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fix(tiles): read a large GeoTIFF tile index in place, doubling the header step - #826
A GeoTIFF on the web whose tile index is larger than about 3 MB is refused, though the header limit is 32 MB:
layer.addUrl https://storage.googleapis.com/mapbiomas-public/initiatives/brasil/collection11/lulc/coverage/brazil_coverage/brazil_coverage-col11_2024.tif
→ the GeoTIFF's header is too large or too scattered to read in place
That file is MapBiomas's land cover of Brazil for 2024: a 713 MB BigTIFF of 154,470 × 146,501 pixels (30 m) in 256-pixel tiles. Each year from 1985 to 2025 is a file like it. Its 346,092 tiles make a 4.15 MB index, all at the start of the file:
TileByteCounts at byte 392, 1.38 MB;
TileOffsets right after, 2.77 MB;
the GeoKeys after them.
Why it was refused.step reads max(b − a, STEP) bytes for each range a..b the layout still needs, and stops after MAX_ROUNDS (12). The intent is that a long array is read in one large step. But the tiff crate reads a tag's values 512 bytes at a time (decode_values in decoder/ifd.rs, a [0u8; 512] buffer), so b − a is never more than 512. Every round therefore read exactly 256 KB. The 12 rounds stopped at 64 KB + 12 × 256 KB ≈ 3.1 MB of header, so MAX_HEADER (32 MB) was never reached. MapBiomas's index needs 18 rounds that way.
This affects any remote GeoTIFF with more than about 260,000 tiles in BigTIFF (12 bytes a tile), or about 390,000 in classic TIFF (8 bytes a tile). These are national 30 m mosaics in 256-pixel tiles.
The fix. The step doubles each round: 256 KB, 512 KB, 1 MB and so on, up to MAX_BYTES (16 MB). It is computed in a new header_read, which also holds the existing checks. Twelve rounds now reach MAX_HEADER well before they run out. MapBiomas's header takes 5 rounds.
Choices in this PR
Doubling, not reading each tag array whole. Reading the arrays whole would mean parsing the IFD's entries before the tiff decoder does. Doubling needs no knowledge of the layout and changes nothing for a cloud-optimised file, whose header is in the first 64 KB read.
The limits are unchanged, and the cap is now strict:MAX_ROUNDS, MAX_HEADER, MAX_BYTES per read, and the 64 × MAX_HEADER bound on where a header part may be. Past them a file is refused with the same message. A read is clamped to what is left of MAX_HEADER, and a range larger than that is refused, so a grown step can't take what is held past the cap. A read also stops where a part already read starts, so no byte is read or held twice; before, the last fixed step could overshoot it by up to 256 KB.
The cost: a header split into a few far-apart pieces now reads more than it needs in its later rounds, at most 16 MB a read. Before, such a header was refused past 3 MB anyway.
Drawing is unchanged. MapBiomas's files have no overviews, so they are drawn only close up: a map tile needing more than 64 file tiles is still "too many GeoTIFF tiles for one map tile". Here that means a view of about 0.2° shows it, one of 1° doesn't.
Testing
a_large_tile_index_is_read_in_a_few_rounds builds the header alone of a tiled GeoTIFF laid out as MapBiomas's: 179,200 pixels a side, 490,000 tiles, a 3.9 MB index. It reads the header range by range as load and step do (layout_by_ranges):
it takes at most 6 reads;
it holds no more bytes than the header has (without stopping at parts already read, 5.9 MB for the 3.9 MB header);
without the fix it fails with "gave up after 12 reads";
a small cloud-optimised file (cog::example) still needs no read past the first;
a read just under the cap is clamped to what is left (1,000 bytes), a range larger than what is left is refused, and so is the read after the cap is reached.
cargo test -p rustgis-tiles passes (164 tests).
cargo fmt --all --check and cargo clippy -p rustgis-tiles --all-targets -- -D warnings are clean.
With rustgis-cli run on this branch, MapBiomas's 2024 file opens in place in about 7 s ("kind": "cog", "inPlace": true). The map exported over Teresina (0.2°) shows the Parnaíba and Poti rivers and the land-cover classes.
The wasm build wasn't checked here, since there is no clang for zstd-sys on this machine. The change has no cfg of its own.
Improved handling of large tile headers so more complex layouts can be read successfully while respecting header size limits.
Kept initial reads efficient for smaller tile headers, avoiding unnecessary additional reads when the header is available immediately.
