Implement a non-blocking SCTP connect

This patch adds a new set of functions - gen_sctp:connect_init/* that initiate
an SCTP connection without blocking for the result. The result is delivered
asynchronously as an sctp_assoc_change event.

The new functions have the same API as documented for gen_sctp:connect/* with
the following exceptions:

* Timeout is only used to supervise resolving Addr (the peer address)
* The possible return values are ok | {error, posix()}

The caller application is responsible for receiving the #sctp_assoc_change{}
event and correctly determining the connect it originated from (for example,
by examining the remote host and/or port). The application should have at
least {active, once} or use gen_sctp:recv to retrieve the connect result.

The implementation of gen_sctp:connect suffers from a number of
shortcomings which the user may avoid by using gen_sctp:connect_init and
adding code to receive the connect result.

First, irrespective of the Timeout value given to gen_sctp:connect, the OS
attempts and retries the SCTP INIT according to various kernel parameters. If
the Timeout value is shorter than the entire attempt then the application will
still receive an sctp_assoc_change event after the {error, timeout} is
returned from the initial call. This could be somewhat confusing (either to
the application or the designer!) especially if the status is
comm_up. Subsequent calls to connect before the OS has finished this process
return {error, ealready} which may also be counter-intuitive.

Second, there is a race-condition (documented in comments in inet_sctp.erl)
that can cause the wrong sctp_assoc_change record to be returned to an
application calling gen_sctp:connect. The race seriously affects connection
attempts when using one-to-many sockets.

1 files changed, 49 insertions, 3 deletions

diff --git a/lib/kernel/test/gen_sctp_SUITE.erl b/lib/kernel/test/gen_sctp_SUITE.erl
index dd7d5f111a..b19ce4d40b 100644
--- a/lib/kernel/test/gen_sctp_SUITE.erl
+++ b/lib/kernel/test/gen_sctp_SUITE.erl
@@ -24,10 +24,11 @@
 %%-compile(export_all).
 
 -export([all/1,init_per_testcase/2,fin_per_testcase/2,
-	 basic/1,xfer_min/1,xfer_active/1,api_open_close/1,api_listen/1]).
+	 basic/1,api_open_close/1,api_listen/1,api_connect_init/1,
+	 xfer_min/1,xfer_active/1]).
 
 all(suite) ->
-    [basic,xfer_min,xfer_active,api_open_close,api_listen].
+    [basic,api_open_close,api_listen,api_connect_init,xfer_min,xfer_active].
 
 init_per_testcase(_Func, Config) ->
     Dog = test_server:timetrap(test_server:seconds(15)),
@@ -325,7 +326,7 @@ api_listen(Config) when is_list(Config) ->
 		  ?line {ok,{Localhost,
 			     Pb,[],
 			     #sctp_assoc_change{
-				  state = comm_lost}}} =
+				  state=comm_lost}}} =
 		      gen_sctp:recv(Sa, infinity);
 	      {error,#sctp_assoc_change{state=cant_assoc}} -> ok
 	  end,
@@ -336,3 +337,48 @@ api_listen(Config) when is_list(Config) ->
     ?line ok = gen_sctp:close(Sa),
     ?line ok = gen_sctp:close(Sb),
     ok.
+
+api_connect_init(doc) ->
+    "Test the API function connect_init/4";
+api_connect_init(suite) ->
+    [];
+api_connect_init(Config) when is_list(Config) ->
+    ?line Localhost = {127,0,0,1},
+
+    ?line {ok,S} = gen_sctp:open(),
+    ?line {ok,Pb} = inet:port(S),
+    ?line try gen_sctp:connect_init(S, Localhost, not_allowed_for_port, [])
+	  catch error:badarg -> ok
+	  end,
+    ?line try gen_sctp:connect_init(S, Localhost, 12345, not_allowed_for_opts)
+	  catch error:badarg -> ok
+	  end,
+    ?line ok = gen_sctp:close(S),
+    ?line {error,closed} = gen_sctp:connect_init(S, Localhost, 12345, []),
+
+    ?line {ok,Sb} = gen_sctp:open(Pb),
+    ?line {ok,Sa} = gen_sctp:open(),
+    ?line case gen_sctp:connect_init(Sa, localhost, Pb, []) of
+	      {error,econnrefused} ->
+		  ?line {ok,{Localhost,
+			     Pb,[],
+			     #sctp_assoc_change{state=comm_lost}}} =
+		      gen_sctp:recv(Sa, infinity);
+	      ok ->
+		  ?line {ok,{Localhost,
+			     Pb,[],
+			     #sctp_assoc_change{state=cant_assoc}}} =
+		      gen_sctp:recv(Sa, infinity)
+	  end,
+    ?line ok = gen_sctp:listen(Sb, true),
+    ?line case gen_sctp:connect_init(Sa, localhost, Pb, []) of
+	      ok ->
+		  ?line {ok,{Localhost,
+			     Pb,[],
+			     #sctp_assoc_change{
+				  state = comm_up}}} =
+		      gen_sctp:recv(Sa, infinity)
+	  end,
+    ?line ok = gen_sctp:close(Sa),
+    ?line ok = gen_sctp:close(Sb),
+    ok.