Lab #2315: gcs-datastore: verify generation-precondition support at setup and in doctor; an emulator or proxy that ignores ifGenerationMatch makes locking silently unsafe

Problem

Same class of gap as #2300, on the GCS side. The GCS datastore's correctness rests on generation preconditions: lock acquisition is an upload with ifGenerationMatch=0 (gcs_client.ts putObjectConditional), and the shard-first index merge uses ifGenerationMatch=<generation>. On a GCS-API endpoint that silently ignores those preconditions (returns 200 instead of 412), putObjectConditional returns a generation for every caller: every writer "acquires" the lock, and index merges lose updates. Nothing reports it.

Today nothing checks for it: GcsDatastoreVerifier.verify() (datastore/gcs/extensions/datastores/_lib/gcs_verifier.ts) issues bucketExists() only, so swamp datastore setup, swamp datastore status and swamp doctor datastores are all green on such an endpoint.

Real GCS honours preconditions. The exposure is emulators and proxies: fake-gcs-server (which datastore/benchmarks already runs against), the GCS JSON-compatible fronts some object stores ship, and anything sitting between swamp and the bucket.

Proposed solution

Port the #2300 probe. Under a unique probe key beneath the already-filtered _control/ prefix:

upload with ifGenerationMatch=0 -> expect success, note the generation
the same request again -> expect PreconditionFailedError (412)
upload with a deliberately stale ifGenerationMatch -> expect 412
delete the probe key, best-effort

Report details.conditionalWrites as supported / ignored / write-denied / inconclusive and fail setup (or report unhealthy) when step 2 or 3 returns 200: "this endpoint ignores generation preconditions; distributed locking would not be safe."

Follow #2300's shape so the two datastores stay comparable:

distinguish a write rejection (403) from an ignored precondition, so narrow-IAM users are not sent hunting for a compatibility bug
memoize the verdict per process, keyed on endpoint/bucket/prefix, with a short TTL - swamp serve streams health on a 1s-floored interval and reaches verify(), so an unmemoized probe becomes sustained write traffic
make the cleanup delete best-effort so it cannot flip a working endpoint to unhealthy; surface a failed cleanup as details.probeCleanup

No swamp-core changes needed: setup, status, doctor and the serve admin handler all already call verify(), and setup already aborts on healthy: false.

Alternatives considered

Leaving it to the S3 probe alone. Doesn't help - the two extensions share no verifier code, and a GCS user gets no signal at all today.

Related: #2300, which adds the equivalent probe to @swamp/s3-datastore and is the reference implementation for this one.

Upstream repository: https://github.com/systeminit/swamp-extensions

Environment

Extension: @swamp/gcs-datastore@2026.09.17.2
Discovered while implementing #2300; deliberately kept out of that PR's scope.

A nested structural swamp still waits on its run's lock when the lock holder is not a same-host ancestor (cross-host worker, --server into a non-ancestor serve)

Deliver workflow signals through a write-once outcome record instead of writing the run record

Forward held-lock identity over --server so a loopback nested swamp does not wait on its caller's lock

A nested structural swamp on a same-host remote worker waits on its own step's lock held by swamp serve

Fail fast when nested structural commands under parallel runs wait on each other's locks

Docs: remove followUpActions from the MethodResult reference in the extension model manual

signal_test: already-settled test fails by chance when a random UUID spells the sender name

Add a wait_for_signal workflow step that pauses a run for a JSON message

run doctor through serve interrupts a live run of another serve instance when no heartbeats are recorded

Flaky test: WalSink replayed segments are delivered before events written after replay leaves one WAL segment

Remove followUpActions: no model or extension can produce them

Workflow schedule watcher and workflow edit symlink lookup ignore the managed config workflows dir

main is red: repository_dirty_coverage_test fails for UnifiedData.collectGarbage(orphaned deferred write)

isProcessGone always reports alive on Linux under the test task permissions, failing 9 tests on main

serve: relative tls cert-file/key-file and --config resolve against the working directory, not the repository

findBySpec/findByTag lose a workflow step's output after a version is deleted, pruned or rolled back

Run tracker keeps some interrupted workflow rows forever: retention waits for markSettled, which several paths never call

run doctor reads every run record on each run to rebuild the run indexes

Serve audit: two sinks with the same name share one emitter cursor, so the second never receives events

Serve audit: a durable sink write that times out is retried while the first call is still pending, so the store can hold duplicate sequences

Flaky test: ModelResolver data accessors for a renamed model return the earlier id's record from findBySpec

data query --limit returns fewer live records than exist and reports limited: false when stale catalog rows fall inside the limit

data get: help example passes --run latest, which is not a recognised run id

workflow reject leaves sibling gates waiting_approval and dependents pending in a failed, retryable run

workflow cancel cannot cancel a suspended run whose workflow file was deleted, and cancel --all silently skips it

