bug: get_public_pmtiles unsigned integer underflow on zero-byte PMTiles file

[evan-zhang11]

Description

The get_public_pmtiles endpoint in backend/src/public.rs performs file_size - 1 using unsigned (u64) arithmetic without checking for zero. If a zero-byte .pmtiles file is uploaded and published, requesting it (or sending a Range header) causes unsigned integer underflow.

Location

backend/src/public.rs, function get_public_pmtiles:

let end: u64 = if parts[1].is_empty() {
    file_size - 1  // underflow when file_size == 0
} else {
    parts[1].parse()...
};

let actual_end = end.min(file_size - 1);  // same underflow

Impact

• Debug builds: Panic (server crash)
• Release builds: wraps to u64::MAX, leading to incorrect Range parsing and potential huge memory allocation

A zero-byte .pmtiles file passes upload validation (the pmtiles arm returns Ok(()) without any validation), and can then be published.

Suggested Fix

Add a guard for empty files before Range parsing:

if file_size == 0 {
    return Ok((
        StatusCode::OK,
        [
            (header::CONTENT_TYPE, "application/octet-stream"),
            (header::CONTENT_LENGTH, "0"),
            (header::ACCEPT_RANGES, "bytes"),
        ],
        Vec::<u8>::new(),
    ).into_response());
}

Also consider adding PMTiles header validation in upload.rs.