feat: add peercert/1 function to get peer SSL certificate (#599) (#813)

Add hackney:peercert/1 function that returns the DER-encoded peer
certificate for SSL connections, similar to how peername/1 returns
the peer address.

The function returns:
- {ok, Cert} where Cert is the DER-encoded certificate binary
- {error, not_ssl} for non-SSL connections
- {error, not_connected} if there's no active connection

Fixes #599

Author: benoitc

src/hackney.erl

@@ -11,6 +11,7 @@
 -export([connect/1, connect/2, connect/3, connect/4,
          close/1,
          peername/1,
+         peercert/1,
          sockname/1,
          request/1, request/2, request/3, request/4, request/5,
          send_request/2,
@@ -401,6 +402,13 @@ normalize_transport(Other) -> Other.
 peername(ConnPid) when is_pid(ConnPid) ->
   hackney_conn:peername(ConnPid).
 
+%% @doc Get the peer SSL certificate.
+%% Returns the DER-encoded certificate of the peer, or an error if the connection
+%% is not SSL or the certificate is unavailable.
+-spec peercert(conn()) -> {ok, binary()} | {error, term()}.
+peercert(ConnPid) when is_pid(ConnPid) ->
+  hackney_conn:peercert(ConnPid).
+
 %% @doc Get the local address and port.
 -spec sockname(conn()) -> {ok, {inet:ip_address(), inet:port_number()}} | {error, term()}.
 sockname(ConnPid) when is_pid(ConnPid) ->

src/hackney_conn.erl

@@ -53,6 +53,7 @@
     %% Socket operations
     setopts/2,
     peername/1,
+    peercert/1,
     sockname/1,
     %% Low-level socket operations (for hackney_request/hackney_response)
     send/2,
@@ -352,6 +353,13 @@ setopts(Pid, Opts) ->
 peername(Pid) ->
     gen_statem:call(Pid, peername).
 
+%% @doc Get the peer SSL certificate.
+%% Returns {ok, Cert} where Cert is the DER-encoded certificate binary,
+%% or {error, Reason} if the connection is not SSL or the certificate is unavailable.
+-spec peercert(pid()) -> {ok, binary()} | {error, term()}.
+peercert(Pid) ->
+    gen_statem:call(Pid, peercert).
+
 %% @doc Get the local address and port.
 -spec sockname(pid()) -> {ok, {inet:ip_address(), inet:port_number()}} | {error, term()}.
 sockname(Pid) ->
@@ -1405,6 +1413,14 @@ handle_common({call, From}, peername, _State, #conn_data{transport = Transport,
     Result = Transport:peername(Socket),
     {keep_state_and_data, [{reply, From, Result}]};
 
+handle_common({call, From}, peercert, _State, #conn_data{transport = Transport, socket = Socket} = _Data)
+  when Socket =/= undefined ->
+    Result = case erlang:function_exported(Transport, peercert, 1) of
+        true -> Transport:peercert(Socket);
+        false -> {error, not_ssl}
+    end,
+    {keep_state_and_data, [{reply, From, Result}]};
+
 handle_common({call, From}, sockname, _State, #conn_data{transport = Transport, socket = Socket} = _Data)
   when Socket =/= undefined ->
     Result = Transport:sockname(Socket),
@@ -1443,6 +1459,9 @@ handle_common({call, From}, {setopts, _Opts}, _State, _Data) ->
 handle_common({call, From}, peername, _State, _Data) ->
     {keep_state_and_data, [{reply, From, {error, not_connected}}]};
 
+handle_common({call, From}, peercert, _State, _Data) ->
+    {keep_state_and_data, [{reply, From, {error, not_connected}}]};
+
 handle_common({call, From}, sockname, _State, _Data) ->
     {keep_state_and_data, [{reply, From, {error, not_connected}}]};
 

src/hackney_ssl.erl

@@ -15,6 +15,7 @@
   setopts/2,
   controlling_process/2,
   peername/1,
+  peercert/1,
   close/1,
   shutdown/2,
   sockname/1]).
@@ -237,6 +238,13 @@ controlling_process(Socket, Pid) ->
 peername(Socket) ->
   ssl:peername(Socket).
 
+%% @doc Return the peer certificate of an SSL connection.
+%% @see ssl:peercert/1
+-spec peercert(ssl:sslsocket()) ->
+  {ok, binary()} | {error, atom()}.
+peercert(Socket) ->
+  ssl:peercert(Socket).
+
 %% @doc Close a TCP socket.
 %% @see ssl:close/1
 -spec close(ssl:sslsocket()) -> ok.