feat(world): merge chunk light after ready, not as a pipeline stage

[soloturn]

AI-assisted change proposal. Filed by agent driven by @soloturn via GDD.

Summary

• Light merging was the last stage of ChunkProcessingPipeline, so a chunk could not become ready until all 26 neighbours needed for its 3x3x3 merge existed too. A neighbour that was slow — or never requested at all — stalled the chunk indefinitely, which is why c645e8154 had to add a 10s idle timeout that force-skips the stage.
• Now a chunk goes ready as soon as its own generation finishes, and LateLightMerger merges it afterwards, incrementally, as neighbours arrive. A chunk at the edge of the loaded world simply stays unmerged — visible, with imperfect edge lighting — instead of never becoming ready. When a neighbour loads later, that neighbour's own merge propagates across the shared face and corrects it.
• Light merging was the only multi-chunk stage: the sole production uses of ChunkTaskProvider.createMulti were the three "Light merging" stages removed here. The pipeline's dependency machinery (blockedPositions, skipBlockedStages, MultiplyRequirementChunkTask) is therefore now dead code, but is left in place — removing it here would double the review surface, and ChunkProcessingPipelineTest still covers the API. Follow-up.
• LightMerger marked nothing dirty. That was harmless while merging ran before the chunk was ready, since its first mesh was still to come and picked the result up for free. Now the chunk is already visible, so without marking, merged light never reaches a mesh — ChunkMeshWorker only re-meshes chunks that are isReady() && isDirty().
• LocalChunkView now marks each chunk it writes to, as AbstractFullWorldView already does for runtime block changes. Marking the whole 27-chunk neighbourhood from the merger instead was the obvious alternative, and is wrong: a chunk sits in 27 neighbourhoods, so it gets re-meshed over and over during a world load while most of those merges never touch its light. That exhausted the 768 MB heap inside mesh generation within minutes of starting a world. Found by running the game — no test here caught it, which is worth weighing when reviewing the test plan below.
• Threading. Merging now touches live chunks rather than chunks only the pipeline can see, and deflated light storage can reallocate on write — a concurrent merge would not be merely a stale-value glitch. It runs on the main thread from each provider's update() under a time budget, matching how runtime light propagation for block changes already works (WorldProviderCoreImpl.processPropagation).

The design is @naalit's, from #4879. That PR targets an unmerged branch predating the engine package restructure and cannot be rebased, so this is a fresh implementation of the same idea with no code carried over — credited as co-author. It differs in two ways: #4879 merged off-thread, and it omitted the dirty-marking above.

Test plan

Both tests were checked to fail against the pipeline-stage merge, not merely pass against this one — a test that passes either way would not be a regression test.

• LateLightMergerMteTest — requests relevance for a single chunk, and separately for an unpadded 3x3x3 region, bounding readiness at 8s: comfortably above generation time (~100-200ms observed), comfortably below the ~10s the old idle-skip needed. Restoring the removed pipeline stage makes both time out:
  Timed out waiting for an isolated chunk (no neighbours requested) to become ready - gave up after 8001 ms
• LateLightMergerTest — drives the merger over a plain map and covers the requeue in mergeAt: queue a position, drop a neighbour before the queue drains, and the position must return to needsMerging rather than be lost. Deleting that single line makes the test fail.
• ./gradlew :engine-tests:unitTest — passes; neither new test appears there (both are correctly MteTest/TteTest, so they run under integrationTest).
• ./gradlew :engine:check :engine-tests:check -x test — checkstyle, PMD and SpotBugs clean on the new files.
• Headless server (:facades:PC:server) ran 20+ minutes with no exceptions. Worth stating plainly: this only shows the engine boots and runs stably with the change — a server with no players has no relevance regions and generates no chunks, so it does not exercise LateLightMerger. That gap is what the tests above exist to close.
• Played in-game by @soloturn on both JoshariasSurvival and CoreSampleGameplay, and it works. This covers the mesh-churn fix above, which is the failure it was checked against — an earlier build that blanket-marked whole neighbourhoods exhausted the heap within minutes of a world load.

