fix: _range() log spam, Content-Length clamping, multipart terminator off-by-one

[troglodyne-bot]

What

Three bugs in _range(), none addressed by open PRs #4–#10.

Why

1. INFO "GET 416" log spam — $self->INFO("GET 416 $fullpath") fired unconditionally before the range validation check, meaning every valid byte-range request logged a false 416. Fail2ban rules watching for 416s would incorrectly ban clients making legitimate ranged GETs.

2. Content-Length not clamped to file size — When a client requests bytes=0-9999 on a 5-byte file, RFC 7233 §2.1 says to silently cap the range end. The streaming write already used List::Util::min($sz, ...) so the actual bytes sent were correct, but Content-Length was computed from the unclamped $range->[1]. Clients received a header promising 10,000 bytes and then only 5 — hanging indefinitely waiting for data that would never come. Content-Range in the response was also wrong (claimed bytes 0-9999/5 instead of bytes 0-4/5).

3. Multipart terminator Content-Length off by one — The Content-Length calculation used length("\n--$CHUNK_SEP\--\n") where \- in a double-quoted Perl string passes through as a literal backslash. The streaming write (fixed separately by PR Fix multipart boundary, HTTP::Body dep, NYTProf overhead, error fallback #4) uses "\n--$CHUNK_SEP--\n" without the backslash. The mismatch made declared Content-Length one byte larger than the actual body for every multipart range response.

How

• Moved INFO("GET 416") inside the failure branch
• Added $range->[1] = $sz - 1 if $range->[1] >= $sz immediately after the //= default, before the Content-Length sum and Content-Range header — one clamp fixes all three header values
• Changed length("\n--$CHUNK_SEP\--\n") → length("\n--$CHUNK_SEP--\n") in the Content-Length accumulation

Testing

t/01-range-response.t — 19 tests:

• 416 for out-of-bounds start and inverted range
• 206 + correct Content-Length/body for single range
• Overlong range: Content-Length clamped, body correct, Content-Range header correct
• Suffix range (bytes=N- with no end): defaults to last byte
• Multipart: Content-Length matches actual body length (regression for terminator off-by-one)