2124](https://gitlab.com/gitlab-org/ops/artifact-registry/-/merge_requests/2124) both edit `internal/format/npm/packument_cache.go`.
Neither one edits this test file or this assertion.
issue
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