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139f30e0f1
* Drop the claim that small bodies skip compression There is no size threshold anywhere in the compression path. encoding_type() gates on the content type and Accept-Encoding only, and apply_ranges() compresses whatever body it is given, so a two-byte text/plain response comes back gzipped at 22 bytes. Say what actually happens and leave the decision to the handler. * Compress static file responses behind an opt-in (Fix #2545) apply_ranges() runs the compressor inside the branch it takes when res.body is non-empty. A response served from a file leaves res.body empty and sets content_length_, so it took the other branch, which writes Content-Length and returns; encoding_type() was computed before the split and never consulted on that side. The same bytes handed to set_content() came back gzipped, which left set_mount_point() and Response::set_file_content() as the one path that missed out. Add Server::set_static_file_compression(), off by default so nothing about an existing server changes. When it is on, the file-backed provider is run through the compressor into res.body ahead of the rest of apply_ranges(), so the response is framed the way set_content() already frames one: it keeps its Content-Length, and HEAD still reports the size a GET would return. Ranges are answered from the identity representation, since RFC 9110 applies Range after content coding and slicing a compressed body would mean compressing the whole file first. The ETag carries the coding it belongs to, so a client that cached the compressed form revalidates against its own validator rather than the identity one. Both the ETag and the body take their coding from static_file_encoding(), so the two cannot disagree. Providers registered with set_content_provider() are left alone. zlib buffers until its window fills, so running one through a compressor would hold back writes that a caller expects to reach the peer as they are produced. The compressed bytes stay in memory until the response has been written, so the peak cost scales with requests in flight. set_static_file_compression_max_length() bounds it, defaulting to 4MB. * Add a minimum size for static file compression Compressing a file that already fits in a single 1500-byte MTU does not get it to the client any sooner, and a file of a few bytes comes back larger than it went in once gzip's header and trailer are added. Every other server draws this line: nginx's gzip_min_length, Caddy's minimum_length, IIS's minFileSizeForComp, CloudFront's 1000-byte floor. The note this replaces told callers to decide in the handler. A response served through set_mount_point() has no handler to decide in, so the floor has to live in the server. It defaults to 1400 bytes, the size that fits inside one MTU with room for headers. set_static_file_compression_min_length() moves it, and CPPHTTPLIB_STATIC_FILE_COMPRESSION_MIN_LENGTH sets the default at compile time. The empty-file case keeps its own early-out so that a zero floor still cannot turn an empty body into a 20-byte gzip stream. The two bounds now read as a pair, so the documentation says what each one is for: the lower bound is about what is worth compressing, the upper bound about what one request is allowed to cost. Every file under test/www except 1MB.txt is below the default floor, so the tests that need a small file compressed lower it explicitly.