Performance

Reproduce with the repo's own diagnostic harness — ManyUsersChunkLoadTest, written for #5150:

./gradlew :engine-tests:integrationTestDiagnostic

It is opt-in (@Tag("diagnostic"), excluded from unitTest and integrationTest), simulates a host plus 8 connected clients exploring separate regions, and prints its own timings. Raw values land in engine-tests/build/test-results/integrationTestDiagnostic/. One run per variant below, same machine, back to back.

metric	develop	this PR	this PR before the x/z fix
solo region relevant	527 ms	452 ms	420 ms
8 concurrent user regions	2749 ms	1400 ms	1437 ms
reload 200 chunks w/ entities	11118 ms	4398 ms	1197 ms
connect 8 clients (control)	43927 ms	45782 ms	45314 ms

The control row is client connection — network and entity setup, untouched here. It moves 3-4% between runs, which gives the noise floor.

Against develop: −49% on eight users loading distinct regions at once, and −60% on the 200-chunk reload. The reload case is the scenario in #5150, the multi-second main-thread stalls after loading a previously-explored save. It fits — reloading 200 chunks at once under the old design meant every chunk waiting on its full neighbourhood, with the outer shell only escaping via the 10s idle-skip.

The third column is kept because the difference is informative rather than noise. Before bfa8ce9, light merging propagated into transposed neighbours, so a good deal of its work went to the wrong chunks. Correcting that makes merging do what it was always supposed to, and on the reload case that costs about 3.2s. Worth knowing which part of the remaining time is the price of correctness rather than of this feature.

Caveat: single runs. Treat the solo figure as noise; the two large ones are well outside it.

Open question for reviewers: is the boundary marking worth keeping?

LocalChunkView.setValueAt marks not only the chunk it writes to but any chunk within one block of that write, because a chunk's mesh samples its neighbours' light to shade the faces along a shared boundary. It was added in response to review feedback. It costs nothing measurable. Measured separately on 67ff49df, the same build with it removed gave 406 ms solo, 1506 ms concurrent and 1464 ms reload — slower on two of the three against that build's 420 / 1437 / 1197, so the difference is inside the noise.

It is worth being precise about how narrow the case is, because it is narrower than it first appears. A stale mesh needs all of:

1. a light value changes on a chunk boundary during a merge;
2. the neighbour's side of that boundary is opaque, so propagation writes nothing into it — otherwise it dirties itself;
3. that neighbour is already ready and meshed; and
4. that neighbour is never later merged as a centre in its own right — otherwise it dirties itself then.

Condition 4 is the narrow one. A chunk is merged as a centre as soon as its own 27-neighbourhood completes, so for everything except the outer shell of the loaded world this only shortens a transient seam. At the shell it is permanent — but a shell chunk stops being one as soon as the player moves toward it and its neighbours load, at which point it self-corrects. So the genuinely permanent case is a seam on a solid face, at render distance, on a frontier nobody walks toward.

Kept because it measures free and closes the case properly. Reverting it is defensible if the simpler write path is preferred — the four conditions above are what would be traded away.

Notes for review

• ChunkRegionFuture.REQUIRED_CHUNK_MARGIN pads every relevance request by one extra shell, with a puzzled // FIXME: Is the complete relevance region not actually loaded‽. That padding turns out to be the same root cause: f324128dd records that "before the ChunkProcessingPipeline fix, padding/neighbour chunks near the edge of a region could never complete their own stages". relevanceRegionBecomesFullyReadyWithoutMargin shows the padding is no longer structurally needed for readiness — but it is left untouched here, since other tests share it and "not needed for readiness" is not "not needed for anything".
• Unrelated pre-existing bug found while writing the tests, deliberately not fixed here: RelevanceSystem.addRelevanceEntity streams a BlockRegion through .sorted(), but that iterator hands out a reused, mutable Vector3i. Since sorted() buffers before emitting, buffered positions alias to a stale value, so a few createOrLoadChunk calls go to the wrong position. Not a hang — updateRelevance()'s follow-up pass uses the defensive-copying getNeededChunks() and still requests the right ones — just wasted work. It is documented in the MTE test's javadoc, which is also why that test does not assert "neighbours stayed unloaded".
• RemoteChunkProvider gets the same treatment for consistency, but multiplayer is untested here.