Limited header reads to the remaining available capacity and prevented requests that exceed the supported header size. These changes preserve existing behavior for common layouts while supporting larger tile indexes within the established limits.
…ader step
The TIFF decoder reads a tag's values 512 bytes at a time, so each header round
read exactly 256 KB and the 12 rounds stopped at about 3 MB, far below
MAX_HEADER (32 MB). A national 30 m mosaic in 256-pixel tiles (MapBiomas's
346,092 tiles, a 4 MB index) was refused as "too large or too scattered".
The step now doubles each round, up to MAX_BYTES.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
No actionable comments were generated in the recent review.
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Walkthrough
The COG header reader now increases its minimum read size exponentially by round, subject to existing limits and remaining header capacity. New tests check large tile-index parsing, the existing example, and header-capacity boundaries.
step delegates read eligibility and length calculation to header_read. The helper applies the round, held-byte, offset, and header-capacity limits, and increases the minimum read size by round.
Large-index parsing tests crates/tiles/src/cog.rs
Test helpers build a large tiled GeoTIFF header and parse requested ranges. Tests check parsing a 490,000-tile index within six additional reads, expected layout values, zero additional reads for the existing example, and header-capacity boundaries.
[Feature]: Fuzz the file readers #779: The issue requests fuzz targets for file readers. This PR changes COG header-read limits but does not add a fuzz target, and the issue’s described target exercises a different reader path.
Suggested reviewers:giswqs
Merge Risk: Minimal · up to d10d4
The larger header reads remain bounded, and parsing can continue across data already held in memory. No concrete merge risk remains in the reviewed changes.
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Docstring coverage is 57.14% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 7 functions across 1 files.
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The title clearly and concisely describes the main change: doubling header reads to support large GeoTIFF tile indexes.
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A rabbit reads the header wide, Each round, the bytes grow by its side. The tile counts fit within the bounds, The tested ranges make their rounds. The COG’s layout comes to light, With capped reads guiding every byte.
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The change is small and correct for its stated purpose. It makes the COG header read step double each round (STEP << rounds), so a large tile index like MapBiomas's 4 MB one no longer runs out of rounds. I read the diff and crates/tiles/src/cog.rs and did not run the tests. I posted one inline comment.
Bugs
None found. STEP << rounds can't overflow because rounds < MAX_ROUNDS (12) is checked first, and MAX_BYTES caps the result. The first round still reads 256 KB, so ordinary COGs behave as before. (high confidence)
Security
Header read can pass the 32 MB cap (crates/tiles/src/cog.rs:371, ~50% confidence): the doubling step isn't limited by the header budget that's left. When have.held() is just under cog::MAX_HEADER, one more read of up to MAX_BYTES can still be requested, so a hostile server can push the total past the cap. The old 256 KB step overshot by at most 256 KB. I suggested clamping to the remaining budget in the inline comment.
Performance
Doubling cuts the round trips for large indexes, which is the point of the PR. Reads for headers that need more than a few rounds are now larger, which is the trade-off. (high confidence)
Quality
Extracting header_read makes the stop conditions testable, and the new test reuses it through layout_by_ranges, so it exercises the same logic as step. The big_index fixture is clear and its layout matches the real file shape.
The test uses panic! and unwrap_or_else, which is fine in test code.
A short comment on why STEP << rounds can't overflow would help future readers (nit).
AGENTS.md
The change adds no unwrap/expect/panic! in non-test code. It has a regression test and doesn't touch wasm, layering, i18n or assets. No violations. (high confidence)
Change size
Compared: the whole PR against main (merge base 45b28ec to PR head 0146b8f)
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🪄 Fix comments: Fix CodeRabbit comments on this PR
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Inline comments:
Review comments at @crates/tiles/src/cog.rs:
- Line 371: Update header_read to enforce MAX_HEADER as a hard memory cap:
calculate the remaining budget from the bytes already held, reject when the
required range exceeds it, and cap the selected read length by that remaining
budget.
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A read is clamped to what is left of the header's cap, so a step grown to
16 MB can't take what is held past it.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
PR #826 replaces the fixed 256 KB header-read step in crates/tiles/src/cog.rs with header_read. The new step starts at STEP and doubles each round. It is capped at MAX_BYTES and at the room left under cog::MAX_HEADER. A new test builds a MapBiomas-style header with a 490,000-tile index.
I found nothing that needs fixing.
Bugs
None found (high confidence).
cog::MAX_HEADER - have.held() can't underflow, because the guard returns None when held() >= MAX_HEADER.
