How to serve static files

livery_static:handler/1,2 serves a directory of files (assets, a built single-page app, downloads) over HTTP, with the right Content-Type, conditional GET (a 304 when the client already has the file), and Range support, and without exposing anything outside the directory. You need it when you want to serve a folder of files rather than write a handler per file.

Mount it on a wildcard route

Mount the handler on a router wildcard route. The *path segment takes the rest of the URL, and the handler turns it into a file under the root:

Router = livery_router:add(
    '_', <<"/assets/*path">>, livery_static:handler("priv/assets"), #{}, Router0
).

GET /assets/css/app.css serves priv/assets/css/app.css with Content-Type: text/css, an ETag, and (if the client sends Range) partial content.

Serve it without the router

If you are not using the router, configure a prefix to strip from the request path:

livery:start_listener(livery_h1, #{
    port => 8080,
    handler => livery_static:handler("public", #{prefix => <<"/">>})
}).

livery_static:handler/2 returns a handler, not a middleware, so it goes under handler, not in the middleware stack.

Options

livery_static:handler("priv/assets", #{
    binding => <<"path">>,                 %% router binding name (default)
    prefix => <<"/assets/">>,              %% fallback when there is no binding
    index => <<"index.html">>,             %% served for a directory; false to disable
    cache_control => [{max_age, 3600}, public],  %% optional Cache-Control
    etag => true,                          %% weak ETag from size+mtime (default)
    range => true                          %% honor Range requests (default)
}).

Behaviour

Content-Type comes from the file extension (text types get ; charset=utf-8), and is application/octet-stream when the extension is unknown.
A weak ETag (W/"size-mtime") is sent; a matching If-None-Match returns 304 Not Modified.
Range: bytes=... returns 206 Partial Content; an unsatisfiable range returns 416 with Content-Range: bytes */<size>.
Responses carry Last-Modified. A Range with If-Range is served only when its date matches Last-Modified exactly; an If-Range with an ETag gets the whole file, since these ETags are weak and the check needs a strong one (RFC 9110 section 13.1.5).
HEAD returns headers (including Content-Length) with no body.
Only GET/HEAD are served; other methods get 405 with Allow.
A directory request serves index if present, else 404 (no directory listing).

Notes

The sub-path is percent-decoded and then checked: any .. segment, absolute path, control byte, or bad escape is refused with 404, so a request can never reach a file outside the root.
Only regular files are served (directories and symlinks get 404), so a symlink inside the root cannot be used to get out of it.