Return 0 content length for range requests for empty files

[Dav1dde]

Motivation

Also came up here: seanmonstar/reqwest#1559 (comment)

Currently tower-http's ServeDir handles range requests for empty files slightly incorrectly, it returns a correctly empty Content-Range but the Content-Length is set to 1. Which causes troubles in reqwest (it tries to read 1 byte which never comes).

For example:

└─ %  curl http://localhost:49488/files/index.android.bundle -i -H "Range: bytes=0-"
HTTP/1.1 206 Partial Content
content-type: application/octet-stream
accept-ranges: bytes
last-modified: Thu, 09 Jan 2025 08:11:11 GMT
content-range: bytes 0-0/0
content-length: 1
date: Wed, 09 Apr 2025 08:36:05 GMT

curl: (18) transfer closed with 1 bytes remaining to read

┌─── ~ 
└─ %  curl http://localhost:49488/files/index.android.bundle -i                     
HTTP/1.1 200 OK
content-type: application/octet-stream
accept-ranges: bytes
last-modified: Thu, 09 Jan 2025 08:11:11 GMT
content-length: 0
date: Wed, 09 Apr 2025 08:36:11 GMT

Solution

For empty files, return a content length of 1, instead of calculating it from the inclusive range.