Improve accept(); allow select(2) on listeners

Changes:

    * Add validation to accept() to ensure file descriptors provided
      refer to SOCK_STREAM sockets in the SOCK_LISTENING state

    * Rather than writing a reply to the patty client socket when
      accepting a SABM or SABME request, write a message to the
      remote end of the listening socket; this avoids issues wherein
      a client receives an unexpected accept() response that may be
      unrelated to another pending request; this also allows client
      code to use select(2) on listening sockets

    * In patty_client_accept(), send the initial request to validate
      that the local socket can accept connections, and then wait for
      a connection to be made, reading a message written to the
      listening socket from the patty server

    * Allow select(2) to notify on accepted connections

include/patty/client.h

@@ -120,11 +120,15 @@ typedef struct _patty_client_accept_request {
 } patty_client_accept_request;
 
 typedef struct _patty_client_accept_response {
-    int fd;
+    int ret;
     int eno;
+} patty_client_accept_response;
+
+typedef struct _patty_client_accept_message {
+    int fd;
     patty_ax25_addr peer;
     char path[PATTY_AX25_SOCK_PATH_SIZE];
-} patty_client_accept_response;
+} patty_client_accept_message;
 
 int patty_client_accept(patty_client *client,
                         int fd,

src/client.c

@@ -312,6 +312,7 @@ int patty_client_accept(patty_client *client,
 
     patty_client_accept_request request;
     patty_client_accept_response response;
+    patty_client_accept_message message;
 
     patty_client_sock *local,
                       *remote;
@@ -341,18 +342,33 @@ int patty_client_accept(patty_client *client,
         goto error_io;
     }
 
+    /*
+     * First, the server will tell us if the fd specified in accept() is indeed
+     * accepting connections.
+     */
     if (read(client->fd, &response, sizeof(response)) < 0) {
         goto error_io;
     }
 
-    if (response.fd >= 0) {
-        memcpy(peer, &response.peer, sizeof(*peer));
+    if (response.ret < 0) {
+        errno = response.eno;
 
-        remote->fd = response.fd;
-
-        strncpy(remote->path, response.path, sizeof(remote->path));
+        return response.ret;
     }
 
+    /*
+     * Next, we will wait for the server to receive a SABM or SABME frame, and
+     * notify us via the listening socket that a connection has been accepted.
+     */
+    if (read(fd, &message, sizeof(message)) < 0) {
+        goto error_io;
+    }
+
+    remote->fd = message.fd;
+
+    memcpy(peer, &message.peer, sizeof(*peer));
+    strncpy(remote->path, message.path, sizeof(remote->path));
+
     if ((pty = open(remote->path, O_RDWR)) < 0) {
         goto error_open;
     }

src/server.c

@@ -514,26 +514,35 @@ int patty_ax25_server_add_route(patty_ax25_server *server,
 }
 
 static int respond_accept(int client,
-                          int fd,
-                          int eno,
-                          patty_ax25_addr *peer,
-                          char *path) {
-    patty_client_accept_response response;
+                          int ret,
+                          int eno) {
+    patty_client_accept_response response = {
+        .ret = ret,
+        .eno = eno
+    };
 
-    memset(&response, '\0', sizeof(response));
+    return write(client, &response, sizeof(response));
+}
 
-    response.fd  = fd;
-    response.eno = eno;
+static int notify_accept(int local,
+                         int remote,
+                         patty_ax25_addr *peer,
+                         char *path) {
+    patty_client_accept_message message;
+
+    memset(&message, '\0', sizeof(message));
+
+    message.fd = remote;
 
     if (peer) {
-        memcpy(&response.peer, peer, sizeof(response.peer));
+        memcpy(&message.peer, peer, sizeof(message.peer));
     }
 
     if (path) {
-        strncpy(response.path, path, sizeof(response.path)-1);
+        strncpy(message.path, path, sizeof(message.path)-1);
     }
 
-    return write(client, &response, sizeof(response));
+    return write(local, &message, sizeof(message));
 }
 
 static int respond_connect(int client, int ret, int eno) {
@@ -770,13 +779,18 @@ static int server_accept(patty_ax25_server *server,
     }
 
     if ((sock = sock_by_fd(server->socks_by_fd, request.fd)) == NULL) {
-        goto error_sock_by_fd;
+        return respond_accept(client, -1, EBADF);
     }
 
-    return 0;
+    if (sock->type != PATTY_AX25_SOCK_STREAM) {
+        return respond_accept(client, -1, EOPNOTSUPP);
+    }
 
-error_sock_by_fd:
-    return respond_accept(client, -1, EBADF, NULL, NULL);
+    if (sock->state != PATTY_AX25_SOCK_LISTENING) {
+        return respond_accept(client, -1, EINVAL);
+    }
+
+    return respond_accept(client, 0, 0);
 
 error_io:
     return -1;
@@ -1228,8 +1242,10 @@ static int handle_sabm(patty_ax25_server *server,
     patty_ax25_sock *local, *remote;
 
     if ((local = sock_by_addr(server->socks_local,
-                              &frame->dest)) == NULL) {
-        return reply_dm(iface, frame, PATTY_AX25_FRAME_FINAL);
+                              &frame->dest)) == NULL
+     || local->type  != PATTY_AX25_SOCK_STREAM
+     || local->state != PATTY_AX25_SOCK_LISTENING) {
+        goto reply_dm;
     }
 
     if ((client = client_by_sock(server, local)) < 0) {
@@ -1254,6 +1270,8 @@ static int handle_sabm(patty_ax25_server *server,
         save_reply_addr(remote, frame);
 
         created = 1;
+    } else if (remote->state != PATTY_AX25_SOCK_PENDING_ACCEPT) {
+        goto reply_dm;
     }
 
     remote->state = PATTY_AX25_SOCK_ESTABLISHED;
@@ -1280,15 +1298,18 @@ static int handle_sabm(patty_ax25_server *server,
 
     fd_watch(server, remote->fd);
 
-    if (respond_accept(client, remote->fd, 0, &frame->src, remote->pty) < 0) {
-        goto error_respond_accept;
+    if (notify_accept(local->fd, remote->fd, &remote->remote, remote->pty) < 0) {
+        goto error_notify_accept;
     }
 
     patty_timer_start(&remote->timer_t3, PATTY_AX25_SOCK_DEFAULT_KEEPALIVE);
 
     return reply_ua(iface, frame, PATTY_AX25_FRAME_FINAL);
 
-error_respond_accept:
+reply_dm:
+    return reply_dm(iface, frame, PATTY_AX25_FRAME_FINAL);
+
+error_notify_accept:
 error_sock_realloc_bufs:
 error_sock_save:
     patty_ax25_sock_destroy(remote);
@@ -1673,9 +1694,7 @@ static int handle_xid(patty_ax25_server *server,
         patty_ax25_sock_params_max(remote);
 
         if (patty_ax25_sock_params_negotiate(remote, &params) < 0) {
-            patty_ax25_sock_destroy(remote);
-
-            return respond_accept(client, -1, errno, NULL, NULL);
+            goto error_sock_params_negotiate;
         }
 
         save_reply_addr(remote, frame);
@@ -1691,6 +1710,9 @@ reply_dm:
     return reply_dm(iface, frame, PATTY_AX25_FRAME_FINAL);
 
 error_sock_save:
+error_sock_params_negotiate:
+    patty_ax25_sock_destroy(remote);
+
 error_sock_new:
 error_client_by_sock:
 error_io: