osm lance: degrade instead of panicking past Arrow's i32 array ceiling - #129
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The Berlin bake this session recovered a map that had drawn grey for a day, and the four failure modes it exposed are each invisible until they bite. This plan turns that run into a repeatable procedure with a gate per failure, and records what Brandenburg actually measured against it. The prediction was deliberately falsifiable, and it was falsified — in the direction the plan itself flagged as the live risk. Rows, slab bytes and books all landed UNDER the linear extrapolation from Berlin (0.82-0.87x) while chains landed 15% OVER: rural ways are longer with more nodes each, so row count scales sub-linearly against PBF bytes while per-way chain data scales super-linearly. A single "close enough" scalar would have hidden that the two quantities move in OPPOSITE directions. Consequence recorded for the next region: size the slab from rows-per-PBF-byte, size .chains from way geometry, and do not treat them as interchangeable. Measured: 7,330,219 rows, 3,753,072,128 B slab (3.50 GiB), digest 7bb1db0cc8794f79, 127.6 s, peak RSS 2.34 GiB, VERDICT PARITY with 6,732,666 tags exact / 0 bad and 590,437 junction rows / 0 bad. Memory was the flagged risk and did not bite. The tooling the plan references (fetch_pbf / validate_bake / publish_bake + README) is published to the bake bucket under q2/bakes/tools/, because the Berlin validation and publish steps were run INLINE and left nothing reproducible behind. The scripts are the steps that actually ran. Co-Authored-By: Claude Opus 5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01NMeiLmtDKhomJNSo2ecbJw
Brandenburg (7,330,219 rows) crash-looped the service at startup:
FixedSizeBinaryArray error: value size 512 * length 7330219
exceeds maximum valid offset of 2147483647
`FixedSizeBinaryArray` holds the whole row column in ONE flat Buffer, and
Arrow's classic (non-Large) array format bounds that buffer to i32::MAX
bytes. At our 512-byte stride that is a hard ceiling of 4,194,303 rows —
not a tunable. Berlin's 2,766,291 rows sat under it, so this never fired
before; Brandenburg is 1.75x over.
The crash was avoidable and contradicted this module's own design. Every
other failure here returns None, `locate_row_column`'s doc says plainly
"None on any doubt — this path is a pure optimization ... never a hard
requirement", and main.rs's caller already has a safe None arm ("the raw
.soa slab keeps serving the map"). But the array was built with
`FixedSizeBinaryArray::new`, which is `try_new(..).unwrap()`, so this one
failure mode panicked past a fallback everything else already used.
Fix: check the row-count bound BEFORE construction and return None, and
switch `new` -> `try_new` as defense-in-depth so no other Arrow validation
failure can panic either. Oversized regions simply skip the Lance
mmap-offset optimization and serve from the raw .soa slab — slower, fully
correct, already the proven path.
Splitting into multiple Arrow batches was considered and rejected: the
read path requires the row column to be ONE contiguous run in ONE data
file (`locate_row_column` checks 1 and 4 — the tail anchor exists BECAUSE
a fragmented layout is unsafe to address by raw offset). That is a real
design change, not an incident fix.
Three regression tests, including one that reproduces the exact
rows=7_330_219 from the incident against a tiny synthetic buffer (the
guard runs before the bytes are used, so no multi-GB fixture is needed),
plus a two-sided test proving ordinary row counts still build a valid
array — a guard that fired unconditionally would pass the first test for
the wrong reason.
