Publish merged realtime state as an atomic snapshot - #1420
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rebuildMergedRealtimeLocked built seven O(N) structures under the write lock, and every API read path took RLock to reach them. Go's RWMutex gives writers preference, so a poll cycle parked incoming readers behind the rebuild. Group the seven into one immutable mergedRealtime and publish it with atomic.Pointer. Writers keep realTimeMutex, which still guards the per-feed maps they rebuild from, so writer hold time is unchanged. Readers load the snapshot with no lock at all and must not mutate it. Reader latency measured while rebuilds run in a loop, 50k entities: p50 7.3ms to 120ns, p99 16.6ms to 992ns, max 29.9ms to 17us. The Mock* helpers write the merged view directly rather than through the feed maps, so they now copy on write under the same lock. Their existing semantics are preserved, including MockAddAlert wiping directly injected vehicles. Refs OneBusAway#670
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📝 WalkthroughWalkthroughThe GTFS manager now stores merged realtime data in immutable snapshots published through ChangesRealtime snapshot publication
Estimated code review effort: 3 (Moderate) | ~25 minutes Merge Risk: 🟡 Moderate · up to Callers can mutate data owned by the published realtime snapshot, risking inconsistent responses or data races. The concurrency test can also report invariant failures incorrectly. These should be corrected before merge. Sequence Diagram(s)sequenceDiagram
participant FeedUpdates
participant GTFSManager
participant AtomicSnapshot
participant APIReaders
FeedUpdates->>GTFSManager: update per-feed realtime maps
GTFSManager->>AtomicSnapshot: rebuild and publish mergedRealtime
APIReaders->>AtomicSnapshot: load immutable snapshot
AtomicSnapshot-->>APIReaders: return realtime data
Suggested reviewers: 🚥 Pre-merge checks | ✅ 3 | ❌ 2❌ Failed checks (2 warnings)
✅ Passed checks (3 passed)
Full details: Linked Issues checkExplanation The PR removes API reader locking and publishes immutable merged state atomically, which addresses reader starvation. However, it retains O(N) snapshot rebuilding and allocation while holding realTimeMutex, so it does not fully implement issue Resolution Move merged snapshot allocation and population outside realTimeMutex, then publish the completed snapshot with an O(1) atomic swap. Alternatively, update issue Full details: Docstring CoverageExplanation Docstring coverage is 52.94% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 17 functions across 10 files. (2 skipped: 2 unsupported.) Thanks for using CodeRabbit! It's free for OSS, and your support helps us grow. If you like it, consider giving us a shout-out. Comment |
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Inline comments:
In `@internal/gtfs/realtime_snapshot_test.go`:
- Around line 105-106: Update the worker goroutines in the realtime snapshot
test around the merged.trips assertions to report the first index or ID mismatch
through a buffered error channel instead of calling require.Less or
require.Equal there; after wg.Wait(), call require.NoError from the test
goroutine and preserve the existing validation behavior.
In `@internal/gtfs/realtime.go`:
- Around line 186-190: Update all realtime getters, including
GetDuplicatedVehiclesForRoute, alert getters, and pointer-returning vehicle/trip
getters, to use a shared deep-copy path that clones nested pointers and slice
fields from mergedRealtime() snapshots. Add a -race regression test that mutates
returned nested data while snapshots are read or rebuilt.
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CLAUDE.mddocs/mutex_contention_analysis.mdinternal/gtfs/agency_filter_test.gointernal/gtfs/gtfs_manager.gointernal/gtfs/gtfs_manager_mock.gointernal/gtfs/gtfs_manager_test.gointernal/gtfs/multi_feed_test.gointernal/gtfs/parallel_realtime_test.gointernal/gtfs/realtime.gointernal/gtfs/realtime_snapshot_test.gointernal/gtfs/realtime_test.gointernal/restapi/vehicles_for_agency_handler_test.go
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…y contract require calls FailNow, which Go only allows on the test goroutine, so the concurrent readers now send the first mismatch back on a channel and the assertion happens after wg.Wait. The snapshot getters hand back snapshot-owned data. That is the point of the change, so the contract is now stated on them rather than left implicit, and a second race test exercises the exported getters instead of only mergedRealtime.