STEP << rounds can't overflow: rounds < 12 gives at most 512 MB in a u64, and .min(MAX_BYTES) bounds the result.
Because a..b is only 512 bytes, the step is what actually drives the read size. The room cap still lets the loop end cleanly: once held() reaches the cap, the next call returns None.
Security
None found. Read sizes stay bounded by MAX_BYTES, MAX_HEADER and MAX_ROUNDS, so a hostile header can't force unbounded reads.
Performance
Low confidence, minor. The doubling step reads up to 16 MB per round, and later rounds read speculatively. For a header that is large but not a tile index, such as many scattered IFDs, this can fetch more than needed. It stays within the 32 MB MAX_HEADER cap, so I'd leave it.
Quality
Nit, low confidence. The test helper layout_by_ranges uses a as usize and (a + len) as usize. That is acceptable in test code, but a + len could overflow for hostile values, which can't happen in this test.
The test is well targeted. It covers the large index, the unchanged single-read case for a normal COG, and the cap boundary in both directions.
AGENTS.md
No violations. No unwrap, expect or panic! in non-test code, and nothing new that is wasm-unfriendly. The panic! calls are inside #[cfg(test)]. No user-visible strings were added.
Change size
Compared: the whole PR against main (merge base 45b28ec to PR head 1186c52).
I reviewed crates/tiles/src/cog.rs, the only file in the PR. I found no bugs. I couldn't run the tests, so this is from reading the code.
Bugs: None found.
header_read computes cog::MAX_HEADER - have.held() only after the held() >= MAX_HEADER guard returns None, so it can't underflow.
STEP << rounds is safe because rounds < MAX_ROUNDS (12) at that point, and the result is clamped by MAX_BYTES and room.
The new early None when b - a > room is stricter than before, because the old code would have started a read that overshot the cap. It gives up one round earlier and saves the request. Since the decoder asks for at most 512 bytes at a time, this should only matter very near the cap. (Confidence: high.)
Security: None. Header sizes stay bounded by MAX_HEADER, MAX_BYTES and MAX_ROUNDS, so a hostile server can't drive unbounded reads or allocations. The doubling step is also capped by room.
Performance: Minor, low confidence.
The read size doubles from 256 KB each round (512 KB, 1 MB, and so on). A header that only slightly exceeds the previous read can now fetch up to roughly twice what it needs, and the cap is MAX_BYTES. This is a reasonable trade for fewer round trips on large tile indexes like MapBiomas's, which has 346,092 tiles.
Quality:
The logic is now in a pure function, header_read, which is easy to test.
The synthetic big_index builder and the layout_by_ranges harness give good regression coverage.
The tests cover the 490,000-tile case and the cap edges: exactly room, one byte over room, and held at the cap.
The panic! and unwrap_or_else calls are in test code only, which AGENTS.md allows.
AGENTS.md: No violations. The change adds no unwrap, expect or unsafe in non-test code, adds no dependencies or assets, and introduces no user-visible strings, so the tl! rules don't apply. The error message is unchanged. The existing cog.rs code appears wasm-safe, and the change adds no std::time use.
Change size
Compared: the whole PR against main (merge base 45b28ec to PR head 119af40).
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Actionable comments posted: 1
🪄 Fix comments: Fix CodeRabbit comments on this PR
Prompt to fix review comments
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Inline comments:
Review comments at @crates/tiles/src/cog.rs:
- Around line 367-377: Update Sparse::held to count the union of stored byte
ranges rather than summing chunk lengths, so overlapping chunks are counted only
once. Keep header_read’s existing limit check based on held() so its
remaining-capacity calculation reflects distinct stored bytes.
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A doubled step could run on over parts already held (the tags at the header's
end, or a later round's), holding those bytes twice and counting them twice
against MAX_HEADER. A read now ends at the next part held.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
I read the whole diff and the Sparse helpers it calls (next_start, held, cog::MAX_HEADER). I found no blocking issues.
Bugs
None found. The overflow and bounds paths all hold:
STEP << rounds can't overflow, since rounds < MAX_ROUNDS (12) is checked first.
MAX_HEADER - have.held() can't underflow, because held() >= MAX_HEADER returns None before it.
next - a can't underflow, because next_start returns only starts greater than a, so the gap is at least 1.
Low confidence, minor: a part already held that starts only a few bytes after a gives a very small read (gap of 1 to 512 bytes). That read spends one of the 12 rounds on little data. It needs unusually scattered tags, and the old code could waste a round the same way.