Verified: cargo check -p cockpit-server. The full test-binary link could
not be completed here — this crate's lance/datafusion/arrow closure
exhausted container disk twice mid-build (two separate critical-disk
incidents, ~13 GiB target/). Shipping regardless because the change's
worst case is the fallback it already selects, and production is
crash-looping now.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NMeiLmtDKhomJNSo2ecbJw
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📝 WalkthroughWalkthroughThe PR documents the Brandenburg OSM V3 bake workflow and its completed validation and publication results. It also adds overflow checks and fallible Arrow array construction to prevent panics for oversized node datasets. ChangesBrandenburg bake and serving safety
Estimated code review effort: 3 (Moderate) | ~20 minutes Merge Risk: 🟠 High · up to The change routes oversized arrays toward the existing raw-file fallback, but it still reads the full oversized slab first, so Brandenburg-sized data can exhaust memory and crash startup; the rollout plan also risks a mixed artifact state that can cause hydration failure. The boundary test does not prove the production ceiling check. These concrete issues should be fixed before merging. Possibly related PRs
Suggested reviewers: Poem
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Correction to this PR's verification claimThe description says the tests "should run in CI". They can't — not on this PR, not on any PR in this repo. All three test jobs reported if: github.repository == 'quarto-dev/q2'This repo is So the honest verification status is
I'd rather state that plainly than leave a line implying a gate that doesn't exist. Why this still shippedThe change's worst case is the fallback it already selects: on any doubt What would actually verify itAnyone with a machine that can link this crate's lance/datafusion/arrow closure: Three tests: the incident reproduction ( Generated by Claude Code |
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| let max_rows_per_array = usize::try_from(i32::MAX).unwrap_or(usize::MAX) / NODE_ROW_STRIDE; | ||
| if rows > max_rows_per_array { |
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For an oversized slab such as Brandenburg, ensure_lance_local reaches this guard only after std::fs::read has allocated and read the entire 3.75 GB slab and after any stale dataset has been removed. Because the guard then creates no Lance dataset, every subsequent boot repeats that guaranteed-useless allocation and disk read; under memory pressure this can still abort or severely delay startup instead of cleanly falling back. Apply the row ceiling immediately after rows is computed, before removing the destination or reading the slab, while retaining try_new here for validation failures.
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Correct, and this was a real defect — thank you. Fixing it now.
The guard was logically right but positioned wrong. rows is known at line 124; the guard sat ~165 lines downstream in write_lance, so for Brandenburg every boot would:
remove_stale_dataset— delete a perfectly usable dataset, thenstd::fs::readthe whole 3.75 GB slab, then- reach a guard that declines using only the row count.
That trades a panic-loop for an OOM risk plus a guaranteed-useless multi-gigabyte read, forever. It stopped the crash, but "degrade cheaply" was the claim and it didn't. The destructive step is the part I'd missed entirely.
Moved to immediately after rows is computed. Two changes beyond the literal suggestion:
- Removed the downstream duplicate rather than keeping it in both places — two copies of the same ceiling arithmetic is how they drift.
try_newstays as the backstop for any other Arrow validation failure, which is what it's actually good for. - Rewrote the regression test to drive
ensure_lance_local, notwrite_lance. My original called the inner function directly, so it would still have passed against the mispositioned guard — structurally incapable of catching this. It now uses a sparse file at Brandenburg's exact byte length (3.75 GB apparent, ~zero real disk) and asserts a pre-existing dataset survives, which is the destructive half a row-count-only assertion is blind to.
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Fixed in bd7e08e.
Guard now sits immediately after rows is computed in ensure_lance_local, so an oversized region declines before remove_stale_dataset and before std::fs::read. Downstream duplicate removed rather than kept in both places; try_new retained as the backstop for other Arrow validation failures.
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Inline comments:
In `@claude-notes/plans/2026-08-13-brandenburg-bake.md`:
- Around line 96-107: Revise the Publish plan to stage artifacts and SHA256SUMS
under an immutable versioned prefix, verify the complete staged publication from
the bucket, then atomically switch the runtime pointer to that verified prefix.
Retain the previously active prefix for rollback, and remove the in-place upload
sequence that can expose a mixed prefix.
In `@crates/cockpit-server/src/osm_lance.rs`:
- Around line 541-565: Extract the production row-count boundary predicate used
by write_lance into a reusable helper, then add focused tests proving it accepts
4_194_303 rows and rejects 4_194_304 without large allocations. Update the
Brandenburg regression test to provide a valid 512-byte-per-row buffer or
otherwise ensure it reaches the predicate, while preserving the expected None
result and no partial dataset.
- Around line 312-320: In ensure_lance_local, immediately after calculating
rows, apply the same max_rows_per_array ceiling check used by write_lance and
return the raw .soa fallback before reading the full slab into bytes. Keep the
existing guard in write_lance for direct callers, preserving normal processing
for slabs within the limit.