A job alone in its level starts after the abort, and its unstarted step is recorded as a real failure instead of settledByAbort

workflow approve racing workflow cancel loses the cancel: the run returns to suspended after cancel reported cancelled

data query: does not follow data rename forwarding that data get follows

Unit tests share fixed /tmp catalog paths, so concurrent runs corrupt them and fail every later run

data query: renamed data items are not found by their old name

data query: model lookup by id and orphan recovery parity with data get

workflow approve on a run that is not suspended prints a fatal error with a stack trace

data query: no way to target a workflow's latest run

data query: binary content lacks contentEncoding/base64 parity with data get

data query: no fallback from spec name to data instance name

data query: no definitionHash or garbageCollection fields in query results

A nested structural swamp spawned by swamp serve skips every lock serve holds, including other runs' locks

data query: confirm catalog parity with data get for custom datastores

data query: single-result --json mode for scripts migrating from data get

data get --workflow silently returns an arbitrary step's data when several steps share a data name

swamp doctor workflows misses nested and .yml extension workflows that the loader reads

auth whoami says the stored API key is no longer valid when auth.json only holds an env-key identity cache

Docs: swamp skill says cancelling a parent run cancels its nested child runs; the child is left suspended

Namespace advice in the slow-lock warning is wrong for a single repo on a local datastore

Method-run records left running by a dead owner are never reaped

Flaky: usecase_sync_characterization_data_test 'data gc (serve)' asserts markDirty order that varies under parallel load

Worker dispatch rebuilds the definition with Definition.create, so a legacy-named model cannot run on a worker

A cancelled workflow exits before its steps stop, leaving their method-run records at running

Nested swamp structural command in a shell step times out on its parent's per-model lock (SWAMP_LOCK_HOLDER_PID is stripped)

Auth gate blocks nested swamp in workflow shell steps when the credential is in SWAMP_API_KEY

swamp workflow create crashes with an [FTL] ZodError stack trace on an invalid (uppercase) workflow name

swamp model create crashes with an [FTL] ZodError stack trace on an invalid (uppercase) model name

model cancel SIGTERMs a running swamp serve process when the method run belongs to a serve-run workflow

model cancel --all on a running workflow leaves its method-run records at running, or records them failed instead of cancelled

model cancel SIGKILLs the owner 2 s after SIGTERM, before the step executor's own 3 s kill grace, so the run never records its own cancellation

workflow cancel and supersede of a suspended run leave its jobs running in the cancelled record

A force-exited workflow run stays running with a dead pid: resume, recover and run doctor --fix cannot clear it

A nested workflow that suspends on manual approval forwards its suspended event into the parent run's stream

workflow cancel SIGKILLs the run 2 s after SIGTERM, cutting off always/completed cleanup jobs mid-step

Warn when datastore setup keeps managedConfig but the new datastore has no config tier

serve: a relative grants-dir resolves against the working directory, not the repository

workflow resume cleanup mode can run an always-gated teardown before a suspended job's approved work

managedConfig: extension writes update the shared lockfile from a stale cache and overwrite peers' entries

datastore setup extension overwrites the shared config tier with the repo-local copy and drops managedConfig from .swamp.yaml

serve daemon enable writes a unit that crash-loops when serve args are invalid (e.g. missing --admins)

Concurrent embedded deno extraction races: stale temp-file cleanup deletes another process's in-flight .deno.tmp file (chmod ENOENT)

Telemetry bridge drops a parent step's invocation when a nested workflow step has the same job and step names

serve daemon: unit sets SWAMP_HOME, which relocates the config dir — token/oauth daemons can't find auth.json and crash-loop

Workflow step that times out on the model lock leaves its output and run-tracker row stuck at 'running'

An extension upgrade that hits a type collision is rolled back to no version: roll back to the prior version with a commit/rollback install handle

Telemetry bridge attributes a parent run's later method invocations to its nested child run

sensitive_data_delivery_test fails where /bin/sh is bash: the final ps is exec'd and reports its own argv

build-attestation rejects every run: evaluated workflows now carry sensitiveFormat, which the provenance check treats as an uncommitted field

DuplicateTypeError names the incoming extension as the existing claimant and reports a false ghost row when it sorts first

Install extensions by stage and swap, with a journal and crash recovery

workflow resume ignores SWAMP_MAX_CONCURRENT_STEPS for job-level concurrency

Tidy up tests and helpers from the serve suspended-run cancel

Concurrent extension installs and removals can interleave in one checkout: add a pulled-extensions lock and LockfileRepository.refresh()

deno run audit prints No description available for every advisory

Add a literal() CEL function so one string can mix swamp expressions with another service template syntax

Property test for deferred bindings fails intermittently on a proto key