Related — and what this unblocks

• refactor: do light merging later, remove ChunkProcessingPipeline #4879 — the draft this ports; can be closed if this lands.
• feat: rework chunk ordering using Reactor #4822 — feat: rework chunk ordering using Reactor. Still open, and the natural thing to do next. It was reviewed favourably in 2021 and stalled rather than being rejected. It replaces the eager push model — every relevant position submitted up front into a PriorityBlockingQueue whose heap order is fixed at insertion time, so ordering goes stale as the player moves — with a bounded pull model that re-sorts by current relevance on each request. This PR makes that materially easier: light merging was the only stage holding chunks pending their neighbours, and the defer-loop in feat: rework chunk ordering using Reactor #4822's processingInfoReactor() exists largely to service exactly that dependency. Two caveats for whoever picks it up: it cannot be rebased (it targets an unmerged branch predating the package restructure in Terasology engine #5192), so treat it as a specification rather than a patch; and its unresolved design question — how to wake a paused pull stream from the main thread — is already argued out in its review thread and should be answered rather than rediscovered.
• Chunk generation order goes stale as the player moves #5361 — the underlying issue behind feat: rework chunk ordering using Reactor #4822, filed so the problem is tracked independently of the PRs that have attempted it: chunks near you generate after chunks far away, especially while moving. It also records a pre-existing RelevanceSystem aliasing bug found while testing this PR, which sits in the very code a pull-based rework would replace.

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📝 Walkthrough

Summary by CodeRabbit

• New Features

  • Improved incremental chunk lighting as chunks become available.
  • Lighting updates now wait for complete neighboring regions and retry when required chunks are temporarily unavailable.
  • Pending lighting work is processed incrementally within available update time.

• Bug Fixes

  • Fixed lighting merges being delayed or skipped when neighboring chunks load or unload.
  • Improved handling of chunk provider resets and world purges.
  • Fixed propagated lighting changes not consistently triggering chunk remeshing.
  • Reduced unnecessary updates for changes away from chunk boundaries.

Walkthrough

Chunks can become ready before their full lighting neighbourhood exists. LateLightMerger defers lighting until required chunks are available. Local and remote providers integrate this lifecycle. Tests cover neighbour unloading and unpadded relevance regions.

Changes

Late lighting flow

Layer / File(s)	Summary
LateLightMerger processing engine/src/main/java/org/terasology/engine/world/chunks/LateLightMerger.java	Tracks ready chunks, queues complete 27-chunk neighbourhoods, processes merges within the caller-supplied budget, requeues missing-neighbour work, and clears unloaded state.
Provider lifecycle integration engine/src/main/java/org/terasology/engine/world/chunks/localChunkProvider/LocalChunkProvider.java, engine/src/main/java/org/terasology/engine/world/chunks/remoteChunkProvider/RemoteChunkProvider.java	Providers notify LateLightMerger on readiness and unload, process pending work during updates, clear state during reset or purge, and remove blocking pipeline light merging.
Propagation dirty tracking engine/src/main/java/org/terasology/engine/world/propagation/LocalChunkView.java	Propagation marks the updated chunk and present neighbouring chunks near boundary writes as dirty.
Late lighting validation engine-tests/src/test/java/org/terasology/engine/world/chunks/LateLightMergerTest.java, engine-tests/src/test/java/org/terasology/engine/integrationenvironment/LateLightMergerMteTest.java	Tests cover missing-neighbour requeueing, dirty marking, isolated chunk readiness, and unpadded 3×3×3 relevance regions.

Estimated code review effort: 3 (Moderate) | ~25 minutes

Merge Risk: 🟡 Moderate · up to 67ff4

The change can dirty the wrong chunk and leave updated lighting absent from the affected mesh, while an expired tick budget may allow an extra expensive main-thread merge. These bounded correctness and frame-time risks should be fixed or explicitly accepted before merge.

Sequence Diagram(s)

sequenceDiagram
  participant ChunkProvider
  participant LateLightMerger
  participant ChunkCache
  participant LightMerger
  ChunkProvider->>LateLightMerger: Notify chunkReady(chunkPos)
  LateLightMerger->>ChunkCache: Check complete 27-chunk neighbourhood
  ChunkProvider->>LateLightMerger: processPending(tickStartTime, tickBudgetMs)
  LateLightMerger->>LightMerger: Merge ready neighbourhood
  LightMerger->>ChunkCache: Propagate light and mark affected chunks dirty

Loading

Poem

A rabbit watched the chunks turn bright,
Neighbours joined the queue just right.
If one hopped off, the merge delayed.
When it returned, the light was made.
“Ready!” cried the rabbit.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Title check	✅ Passed	The title clearly and concisely describes moving chunk light merging to post-readiness processing.
Description check	✅ Passed	The description directly explains the implementation, rationale, tests, performance results, and related follow-up work.