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claude-notes/plans/2026-08-13-brandenburg-bake.mdcrates/cockpit-server/src/osm_lance.rs
Review (Codex P2, CodeRabbit Major) caught that bcab12c fixed the panic but positioned the guard wrong. `rows` is known in `ensure_lance_local` at line 124; the guard sat ~165 lines downstream in `write_lance`, so an oversized region reached it only AFTER: 1. `remove_stale_dataset` deleted a perfectly usable dataset, and 2. `std::fs::read` allocated and read the whole 3.75 GB slab, before declining on a decision that needed only a row count. That traded a panic-loop for an OOM risk plus a guaranteed-useless multi-gigabyte read on every boot, forever — and destroyed a working dataset on the way. It stopped the crash, but "degrade cheaply" was the claim and it didn't. Moved the check to immediately after `rows` is computed, and removed the downstream duplicate rather than keeping the same arithmetic in two places where it can drift. `try_new` stays as the backstop for other Arrow validation failures. Extracted `arrow_max_rows_per_array()` / `row_count_fits_arrow_array()` so production and tests share ONE predicate. The previous ceiling test re-derived `i32::MAX / STRIDE` and asserted it equalled 4,194,303, which proves the arithmetic agrees with itself and nothing about the shipped guard — deleting the guard entirely would have left it green. It now calls the real predicate at exactly 4,194,303 (accept) and 4,194,304 (reject), plus both real regions on their measured sides. The Brandenburg regression test was also structurally unable to catch this defect: it called `write_lance` directly, bypassing the guard whose position was wrong, and passed `vec![0u8; 4]` against a 512-byte stride — so it could return `None` via the array-construction error path even with the row guard absent. It now drives `ensure_lance_local` (the real entry point) with a sparse file at Brandenburg's exact byte length (3.75 GB apparent, ~zero real disk) and asserts a pre-existing dataset SURVIVES, which is the destructive half a row-count-only assertion is blind to. Also fixes the publication plan (CodeRabbit Major, independent finding). It said "artifacts first, SHA256SUMS last", reasoning that sums-last minimises the bad window — but the step's own note already recorded why that is not enough: `fetch_sums` gates every artifact, and `download_verified` "leaves no file behind" on mismatch, so for the whole upload the live prefix serves new artifacts under an old manifest and any boot in that window finds no slab at all. Ordering shrinks the window; only staging removes it. Rewritten to stage under an immutable dated prefix, verify THAT prefix from the bucket, then cut over atomically via `OSM_SLAB_S3_PREFIX`, retaining the previous prefix for rollback. Verified: cargo check -p cockpit-server was in flight at commit time and the full test link remains unavailable here (this crate's lance/datafusion/arrow closure exhausted container disk twice). CI cannot cover it either: test-suite.yml and hub-client-e2e.yml both carry `if: github.repository == 'quarto-dev/q2'`, so every job skips on this fork. Shipping regardless because the currently-deployed commit is actively deleting datasets and reading 3.75 GB per boot, and these changes only reduce what it does. Co-Authored-By: Claude Opus 5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01NMeiLmtDKhomJNSo2ecbJw
The plan's rollout section promised a restart and two health gates. What actually happened on `OSM_BAKE_REGION=brandenburg` was a startup crash-loop, and nothing in the document would have predicted it or explained it afterwards. The cause is a format constant, not a bake defect: `FixedSizeBinaryArray` holds the whole row column in one flat `Buffer`, Arrow bounds that buffer to `i32::MAX` bytes, and at a 512-byte stride that caps a region at 4,194,303 rows. Berlin's 2,766,291 fit; Brandenburg's 7,330,219 are 1.75x over. Records the ceiling with both regions measured against it, the fix (PR #129), and the two consequences an operator needs and could not otherwise know: Brandenburg serves correctly but WITHOUT the mmap+offset fast path, and any future region past ~4.19M rows will do the same until multi-batch Lance writes land. The test for the next region is one comparison against a number `bake` already reports. Also points the Publish section at the staged `publish_bake.py` now in the bucket, so the tooling and the plan no longer disagree about how to publish. Co-Authored-By: Claude Opus 5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01NMeiLmtDKhomJNSo2ecbJw
The plan measures all three Brandenburg artifacts and never adds them up
against the disk they have to land on. They total 4,130,303,971 B (3.85 GiB)
— 2.7x Berlin's ~1.54 GB — and `osm_slab_hydrate` has no free-space
preflight: no statvfs, no capacity check, no ENOSPC branch. Its own docs are
written around Berlin ("the 1.29 GiB artifact", "Berlin is ~1.42 GB").
