Add backpressure capability to net.

examples/tcpserver.janet

@@ -1,14 +1,19 @@
 (defn handler
   "Simple handler for connections."
   [stream]
-  (defer (:close stream)
-    (def id (gensym))
-    (def b @"")
-    (print "Connection " id "!")
-    (while (:read stream 1024 b)
-      (:write stream b)
-      (buffer/clear b))
-    (printf "Done %v!" id)))
+  (def id (gensym))
+  (def b @"")
+  (print "Connection " id "!")
+  (while (:read stream 1024 b)
+    (:write stream b)
+    (buffer/clear b))
+  (printf "Done %v!" id))
 
 (print "Starting echo server on 127.0.0.1:8000")
-(net/server "127.0.0.1" "8000" handler)
+
+(def server (net/server "127.0.0.1" "8000"))
+
+# Run server.
+(while true
+  (with [conn (:accept server)]
+    (handler conn)))

src/core/net.c

@@ -52,6 +52,7 @@
 
 #define JANET_STREAM_READABLE 0x200
 #define JANET_STREAM_WRITABLE 0x400
+#define JANET_STREAM_ACCEPTABLE 0x800
 
 static int janet_stream_close(void *p, size_t s);
 static int janet_stream_mark(void *p, size_t s);
@@ -335,6 +336,38 @@ JanetAsyncStatus net_machine_simple_server(JanetListenerState *s, JanetAsyncEven
     return JANET_ASYNC_STATUS_NOT_DONE;
 }
 
+/* State machine for accepting connections. */
+
+typedef struct {
+    JanetListenerState head;
+} NetStateAccept;
+
+JanetAsyncStatus net_machine_accept(JanetListenerState *s, JanetAsyncEvent event) {
+    switch (event) {
+        default:
+            break;
+        case JANET_ASYNC_EVENT_CLOSE:
+            janet_schedule(s->fiber, janet_wrap_nil());
+            return JANET_ASYNC_STATUS_DONE;
+        case JANET_ASYNC_EVENT_READ: {
+            JSock connfd = accept(s->pollable->handle, NULL, NULL);
+            if (JSOCKVALID(connfd)) {
+                JanetStream *stream = make_stream(connfd, JANET_STREAM_READABLE | JANET_STREAM_WRITABLE);
+                Janet streamv = janet_wrap_abstract(stream);
+                janet_schedule(s->fiber, streamv);
+                return JANET_ASYNC_STATUS_DONE;
+            }
+            break;
+        }
+    }
+    return JANET_ASYNC_STATUS_NOT_DONE;
+}
+
+JANET_NO_RETURN static void janet_sched_accept(JanetStream *stream) {
+    janet_listen(stream, net_machine_accept, JANET_ASYNC_EVENT_READ, sizeof(NetStateAccept));
+    janet_await();
+}
+
 /* Adress info */
 
 /* Needs argc >= offset + 2 */
@@ -392,10 +425,10 @@ static Janet cfun_net_connect(int32_t argc, Janet *argv) {
 }
 
 static Janet cfun_net_server(int32_t argc, Janet *argv) {
-    janet_fixarity(argc, 3);
+    janet_arity(argc, 2, 3);
 
     /* Get host, port, and handler*/
-    JanetFunction *fun = janet_getfunction(argv, 2);
+    JanetFunction *fun = janet_optfunction(argv, argc, 2, NULL);
 
     struct addrinfo *ai = janet_get_addrinfo(argv, 0);
 
@@ -441,11 +474,26 @@ static Janet cfun_net_server(int32_t argc, Janet *argv) {
     }
 
     /* Put sfd on our loop */
-    JanetStream *stream = make_stream(sfd, 0);
-    NetStateSimpleServer *ss = (NetStateSimpleServer *) janet_listen(stream, net_machine_simple_server,
-                               JANET_ASYNC_EVENT_READ, sizeof(NetStateSimpleServer));
-    ss->function = fun;
-    return janet_wrap_abstract(stream);
+    if (NULL == fun) {
+        JanetStream *stream = make_stream(sfd, JANET_STREAM_ACCEPTABLE);
+        return janet_wrap_abstract(stream);
+    } else {
+        /* Server with handler */
+        JanetStream *stream = make_stream(sfd, 0);
+        NetStateSimpleServer *ss = (NetStateSimpleServer *) janet_listen(stream, net_machine_simple_server,
+                                   JANET_ASYNC_EVENT_READ, sizeof(NetStateSimpleServer));
+        ss->function = fun;
+        return janet_wrap_abstract(stream);
+    }
+}
+
+static Janet cfun_stream_accept(int32_t argc, Janet *argv) {
+    janet_fixarity(argc, 1);
+    JanetStream *stream = janet_getabstract(argv, 0, &StreamAT);
+    if (!(stream->flags & JANET_STREAM_ACCEPTABLE) || (stream->flags & JANET_POLL_FLAG_CLOSED)) {
+        janet_panic("got non acceptable stream");
+    }
+    janet_sched_accept(stream);
 }
 
 static Janet cfun_stream_read(int32_t argc, Janet *argv) {
@@ -511,6 +559,7 @@ static const JanetMethod stream_methods[] = {
     {"read", cfun_stream_read},
     {"write", cfun_stream_write},
     {"flush", cfun_stream_flush},
+    {"accept", cfun_stream_accept},
     {NULL, NULL}
 };
 
@@ -523,10 +572,17 @@ static int janet_stream_getter(void *p, Janet key, Janet *out) {
 static const JanetReg net_cfuns[] = {
     {
         "net/server", cfun_net_server,
-        JDOC("(net/server host port handler)\n\n"
+        JDOC("(net/server host port &opt handler)\n\n"
              "Start a TCP server. handler is a function that will be called with a stream "
              "on each connection to the server. Returns a new stream that is neither readable nor "
-             "writeable.")
+             "writeable. If handler is not provided, net/accept must be used to get the next connection "
+             "to the server.")
+    },
+    {
+        "net/accept", cfun_stream_accept,
+        JDOC("(net/accept stream)\n\n"
+             "Get the next connection on a server stream. This would usually be called in a loop in a dedicated fiber. "
+             "Returns a new duplex stream which represents a connection to the client.")
     },
     {
         "net/read", cfun_stream_read,