How to handle file uploads (multipart/form-data)

Problem

A client POSTs multipart/form-data (a browser form with files, or a multimodal request) and you need the fields and uploaded files without buffering the whole request in memory.

Solution

Use livery_multipart. Pull parts one at a time and stream each part's body, so a large upload never sits in RAM:

upload(Req) ->
    {ok, MP0} = livery_multipart:new(Req),
    loop(MP0).

loop(MP) ->
    case livery_multipart:next_part(MP, 5000) of
        {part, #{name := Name, filename := File}, MP1} ->
            {ok, MP2} = consume(MP1, Name, File),
            loop(MP2);
        {done, _MP1} ->
            livery_resp:text(200, <<"ok">>);
        {error, Reason, _MP1} ->
            livery_resp:text(400, atom_to_binary(element(1, {Reason, x})))
    end.

consume(MP, _Name, _File) ->
    case livery_multipart:read_part(MP, 5000) of
        {ok, Chunk, MP1} -> %% write Chunk to disk / forward / hash
            consume(MP1, _Name, _File);
        {done, MP1} -> {ok, MP1};
        {error, _, MP1} -> {ok, MP1}
    end.

next_part/2 returns each part's name, filename, content_type, and raw headers (parsed from Content-Disposition). read_part/2 streams that part's bytes; calling next_part again skips any unread remainder.

Small forms: read everything at once

When the parts are small, read_all/1,2 collects them into memory under the limits:

{ok, Parts} = livery_multipart:read_all(Req),
%% Parts :: [#{name, filename, content_type, headers, body}]

Limits

All buffering is bounded. Override the defaults via the options map on new/2 / read_all/2:

livery_multipart:read_all(Req, #{
    max_parts => 50,             %% default 1000
    max_part_size => 5_242_880,  %% read_all per-part bytes; default 10 MiB
    max_header_bytes => 16_384,  %% per-part header block; default 64 KiB
    max_header_count => 32,      %% header fields per part; default 64
    max_body => 52_428_800,      %% total bytes consumed; default 100 MiB
    part_timeout => 5000         %% per read; default 5000 ms
}).

Security: sanitize the filename

filename is returned exactly as the client sent it and the parser never touches the filesystem. A hostile client can send ../../etc/passwd. If you write uploads to disk, confine the path yourself (basename + a fixed directory); never join the raw filename onto a path.