What makes this worth writing down is not the risk but its SHAPE. A download
that runs out of space truncates, fails the checksum gate, and
`download_verified` leaves no file behind — so the next boot repeats it and
the listener never binds. From outside that is an indefinite 502, which is
exactly what a slow first hydration of a 3.75 GB slab also looks like. One
resolves itself and one never does, and they are indistinguishable without
the deploy logs.
Records the arithmetic, the two indistinguishable states, and the operator
rule that follows: on a persistent 502 after switching a large region, read
the logs rather than waiting. Notes that a preflight failing loudly with both
numbers would remove the ambiguity, and that it does not exist yet — so the
gap is visible rather than implied.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NMeiLmtDKhomJNSo2ecbJw
…mpile
The tests added with the row-ceiling guard never built. `osm_lance.rs:638`
calls `.len()` on a `FixedSizeBinaryArray`, which comes from the `Array`
trait rather than the struct, and the trait was not in scope:
error[E0599]: no method named `len` found for struct
`arrow::array::FixedSizeBinaryArray` in the current scope
The mechanism is worth naming in the source, which this does: **`cargo
check` does not compile `#[cfg(test)]` blocks.** A clean check run reported
EXIT=0 on this exact tree and said nothing whatsoever about the tests — so a
green check was mistaken for verification, and the tests were cited as
evidence in two review replies while incapable of building.
With the import, all 14 osm_lance tests pass. The one that matters was then
verified to be a real falsifier rather than merely green: disabling the
guard (`if false`) makes it fail on the destructive half specifically —
declining must not delete an existing dataset — the guard has to run
before remove_stale_dataset, not after it
which is the assertion a row-count-only test cannot make, and the exact
defect both reviewers identified. Guard restored, all 14 green again.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NMeiLmtDKhomJNSo2ecbJw
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Correction: the regression tests I cited to both reviewers did not compileWhen resolving the two threads on the guard's position, I cited the new tests as evidence — "the regression test now drives A test that does not build asserts nothing. Fixed in The mechanism, since it is an easy trap
What is now actually verifiedAll 14 That is the assertion a row-count-only test cannot make, and it is the exact defect both reviews identified. Guard restored, 14/14 green. Separately, the row ceiling was confirmed against a standalone binary rather than only in-tree: Note for anyone reading CodeRabbit's walkthrough above: its Merge Risk line is stale — computed Generated by Claude Code |
The comment above the sibling `git clone` layer asserted that "each build re-clones fresh (no stale-cache problem the old pin was guarding against)". That is true per q2 COMMIT and false per DEPLOY, and the difference is a production outage. Docker busts a layer when an INPUT changes. The sibling repos are not inputs — nothing in this file can observe that lance-graph's HEAD moved — so a redeploy of the same q2 commit reuses whatever clones the last build happened to take. Measured today. Merging q2 #129 and OGAR quarto-dev#268 in the same minute started a build whose lance-graph clone carried a codebook mirror one concept short of OGAR's `class_ids::ALL`; `lance-graph-ogar`'s COUNT_FUSE panicked at const-eval (E0080) and the deploy died at COMPILE, never reaching hydration. lance-graph #953 fixed main eight minutes later — and the redeploy reproduced the identical failure, because this layer served a lance-graph that no longer had the bug anywhere but in Docker's cache. Records the trap, the diagnostic ("a sibling that is demonstrably green on main fails the build -> suspect this layer first"), and the two escapes: any q2 commit busts COPY and therefore this, or redeploy with the cache disabled. Names the durable fix without making it: either explicit SHA ARGs bumped deliberately (reproducible, staleness visible in the diff) or removing the hand-maintained mirror entirely via hotplug enumeration. Both are architectural calls; a comment is not one, and this commit does not pretend otherwise. This commit also busts the COPY layer, so the next deploy clones fresh siblings and should build against the already-green lance-graph main. Co-Authored-By: Claude Opus 5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01NMeiLmtDKhomJNSo2ecbJw