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• Create PR with unit tests
• Commit unit tests in branch feat/late-light-merging

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[coderabbitai]

Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

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Inline comments:
In
`@engine/src/main/java/org/terasology/engine/world/propagation/LocalChunkView.java`:
- Around line 57-63: Update the light-write handling in LocalChunkView to expand
the affected block region by one block around pos and dirty every intersecting
chunk in the local 3x3x3 view, rather than only calling chunk.setDirty(true) for
the containing chunk. Mirror the affected-chunk logic used by
AbstractFullWorldView.setValueAt while preserving the existing behavior for the
changed chunk.

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• engine-tests/src/test/java/org/terasology/engine/world/chunks/LateLightMergerTest.java
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• engine/src/main/java/org/terasology/engine/world/propagation/LocalChunkView.java

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• engine-tests/src/test/java/org/terasology/engine/world/chunks/LateLightMergerTest.java

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[soloturn]

Background: why this differs from #4879 in exactly two places

Expanding on the two differences noted in the description, since both are easy to read as nitpicks and neither is.

Both come from the same shift. Before this change, light merging touched chunks nobody else could see. Afterwards it touches chunks that are live in the world. #4879 kept the old assumptions, which were correct for the old design.

1. Merging off-thread

#4879 does this in processReadyChunk:

chunk.markReady();
GameScheduler.scheduleParallel("light merging",
        () -> ...forEach(this::tryLightMerging));

That was safe while merging was a pipeline stage: the chunk wasn't in chunkCache, wasn't markReady(), and had never been meshed. It was private to the pipeline, so mutating it concurrently raced with nothing.

Once merging moves after readiness, the same code mutates a chunk that is simultaneously:

• being tessellated by the renderer — ChunkMeshWorker generates meshes off the main thread (only the GL upload is main-thread: "Does GL stuff, must be on main thread!"), and
• readable by gameplay on the main thread.

The data being mutated is not a plain array either. Chunk light lives in a TeraArray, which after deflate() is a sparse representation, so a write can reallocate — promoting a sparse row to a dense one. A concurrent reader therefore risks observing the structure mid-swap, not merely a stale byte.

There is a second race in that approach: needsLightMerging is a plain Sets.newHashSet(), written both from the parallel scheduler and from processReadyChunk/unloadChunkInternal on the main thread, unsynchronised.

This PR runs the merge on the main thread from each provider's update() under a time budget, following the precedent already in the engine — WorldProviderCoreImpl.processPropagation(), runtime light propagation for block changes, has always run on the main thread. The trade-off is real and deliberate: it costs frame time, which is what the budget bounds. Off-thread would be faster but needs locking that doesn't exist today. If profiling later justifies it, that's a separate, measurable change.

2. No dirty marking

LightMerger.merge writes light values but marks nothing dirty, and LocalChunkView.setValueAt — the actual write path — didn't either.

Harmless in the old design, for the same reason as above: merging was the last stage before readyChunks::add, so the chunk had never been meshed and its first mesh picked the merged light up for free.

Afterwards it silently defeats the feature, because of how re-meshing is triggered. ChunkMeshWorker.update() only re-emits a chunk when:

chunk.isReady() && chunk.isDirty()

By then the chunk is already ready and already meshed. With nothing setting the dirty flag, merged light sits in the chunk's data and never reaches a mesh — you would get the visible half of the feature (chunks appearing sooner) and permanently lose the other half, so edge seams would never heal.

Worth recording the middle step, because it is the part that actually cost something. The first correction here was to mark all 27 chunks dirty from the merger. That is correct but ruinous: a chunk belongs to 27 neighbourhoods, so it gets re-meshed over and over during a world load, and that exhausted the 768 MB heap inside mesh generation within minutes. The right place is the write site, LocalChunkView.setValueAt, exactly as AbstractFullWorldView already does for runtime block changes — so only chunks whose light actually moved are re-meshed.

The sequence, in short: no marking → invisible; over-marking → OOM; marking where the write happens → correct. Only the middle one was caught by running the game; neither of the tests here would have found it.

[coderabbitai]

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Inline comments:
In
`@engine/src/main/java/org/terasology/engine/world/propagation/LocalChunkView.java`:
- Around line 89-94: Align LocalChunkView’s chunk-coordinate mapping with
LightMerger.merge’s x-then-y-then-z ordering, ensuring z is the fastest-changing
index. Update chunkIndexOf and all consumers, including getValueAt, setValueAt,
and the affected-chunk dirtying loop, consistently rather than changing only the
loop.

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Inline comments:
In
`@engine/src/main/java/org/terasology/engine/world/chunks/LateLightMerger.java`:
- Around line 82-112: Update LateLightMerger.processPending to check elapsed
time against tickBudgetMs before polling readyToMerge or calling mergeAt. When
the shared budget is exhausted, exit while leaving the queued position intact;
otherwise dequeue and merge as before, retaining the existing backlog logging
behavior.

