Support for little endian packet sizes in gen_tcp

[hauleth]

Is your feature request related to a problem? Please describe.

I want to implement protocol that uses gen_tcp and uses little endian for packet length definitions.

Describe the solution you'd like

Something like:

{ok, Socket} = gen_tcp:connect(Node, Port, [
  {packet, {4, little}}
]).

Or:

{ok, Socket} = gen_tcp:connect(Node, Port, [
  {packet, 4},
  {packet_endianess, little}
]).

Would make it quite handy.

Describe alternatives you've considered

It is still possible to do it manually right now via {active, false} or similar to capture the length and doing everything manually, it is just a little bit more convenient to have it as a part of packet option.

[sverker]

This is not high prio for us but would be reasonable easy to implement.
Do git grep TCP_PB_4 and add corresponding code for TCP_PB_LITTLE_4 and maybe also TCP_PB_LITTLE_2 for completeness.

[tonyrog]

Do not forget the https://www.erlang.org/eeps/eep-0034 while you are at it :-)

[ptome]

I was considering creating a PR for this, but EEP-34 mentions that the author implemented it. Is there an existing PR for this feature?

I like the use of negative numbers for the PacketType described in EEP-34 because we can keep the same type. If we use an atom, something like '4_little' would require quotes, since it starts with a digit, and little_4 breaks symmetry (subjective).

For gen_tcp:connect options, something like {packet, 4, little} or {packet, {4, little}} also introduces some asymmetry, since packet type is also used in erlang:decode_packet. It would create a tuple parameter, when all others are atoms or integers, depending on how we pass on the arguments.

[hauleth]

I would go with {4, little} as it would allow us to extend it in future, as I see one more improvement possible there. Because in general we have 2 configuration options:

• endianess
• whether size include bytes for size itself

So in total there are 4 combinations for each of possible sizes. We could leave negative numbers for inclusive/exclusive sizes and have textual flag for endianness. That allows us to cover all possible combinations:

• {N, big} (the same as current N) - big-endian, size exclusive
• {-N, big} - big-endian, size inclusive
• {N, little} - little-endian, size exclusive
• {-N, little} - little-endian, size inclusive

I do not see how that would be a problem for erlang:decode_packet/3 as it could be implemented there as well in exactly the same format as described there.

[wojtekmach]

I think this proposal or EEP-34 is a very welcome change.

For anyone interested, here's a proof-of-concept for 3 byte little-endian packet headers using the {packet, -N} notation per https://www.erlang.org/eeps/eep-0034#new-packet-types. An example use case is parsing MySQL packets 1, here focused on the handshake 2.

MySQL packets are <<Len:24/little, Seq:8, Payload:Len/binary>>. The handshake is (essentially) <<10, ServerVsn/binary, 0, ...>>.

$ docker run --rm -p 3306:3306 -e MYSQL_ALLOW_EMPTY_PASSWORD=true mysql

$ cat mysql.escript

#!/usr/bin/env escript

main(_) ->
  connect1(),
  connect2(),
  ok.

connect1() ->
  {ok, S} = gen_tcp:connect("localhost", 3306, [binary, {active, false}]),
  {ok, Data} = gen_tcp:recv(S, 0),
  <<Len:24/little, _Seq:8, Rest0:Len/binary>> = Data,
  <<10, Rest1/binary>> = Rest0,
  [ServerVsn, _Rest] = binary:split(Rest1, <<0>>),
  io:format("server version: ~s~n", [ServerVsn]).

connect2() ->
  {ok, S} = gen_tcp:connect("localhost", 3306, [binary, {active, false}, {packet, -3}]),
  {ok, Data} = gen_tcp:recv(S, 0),
  <<_Seq:8, 10, Rest/binary>> = Data,
  [ServerVsn, _Rest] = binary:split(Rest, <<0>>),
  io:format("server version: ~s~n", [ServerVsn]).

$ ./mysql.escript
server version: 8.2.0
server version: 8.2.0

Below is a diff, also available at https://github.com/wojtekmach/otp/commits/wm-little/.