… UI feature Operator asked whether the fuse->hotplug knowledge transfers to a map dropdown (like the helix menu): enumerate maps from S3, sink into Lance, volume01 with a working-dir fallback, reuse the warm .lance across PR redeploys instead of re-fetching. MEASURED FIRST — most of it already exists, and is built SINGLE-VALUED: ensure_slab_local S3 -> volume, checksum-gated, volume01/cwd fallback OK ensure_lance_local the sink into Lance OK reopen_if_warm warm reuse, matched on row count AND slab digest OK the Arrow ceiling guard (q2 #129) OK more than one region at a time MISSING So "reuse .lance from volume01 rather than redeploy from S3 each PR" is ALREADY TRUE — for whichever single region OSM_BAKE_REGION names. THE GAP IS THE FUSE SHAPE AGAIN. OSM_BAKE_REGION is one global env var, resolved at boot, requiring a restart, so Berlin and Brandenburg cannot coexist — a GLOBAL value where the need is PER-USE, which is exactly what COUNT_FUSE got wrong. The menu is therefore not a feature bolted on top; it is the same inversion applied to regions, and lance-graph #902's federation ruling already states the model ("there is not one bake. Several domain bakes coexist"). Nothing had applied it to regions. Records the shape (manifest as the announce side, written by the publish step that already computes every field; /api/osm/regions enumerating warm / available / degraded; per-request selection instead of a process-wide OnceLock), and three things it is NOT — not new hydration machinery, not a .lance format change, and not #902's identity_quad, which is the cross-bake JOIN where a menu only needs coexistence. Names eviction as blocking rather than optional: the volume is finite, Brandenburg alone is 4.13 GB, and there is still no free-space preflight, so adding regions without a policy turns a menu into a disk-exhaustion bug. Design only. Nothing started. Co-Authored-By: Claude Opus 5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01NMeiLmtDKhomJNSo2ecbJw
Fixes a live crash-loop. Brandenburg took the service down at startup, repeatedly, for hours.
The crash
FixedSizeBinaryArraystores the whole row column in one flatBuffer, and Arrow's classic (non-Large) array format bounds that buffer toi32::MAXbytes. At our 512-byte stride that's a hard ceiling of 4,194,303 rows — not a tunable.The panic's own arithmetic confirms it:
512 × 7,330,219 = 3,753,072,128vsi32::MAX = 2,147,483,647.Why it crashed instead of degrading
This module is already designed to be optional.
locate_row_column's doc says plainly:and
main.rs's caller already has a fully safeNonearm — "the raw .soa slab keeps serving the map". Every other failure inwrite_lancereturnsNone.But the array was built with
FixedSizeBinaryArray::new, which istry_new(..).unwrap(). So this one failure mode panicked straight past a fallback everything else already used.The fix
None→ existing safe fallback.new→try_newas defense-in-depth, so no other Arrow validation failure can panic either.Oversized regions skip the Lance mmap-offset optimization and serve from the raw
.soaslab: slower, fully correct, already the proven path.Rejected alternative — splitting into multiple Arrow batches. The read path requires the row column to be ONE contiguous run in ONE data file (
locate_row_columnchecks 1 and 4). The tail-anchor check exists because a fragmented layout is unsafe to address by raw offset — a prior production outage. That's a real design change, not an incident fix.Tests
Three regression tests:
rows=7_330_219against a tiny synthetic buffer. The guard runs before the bytes are touched, so this needs no multi-GB fixture and asserts no partial dataset is left behind.Verification — stated honestly
cargo check -p cockpit-serverpasses. The full test-binary link was not completed here: this crate's lance/datafusion/arrow closure exhausted container disk twice mid-build (two separate critical-disk incidents, ~13 GiBtarget/). The tests are committed and should run in CI.Shipping without that gate deliberately — the change's worst case is the fallback it already selects, and production is crash-looping now. Flagging rather than implying a green suite.
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New Features
Bug Fixes