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• engine-tests/src/test/java/org/terasology/engine/world/chunks/LateLightMergerTest.java
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• engine/src/main/java/org/terasology/engine/world/chunks/localChunkProvider/LocalChunkProvider.java
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• engine/src/main/java/org/terasology/engine/world/chunks/localChunkProvider/LocalChunkProvider.java

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[soloturn]

@BenjaminAmos if you could also verify the single commits? the original made it quick. this one: https://github.com/MovingBlocks/Terasology/compare/346ad391036bbae01375c9d3a683a5951454e7a7..1bad54f7702bcdbf71cb52ede20f4d81a1a97b90 seemed not have it visually improved, just slowed down again.

then, transposing x and z seemed to have the opposite effect. it not looks better - but i am not too sure how it should look like?

[BenjaminAmos]

This is a first-pass review - This has been reviewed commit-by-commit but I have not scrutinised it completely thoroughly yet.

[soloturn]

Re-mesh cost of late light merging, measured

Since "does this make it slow" is the open question on this PR, here is a number for the part that actually costs something — re-meshing.

I simulated a 147-chunk load (7x3x7, one flat surface under open sky) driving the real LateLightMerger bookkeeping: chunks arrive in a fixed-seed shuffled order in per-tick batches, processPending drains once per tick, and every chunk left dirty is then tessellated for real through ChunkTessellator.generateMesh — the same call ChunkMeshWorker makes, everything short of the GL upload.

304 re-meshes for 147 chunks. 147 of those are each chunk's mandatory first mesh, since a ChunkImpl starts dirty. So late light merging costs ~1.07 extra re-meshes per chunk.

One is the floor. The whole premise is that a chunk becomes visible before its lighting is final, so correcting it afterwards necessarily means meshing it a second time. It cannot cost less than one and still do anything.

Where the time actually goes

• one merge: ~2.7ms
• one re-mesh: ~3.5-4.4ms, and a merge dirties 18 of its 27 chunks

The merge itself is a small fraction of what it ultimately triggers, and the trigger count is already at its minimum.

Things I tried that turned out not to matter

All measured, all approximately zero. Listing them so nobody spends time re-deriving them:

• Sunlight regen marking chunks dirty. No mesh samples regen — BlockMeshPart.appendLightData reads only getSunlight and getLight — so those marks look like pure waste. But dirty-chunks-per-merge is 18.0 with and without them: regen and sunlight propagate through the same columns, so regen's dirty set was already a subset of sunlight's.
• deflateSunlight() at the end of every merge, on a chunk that is not final, since later neighbour merges write to it again. 1us against a 2741us merge.
• chunkReady's O(27^2) neighbourhood scan — up to 729 map lookups and ~750 Vector3i allocations per arriving chunk. 2us per arrival.
• Skipping the boundary dirty-mark for a neighbour already queued for its own merge, since that merge re-meshes it anyway. This one is real but small: 304 -> 291 re-meshes, about 5%, in exchange for threading a predicate through LightMerger and LocalChunkView and keeping a second set in lockstep with readyToMerge. Not proposing it here — a follow-up at most.

Caveat

This is a harness, not the running game: no renderer, no GL upload, uniform terrain, and 147 chunks rather than a real view distance. It bounds the re-mesh count, which is the structural question the PR raises. It says nothing about frame pacing in a real session — that needs a profiler on an actual client, which I have not run.

[soloturn]

Ran the existing ManyUsersChunkLoadTest diagnostic (integrationTestDiagnostic, tagged diagnostic - not part of normal CI, opt-in timing tool for #5150) on this branch and, separately, on an unmerged develop baseline, for a direct before/after comparison.

Metric	this branch	develop baseline	Δ
solo region relevant	511ms	583ms	~12% faster
connect 8 clients	44028ms	43514ms	same (unrelated network/join overhead)
8 concurrent regions relevant	1149ms	1971ms	~42% faster
reload 200 chunks w/ entities	3120ms	12162ms	~4x faster (74% reduction)

The reload number lines up directly with #5150's original complaint (multi-second main-thread stalls reloading a saved world with entities) - 12.2s down to 3.1s.

Caveat: single run each side, not repeated for statistical confidence. The size of the gap (4x on the headline number) makes it unlikely to be pure noise, but treat this as a strong signal rather than a certified benchmark until re-run a few more times.