lib/raster3d/tilewrite: Use >= for tile index bounds check

[echoix]

The bounds check uses > instead of >= for the upper bound. A tileIndex equal to map->nTiles is out of range (valid indices are 0 to map->nTiles - 1), so this condition should use >= map->nTiles to correctly reject that value.

[wenzeslaus]

Here is the Claude Code with Fable 5 review you asked for (I'm also adding a test):

Review of PR #7723 — lib/raster3d/tilewrite: Use >= for tile index bounds check

Overview

A one-line fix in Rast3d_write_tile() (lib/raster3d/tilewrite.c:135) changing the upper-bound check on tileIndex from > map->nTiles to >= map->nTiles. Valid tile indices are 0 to nTiles - 1, so the old check let exactly one out-of-range value (tileIndex == nTiles) slip through.

Correctness — verified, the fix is right

• The old check permits real out-of-bounds access. map->index and map->tileLength are allocated with exactly nTiles elements (lib/raster3d/index.c:165-166). With tileIndex == nTiles, the function would read map->index[nTiles] (tilewrite.c:139), then write it (tilewrite.c:143) and write map->tileLength[nTiles] (tilewrite.c:178) — a one-element heap overflow past both arrays.
• This is a regression fix, not a new behavior. Git history shows the check was originally >= and was accidentally flipped to > in 2013 by commit 84a06c3, whose stated purpose was removing LRE compression — the change to this line was unrelated collateral in that large commit. The PR restores the original check.
• It aligns with every sibling check in the library, all of which use the >= convention:
  • lib/raster3d/tileread.c:154 — (tileIndex >= map->nTiles) || (tileIndex < 0)
  • lib/raster3d/tileio.c:82 — same
  • lib/raster3d/tilemath.c:198 — Rast3d_tile_index_in_range() returns (tileIndex < map->nTiles) && (tileIndex >= 0)
• No caller relies on the old behavior. Internal callers derive indices from tile coordinates via Rast3d_tile2tile_index(), which yields in-range values for valid coordinates, so core code doesn't currently hit the gap. The check guards the public API (Rast3d_write_tile is exported and documented), where an addon or external caller could pass any index — the fix makes the guard actually cover the one value that overflows by a single element.

Code quality and conventions

• The diff is minimal and correctly scoped; the error path (Rast3d_fatal_error) is unchanged and consistent with the read-side counterpart.
• PR title works as the commit message: the lib/raster3d/tilewrite: prefix matches the Libraries category regexp in utils/release.yml ((grass_|lib)[^ ]*: ), capitalization and imperative mood are correct.
• The PR description accurately states the problem — no overclaiming.

Optional suggestion (not blocking)

• The library already provides Rast3d_tile_index_in_range(map, tileIndex) for exactly this test, so the check could be if (!Rast3d_tile_index_in_range(map, tileIndex)). However, tileread.c and tileio.c hand-roll the same condition, so the hand-rolled form is the prevailing idiom here; keeping the diff minimal for a targeted regression fix is a defensible choice. If consolidating, doing all three call sites in a follow-up would be cleaner than doing one here.

Test coverage

• No test added. That's acceptable here: triggering the guard calls Rast3d_fatal_error(), which exits the process, and the repository has no death-test harness for C library code. The invalid index is also unreachable through the tools' Python-facing surface without deliberately misusing the C API.

Risks

• Essentially none. The only behavior change is that tileIndex == nTiles now fails fast with a clear fatal error instead of silently corrupting heap memory. No performance implications.

Verdict: approve. A correct, well-scoped fix that restores a bounds check accidentally weakened in 2013 and brings tilewrite.c in line with the rest of the raster3d library.

Added test

Commit 4693358 adds lib/raster3d/tests/lib_raster3d_tilewrite_test.py. Since Rast3d_fatal_error() exits the calling process, the test runs the call in a subprocess: a small script uses the grass.lib.raster3d ctypes bindings to open a new 3D raster with Rast3d_open_cell_new() in non-cache mode, reads nTiles and tileSize from the RASTER3D_Map struct, and calls Rast3d_write_tile() with a chosen tile index.

• test_write_tile_rejects_index_past_end passes tileIndex == nTiles and asserts a non-zero exit with the "tileIndex out of range" message. Without the fix in this PR, the invalid write is silently accepted (exit code 0) and the test fails; with the fix it passes.
• test_write_tile_accepts_last_index writes the last valid index (nTiles - 1) as a positive control, so the bounds test cannot pass for an unrelated reason.

Verified against a local Autotools build: without the change, the bounds test fails and the control passes; after applying the change and rebuilding lib/raster3d, both tests pass.