Fix Windows read past EOF looping forever instead of erroring

[cberner]

Problem

On Windows, any read that extends past the end of the file hangs forever instead of returning an error. Opening a database file that has a valid magic number but is truncated below the 320-byte header (e.g. an interrupted copy or restore), or a corrupt root page number pointing past EOF during repair, spins indefinitely.

Root cause

In the Windows FileBackend::read (src/tree_store/page_store/file_backend/optimized.rs), seek_read returns Ok(0) at end-of-file. The retry loop only advances offset/data_offset by the number of bytes read, so a zero-byte read leaves both counters unchanged and the while data_offset < out.len() loop never terminates and never errors.

The Unix (read_exact_at) and WASI backends both correctly return UnexpectedEof in this case, and the StorageBackend::read trait contract requires it: "If out.len() + offset exceeds the length of the storage an appropriate Error must be returned."

CI and the fuzzer run on Linux with the in-memory backend, so this was never exercised.

Fix

Treat a zero-length read as UnexpectedEof, mirroring the WASI backend's error kind and message, so reading past the end of the file fails instead of spinning.

Test

read_past_eof_errors (in tests/integration_tests.rs) writes an 8-byte file and asserts that both a read fully past EOF and a read that starts inside the file but extends past EOF (the truncated-header scenario) return UnexpectedEof. It runs on every platform, including the windows-latest CI runner — before this fix that path would hang (surfacing as a CI timeout); after it, it passes.

Verification

• cargo check / clippy --deny warnings for the x86_64-pc-windows-gnu target — the Windows-gated fix and test compile and lint clean (Linux builds skip this #[cfg(windows)] code entirely).
• cargo fmt --check, Linux clippy --all-targets --all-features --deny warnings — clean.
• Full cargo test --all-features — all pass.

A user-facing CHANGELOG.md entry is included under the unreleased 4.2.0 section.

🤖 Generated with Claude Code

https://claude.ai/code/session_01GeBFo3vreYtX9Z4BW2Nx2z

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.19%. Comparing base (8e04e95) to head (51bcd12).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1291      +/-   ##
==========================================
+ Coverage   90.18%   90.19%   +0.01%     
==========================================
  Files          36       36              
  Lines       16227    16227              
==========================================
+ Hits        14634    14636       +2     
+ Misses       1593     1591       -2     

Files with missing lines	Coverage Δ
...rc/tree_store/page_store/file_backend/optimized.rs	77.27% <ø> (ø)

... and 1 file with indirect coverage changes

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