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Fixes #670
docs/mutex_contention_analysis.mdalready names this one:This does the reader half of that.
The problem
rebuildMergedRealtimeLockedbuilds seven O(N) structures underrealTimeMutex.Lock(): the merged trip and vehicle slices, three index maps, the duplicated-vehicle-by-route map, and a four-bucket alert index. Every API read path then tookRLockto reach those fields. Go'sRWMutexgives writers preference, so once a poll cycle starts a rebuild, incoming readers queue behind it.The change
The seven fields become one immutable
mergedRealtime, published withatomic.Pointer. Readers callmanager.mergedRealtime()and take no lock at all.Writers still hold
realTimeMutex. It continues to guard the per-feed maps the rebuild reads from, and the documentedstaticMutex → realTimeMutexordering is untouched. Writer hold time is unchanged. This PR does not try to move the rebuild itself out of the lock, which needs a third mutex ordered againstrealTimeMutexand is really per-feed locking, #479.Measured
Reader latency sampled 3,000 times while rebuilds run in a tight loop:
Being straight about that benchmark: it rebuilds in a loop, which is far more aggressive than the real 30s poll, so the p50 column is not what production looks like. The honest reading is the tail. A reader that happens to arrive during a rebuild used to wait milliseconds and now waits for nothing, because it never takes the lock. I have deliberately not repeated the doc's "75%+ failure rate" figure, since I did not reproduce it.
The awkward part: the mocks
Mock*helpers write the merged view directly instead of going through the feed maps, so they became copy-on-write: load, clone, mutate, store, still underrealTimeMutexso concurrent mock writers serialise.I deliberately did not "fix" them to route through
feedVehicles/feedTrips.vehicles_for_agency_handler_test.go:188documents the current quirk in as many words, thatMockAddAlerttriggers a rebuild which wipes any directly injected vehicle, and roughly 170 mock call sites across the restapi tests sit on that behaviour. Changing it would turn this into a repo-wide test migration.Tests
internal/gtfs/realtime_snapshot_test.go:TestMergedSnapshotIsImmutableAfterPublishholds a snapshot across a rebuild and asserts it did not grow. This is the invariant the whole change rests on.TestConcurrentReadsDuringRebuildruns 8 readers against 40 rebuilds and checks every lookup index is consistent with the same snapshot's slice, so a torn read fails.TestMergedRealtimeBeforeFirstPublishcovers the zero-valueManagerthat several tests construct, which now reads as empty instead of hitting nil maps.BenchmarkRealtimeReadDuringRebuildfor the numbers above.Checks, all clean:
go vet -tags "sqlite_fts5 sqlite_math_functions" ./...go vet -tags "purego" ./...go fmt ./...make test, all 14 packagesgo test -raceon./internal/gtfs/and./internal/restapi/The race run matters more than usual here. Previously the readers were safe because they held
RLock; now they are safe because the snapshot is never mutated after publish, and that is a claim worth having the detector check.Size and sequencing
366 insertions / 168 deletions across 12 files, which is over the 200-line guidance. Removing the fields does not compile without the mock and test migration in the same commit. Say the word if you would rather review it in two passes and I will split it into snapshot-plus-readers, then mocks-plus-tests.
I also updated
CLAUDE.md, which still listed the merged fields as protected byrealTimeMutexand mentioned arealTimeAlertsfield that no longer exists, and added a status line todocs/mutex_contention_analysis.md.One incidental cleanup:
GetVehicleForTriphad a manualRUnlockon two branches rather thandefer, which CONTRIBUTING warns against. The conversion deletes those locks entirely.Note #1286 also edits
MockAddDuplicatedVehicle. I will rebase onto it whenever it lands rather than race it.Summary by CodeRabbit
Improvements
Documentation