commit fb3411ec6aeb84f9c7ef1116108f90a5a1e5845a
Author: Wojtek Mach <wojtek@wojtekmach.pl>
Date:   Fri Jan 5 21:58:00 2024 +0100

    wip

diff --git a/erts/emulator/beam/erl_bif_port.c b/erts/emulator/beam/erl_bif_port.c
index d58dd3a531..0bad398e4b 100644
--- a/erts/emulator/beam/erl_bif_port.c
+++ b/erts/emulator/beam/erl_bif_port.c
@@ -1512,6 +1512,7 @@ BIF_RETTYPE decode_packet_3(BIF_ALIST_3)
     case make_small(1): type = TCP_PB_1; break;
     case make_small(2): type = TCP_PB_2; break;
     case make_small(4): type = TCP_PB_4; break;
+    case make_small(-3): type = TCP_PB_LITTLE_3; break;
     case am_asn1: type = TCP_PB_ASN1; break;
     case am_sunrm: type = TCP_PB_RM; break;
     case am_cdr: type = TCP_PB_CDR; break;
diff --git a/erts/emulator/beam/packet_parser.c b/erts/emulator/beam/packet_parser.c
index a349c3ff84..428b151a4b 100644
--- a/erts/emulator/beam/packet_parser.c
+++ b/erts/emulator/beam/packet_parser.c
@@ -281,6 +281,13 @@ int packet_get_length(enum PacketParseType htype,
         plen = get_int32(ptr);
         goto remain;
 
+    case TCP_PB_LITTLE_3:
+        /* TCP_PB_LITTLE_3:    [L2,L1,L0 | Data] */
+        hlen = 3;
+        if (n < hlen) goto more;
+        plen = get_little_int24(ptr);
+        goto remain;
+
     case TCP_PB_RM:
         /* TCP_PB_RM:    [L3,L2,L1,L0 | Data] 
         ** where MSB (bit) is used to signal end of record
diff --git a/erts/emulator/beam/packet_parser.h b/erts/emulator/beam/packet_parser.h
index 633e3794c5..ef462ed072 100644
--- a/erts/emulator/beam/packet_parser.h
+++ b/erts/emulator/beam/packet_parser.h
@@ -44,7 +44,8 @@ enum PacketParseType {
     TCP_PB_HTTPH    = 11,
     TCP_PB_SSL_TLS  = 12,
     TCP_PB_HTTP_BIN = 13,
-    TCP_PB_HTTPH_BIN = 14
+    TCP_PB_HTTPH_BIN = 14,
+    TCP_PB_LITTLE_3  = 15,
 };
 
 typedef struct http_atom {
@@ -150,9 +151,10 @@ ERTS_GLB_INLINE
 void packet_get_body(enum PacketParseType htype, const char** bufp, int* lenp)
 {
     switch (htype) {
-    case TCP_PB_1:  *bufp += 1; *lenp -= 1; break;
-    case TCP_PB_2:  *bufp += 2; *lenp -= 2; break;
-    case TCP_PB_4:  *bufp += 4; *lenp -= 4; break;
+    case TCP_PB_1:        *bufp += 1; *lenp -= 1; break;
+    case TCP_PB_2:        *bufp += 2; *lenp -= 2; break;
+    case TCP_PB_4:        *bufp += 4; *lenp -= 4; break;
+    case TCP_PB_LITTLE_3: *bufp += 3; *lenp -= 3; break;
     case TCP_PB_FCGI:
 	*lenp -= ((struct fcgi_head*)*bufp)->paddingLength;
         break;
diff --git a/erts/emulator/beam/sys.h b/erts/emulator/beam/sys.h
index 8f4bc5e0a1..2910b67f22 100644
--- a/erts/emulator/beam/sys.h
+++ b/erts/emulator/beam/sys.h
@@ -1213,6 +1213,10 @@ ERTS_GLB_INLINE size_t sys_strlen(const char *s)
                       (((byte*) (s))[2] << 8)  | \
                       (((byte*) (s))[3]))
 
+#define get_little_int24(s) ((((byte*) (s))[2] << 16)  | \
+			     (((byte*) (s))[1] << 8) | \
+			     (((byte*) (s))[0]))
+
 #define get_little_int32(s) ((((byte*) (s))[3] << 24) | \
 			     (((byte*) (s))[2] << 16)  | \
 			     (((byte*) (s))[1] << 8) | \
@@ -1236,6 +1240,11 @@ ERTS_GLB_INLINE size_t sys_strlen(const char *s)
                             ((byte*)(s))[2] = (byte)(i)         & 0xff;} \
                         while (0)
 
