Fix returned error from gen_tcp:accept/1,2 when running out of ports

The {error, enfile} return value is badly misleading and confusing for
this case, since the Posix ENFILE errno value has a well-defined meaning
that has nothing to do with Erlang ports. The fix changes the return
value to {error, system_limit}, which is consistent with e.g. various
file(3) functions. inet:format_error/1 has also been updated to support
system_limit in the same manner as file:format_error/1.

5 files changed, 60 insertions, 7 deletions

diff --git a/lib/kernel/doc/src/gen_tcp.xml b/lib/kernel/doc/src/gen_tcp.xml
index 8a5d40bb16..9f362841ce 100644
--- a/lib/kernel/doc/src/gen_tcp.xml
+++ b/lib/kernel/doc/src/gen_tcp.xml
@@ -235,7 +235,9 @@ do_recv(Sock, Bs) ->
         <p>Returns <c>{ok, <anno>Socket</anno>}</c> if a connection is established,
           or <c>{error, closed}</c> if <c><anno>ListenSocket</anno></c> is closed,
           or <c>{error, timeout}</c> if no connection is established
-          within the specified time. May also return a POSIX error
+          within the specified time,
+          or <c>{error, system_limit}</c> if all available ports in the
+          Erlang emulator are in use. May also return a POSIX error
           value if something else goes wrong, see inet(3) for possible
           error values.</p>
         <p>Packets can be sent to the returned socket <c><anno>Socket</anno></c>
diff --git a/lib/kernel/doc/src/inet.xml b/lib/kernel/doc/src/inet.xml
index 1a05b4ba99..86950b3ecc 100644
--- a/lib/kernel/doc/src/inet.xml
+++ b/lib/kernel/doc/src/inet.xml
@@ -149,7 +149,7 @@ fe80::204:acff:fe17:bf38
       <fsummary>Return a descriptive string for an error reason</fsummary>
       <desc>
         <p>Returns a diagnostic error string. See the section below
-          for possible <c><anno>Posix</anno></c> values and the corresponding
+          for possible Posix values and the corresponding
           strings.</p>
       </desc>
     </func>
diff --git a/lib/kernel/src/gen_tcp.erl b/lib/kernel/src/gen_tcp.erl
index 4d6c7f5f1d..02a6190034 100644
--- a/lib/kernel/src/gen_tcp.erl
+++ b/lib/kernel/src/gen_tcp.erl
@@ -194,7 +194,7 @@ listen(Port, Opts) ->
 -spec accept(ListenSocket) -> {ok, Socket} | {error, Reason} when
       ListenSocket :: socket(),
       Socket :: socket(),
-      Reason :: closed | timeout | inet:posix().
+      Reason :: closed | timeout | system_limit | inet:posix().
 
 accept(S) ->
     case inet_db:lookup_socket(S) of
diff --git a/lib/kernel/src/inet.erl b/lib/kernel/src/inet.erl
index 49f64a9236..4de3c1c6b1 100644
--- a/lib/kernel/src/inet.erl
+++ b/lib/kernel/src/inet.erl
@@ -1218,11 +1218,13 @@ port_list(Name) ->
 %%  utils
 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
 
--spec format_error(Posix) -> string() when
-      Posix :: posix().
+-spec format_error(Reason) -> string() when
+      Reason :: posix() | system_limit.
 
 format_error(exbadport) -> "invalid port state";
 format_error(exbadseq) ->  "bad command sequence";
+format_error(system_limit) ->
+    "a system limit was hit, probably not enough ports";
 format_error(Tag) ->
     erl_posix_msg:message(Tag).
 
diff --git a/lib/kernel/test/gen_tcp_misc_SUITE.erl b/lib/kernel/test/gen_tcp_misc_SUITE.erl
index 3da4b07c05..d9bba0dbb0 100644
--- a/lib/kernel/test/gen_tcp_misc_SUITE.erl
+++ b/lib/kernel/test/gen_tcp_misc_SUITE.erl
@@ -39,7 +39,8 @@
 	 accept_timeouts_in_order/1,accept_timeouts_in_order2/1,
 	 accept_timeouts_in_order3/1,accept_timeouts_mixed/1, 
 	 killing_acceptor/1,killing_multi_acceptors/1,killing_multi_acceptors2/1,
-	 several_accepts_in_one_go/1,active_once_closed/1, send_timeout/1, send_timeout_active/1, 
+	 several_accepts_in_one_go/1, accept_system_limit/1,
+	 active_once_closed/1, send_timeout/1, send_timeout_active/1,
 	 otp_7731/1, zombie_sockets/1, otp_7816/1, otp_8102/1,
          otp_9389/1]).
 
@@ -71,7 +72,7 @@ all() ->
      accept_timeouts_in_order, accept_timeouts_in_order2,
      accept_timeouts_in_order3, accept_timeouts_mixed,
      killing_acceptor, killing_multi_acceptors,
-     killing_multi_acceptors2, several_accepts_in_one_go,
+     killing_multi_acceptors2, several_accepts_in_one_go, accept_system_limit,
      active_once_closed, send_timeout, send_timeout_active, otp_7731,
      zombie_sockets, otp_7816, otp_8102, otp_9389].
 
@@ -1837,6 +1838,54 @@ wait_until_accepting(Proc,N) ->
     end.
 
 
+accept_system_limit(suite) ->
+    [];
+accept_system_limit(doc) ->
+    ["Check that accept returns {error, system_limit} "
+     "(and not {error, enfile}) when running out of ports"];
+accept_system_limit(Config) when is_list(Config) ->
+    ?line {ok, LS} = gen_tcp:listen(0, []),
+    ?line {ok, TcpPort} = inet:port(LS),
+    ?line Connector = spawn_link(fun () -> connector(TcpPort) end),
+    ?line ok = acceptor(LS, false, []),
+    ?line Connector ! stop,
+    ok.
+
+acceptor(LS, GotSL, A) ->
+    case gen_tcp:accept(LS, 1000) of
+	{ok, S} ->
+	    acceptor(LS, GotSL, [S|A]);
+	{error, system_limit} ->
+	    acceptor(LS, true, A);
+	{error, timeout} when GotSL ->
+	    ok;
+	{error, timeout} ->
+	    error
+    end.
+
+connector(TcpPort) ->
+    ManyPorts = open_ports([]),
+    ConnF = fun (Port) ->
+		    case catch gen_tcp:connect({127,0,0,1}, TcpPort, []) of
+			{ok, Sock} ->
+			    Sock;
+			_Error ->
+			    port_close(Port)
+		    end
+	    end,
+    R = [ConnF(Port) || Port <- lists:sublist(ManyPorts, 10)],
+    receive stop -> R end.
+
+open_ports(L) ->
+    case catch open_port({spawn_driver, "ram_file_drv"}, []) of
+	Port when is_port(Port) ->
+	    open_ports([Port|L]);
+	{'EXIT', {system_limit, _}} ->
+	    {L1, L2} = lists:split(5, L),
+	    [port_close(Port) || Port <- L1],
+	    L2
+    end.
+
 
 active_once_closed(suite) ->
     [];