Low confidence, minor: the new "range larger than what is left" refusal happens before any read. A header that would go over MAX_HEADER is now refused earlier than before. This matches the PR's stated intent and uses the same error message.
Security
No new concerns. Header growth stays bounded by MAX_ROUNDS, MAX_HEADER (32 MB), MAX_BYTES (16 MB per read) and the 64 × MAX_HEADER bound on where a header part may start. The doubling step can't be used to request more than 16 MB per read or more than 32 MB held in total.
Performance
Doubling cuts the rounds needed for large tile indexes from 18 or more to about 5, so far fewer network requests. For a header split into a few far-apart pieces, later rounds can now read up to 16 MB more than needed. The PR states this trade-off.
Quality
header_read pulls the guard checks and the length calculation into one function, so the test can call it directly.
The test builds a MapBiomas-like header and covers the cap, the clamp and the refusal cases.
The test uses panic! and unwrap_or_else, and the a as usize cast is unchecked. Both are fine in test code.
The doc comment on STEP and the one on header_read are accurate.
AGENTS.md
No unwrap, expect or panic! in non-test code; the new code uses saturating_sub and map_or.
There is no cfg and no std::time, so the wasm build is unaffected. The PR notes it didn't check the wasm build locally for lack of clang.
No new user-visible strings, and no new dependencies or assets.
Tests are included. This fix has no hostile-parameter angle that hostile_params_never_panic would need extended for.
Change size
Compared: the whole PR against main (merge base 45b28ec to PR head d10d421)
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Summary
A GeoTIFF on the web whose tile index is larger than about 3 MB is refused, though the header limit is 32 MB:
That file is MapBiomas's land cover of Brazil for 2024: a 713 MB BigTIFF of 154,470 × 146,501 pixels (30 m) in 256-pixel tiles. Each year from 1985 to 2025 is a file like it. Its 346,092 tiles make a 4.15 MB index, all at the start of the file:
TileByteCountsat byte 392, 1.38 MB;TileOffsetsright after, 2.77 MB;Why it was refused.
stepreadsmax(b − a, STEP)bytes for each rangea..bthe layout still needs, and stops afterMAX_ROUNDS(12). The intent is that a long array is read in one large step. But thetiffcrate reads a tag's values 512 bytes at a time (decode_valuesindecoder/ifd.rs, a[0u8; 512]buffer), sob − ais never more than 512. Every round therefore read exactly 256 KB. The 12 rounds stopped at 64 KB + 12 × 256 KB ≈ 3.1 MB of header, soMAX_HEADER(32 MB) was never reached. MapBiomas's index needs 18 rounds that way.This affects any remote GeoTIFF with more than about 260,000 tiles in BigTIFF (12 bytes a tile), or about 390,000 in classic TIFF (8 bytes a tile). These are national 30 m mosaics in 256-pixel tiles.
The fix. The step doubles each round: 256 KB, 512 KB, 1 MB and so on, up to
MAX_BYTES(16 MB). It is computed in a newheader_read, which also holds the existing checks. Twelve rounds now reachMAX_HEADERwell before they run out. MapBiomas's header takes 5 rounds.Choices in this PR
tiffdecoder does. Doubling needs no knowledge of the layout and changes nothing for a cloud-optimised file, whose header is in the first 64 KB read.MAX_ROUNDS,MAX_HEADER,MAX_BYTESper read, and the64 × MAX_HEADERbound on where a header part may be. Past them a file is refused with the same message. A read is clamped to what is left ofMAX_HEADER, and a range larger than that is refused, so a grown step can't take what is held past the cap. A read also stops where a part already read starts, so no byte is read or held twice; before, the last fixed step could overshoot it by up to 256 KB.Testing
a_large_tile_index_is_read_in_a_few_roundsbuilds the header alone of a tiled GeoTIFF laid out as MapBiomas's: 179,200 pixels a side, 490,000 tiles, a 3.9 MB index. It reads the header range by range asloadandstepdo (layout_by_ranges):cog::example) still needs no read past the first;cargo test -p rustgis-tilespasses (164 tests).cargo fmt --all --checkandcargo clippy -p rustgis-tiles --all-targets -- -D warningsare clean.rustgis-cli runon this branch, MapBiomas's 2024 file opens in place in about 7 s ("kind": "cog","inPlace": true). The map exported over Teresina (0.2°) shows the Parnaíba and Poti rivers and the land-cover classes.cfgof its own.🤖 Generated with Claude Code
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