Migrating from Ranch 2.3 to 2.4

Ranch 2.4 adds PROXY protocol controls and acceptor diagnostics, and fixes connection accounting, local socket cleanup, and PROXY header parsing.

Ranch 2.4 is tested with Erlang/OTP 27.0 onward.

Features added

• The trusted_proxies transport option limits which immediate peers may send a PROXY protocol header. Each entry is {inet:ip4_address(), 0..32} or {inet:ip6_address(), 0..128}. When the option is absent, any immediate peer may send a header. An empty list rejects every peer.
• ranch:remove_connection/2 drops the given pid when the process is under a supervisor.
• Optional transport callbacks peercert/1 and negotiated_protocol/1 were added. ranch_ssl implements them.
• PROXY protocol version 2 parses and builds the SSL TLV subtypes group, sig_scheme, and client_cert. client_cert carries the client certificate in ASN.1 DER form.
• ranch:trace_acceptors/1,2 samples the {error, Reason} values from Transport:accept/2.

Bugs fixed

• Potential incompatibility: Before listening, and again from ranch:stop_listener/1, Ranch deletes a {local, Path} file only when it is a socket or FIFO. A regular file, directory, symlink, or device is left in place and must not already exist.
• Potential incompatibility: ranch:child_spec/5 raises {badarg, {bad_option, Key}} when a transport option is invalid.
• Potential incompatibility: A PROXY protocol version 1 line longer than 107 bytes is rejected before it is decoded. This includes a TCP6 line whose addresses would otherwise parse.
• PROXY protocol version 2 reading and parsing was strenghtened.
• ranch:set_transport_options/2 left the stored max_connections limit unchanged. ranch:get_max_connections/1 and a restarted connection supervisor now see the new value.
• A release upgrade now fails when an acceptor does not stop or restart using the Ranch-provided appup files.