+#define put_little_int24(i, s) do {((byte*)(s))[2] = (byte)((i) >> 16) & 0xff;  \
+                                   ((byte*)(s))[1] = (byte)((i) >> 8)  & 0xff;  \
+                                   ((byte*)(s))[0] = (byte)(i)         & 0xff;} \
+                               while (0)
+
 #define get_int16(s) ((((byte*)  (s))[0] << 8) | \
                       (((byte*)  (s))[1]))
 
diff --git a/erts/emulator/drivers/common/inet_drv.c b/erts/emulator/drivers/common/inet_drv.c
index 66a5f77d5d..7f6467cb5f 100644
--- a/erts/emulator/drivers/common/inet_drv.c
+++ b/erts/emulator/drivers/common/inet_drv.c
@@ -13147,6 +13147,10 @@ static int tcp_sendv(tcp_descriptor* desc, ErlIOVec* ev)
          put_int32(len, buf);
          h_len = 4;
          break;
+     case TCP_PB_LITTLE_3:
+         put_little_int24(len, buf);
+         h_len = 3;
+         break;
      default:
          h_len = 0;
          break;
@@ -13259,9 +13263,13 @@ static int tcp_send(tcp_descriptor* desc, char* ptr, ErlDrvSizeT len)
 	put_int16(len, buf);
 	h_len = 2; 
 	break;
-    case TCP_PB_4: 
+    case TCP_PB_4:
 	put_int32(len, buf);
-	h_len = 4; 
+	h_len = 4;
+	break;
+    case TCP_PB_LITTLE_3:
+	put_little_int24(len, buf);
+	h_len = 3;
 	break;
     default:
 	h_len = 0;
diff --git a/erts/preloaded/ebin/prim_inet.beam b/erts/preloaded/ebin/prim_inet.beam
index 5f8d5b8709..df9102386b 100644
Binary files a/erts/preloaded/ebin/prim_inet.beam and b/erts/preloaded/ebin/prim_inet.beam differ
diff --git a/erts/preloaded/src/prim_inet.erl b/erts/preloaded/src/prim_inet.erl
index a9264d551a..5c0bb1bae6 100644
--- a/erts/preloaded/src/prim_inet.erl
+++ b/erts/preloaded/src/prim_inet.erl
@@ -1495,6 +1495,8 @@ attach(S) when is_port(S) ->
 %%
 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
 
+is_sockopt_val(packet, -3) ->
+    true;
 is_sockopt_val(Opt, Val) ->
     Type = type_opt(set, Opt),
     try type_value(set, Type, Val)
@@ -1702,6 +1704,7 @@ type_opt_1(packet) ->
 	   {1, ?TCP_PB_1},
 	   {2, ?TCP_PB_2},
 	   {4, ?TCP_PB_4},
+	   {-3, ?TCP_PB_LITTLE_3},
 	   {raw,?TCP_PB_RAW},
 	   {sunrm, ?TCP_PB_RM},
 	   {asn1, ?TCP_PB_ASN1},
diff --git a/lib/kernel/src/gen_tcp_socket.erl b/lib/kernel/src/gen_tcp_socket.erl
index 38be596b44..756b3610ed 100644
--- a/lib/kernel/src/gen_tcp_socket.erl
+++ b/lib/kernel/src/gen_tcp_socket.erl
@@ -68,6 +68,9 @@
 -define(header(Packet, Size),
         (Size):(Packet)/unit:8-integer-big-unsigned).
 
+-define(little_header(Packet, Size),
+        (Size):(Packet)/unit:8-integer-little-unsigned).
+
 -define(badarg_exit(Error),
         case begin Error end of
             {error, badarg} -> exit(badarg);
@@ -512,6 +515,13 @@ send(?MODULE_socket(Server, Socket), Data) ->
                     Header_Data = [Header, Data],
                     Result = socket_send(Socket, Header_Data, SendTimeout),
                     send_result(Server, Header_Data, Meta, Result);
+                Packet =:= -3 ->
+                    Size = iolist_size(Data),
+		    %% ?DBG([{packet, Packet}, {data_size, Size}]),
+                    Header = <<?little_header(3, Size)>>,
+                    Header_Data = [Header, Data],
+                    Result = socket_send(Socket, Header_Data, SendTimeout),
+                    send_result(Server, Header_Data, Meta, Result);
                 true ->
                     Result = socket_send(Socket, Data, SendTimeout),
                     send_result(Server, Data, Meta, Result)
diff --git a/lib/kernel/src/inet_int.hrl b/lib/kernel/src/inet_int.hrl
index 2f50f2c23c..70963bffb1 100644
--- a/lib/kernel/src/inet_int.hrl
+++ b/lib/kernel/src/inet_int.hrl
@@ -212,6 +212,7 @@
 -define(TCP_PB_SSL_TLS, 12).
 -define(TCP_PB_HTTP_BIN,13).
 -define(TCP_PB_HTTPH_BIN,14).
+-define(TCP_PB_LITTLE_3, 15).
 
 
 %% getstat, INET_REQ_GETSTAT

[sverker]

I prefer {4, little} . I do not like negative values meaning little endian.