[Qemu-devel] [PATCH] handle bind(), listen() race

[Simon Rowe]

Hello, we've seen a very occasional failure in the startup of qemu where the 
call to inet_listen() for the VNC port fails with EADDRINUSE.

I believe there is a race condition when two qemu processes both bind to the 
same port, in one the subsequent call to listen() will succeed and the other 
fail. Below is a patch which appears to deal with this scenario but we would 
appreciate any comments on it.

Thanks
                Simon


diff -ur qemu-0.14.0-org/qemu-sockets.c qemu-0.14.0/qemu-sockets.c
--- qemu-0.14.0-org/qemu-sockets.c      2011-02-16 14:44:05.000000000 +0000
+++ qemu-0.14.0/qemu-sockets.c  2011-04-26 10:41:14.472067346 +0100
@@ -158,6 +158,7 @@
 
     /* create socket + bind */
     for (e = res; e != NULL; e = e->ai_next) {
+rebind:
         getnameinfo((struct sockaddr*)e->ai_addr,e->ai_addrlen,
                        uaddr,INET6_ADDRSTRLEN,uport,32,
                        NI_NUMERICHOST | NI_NUMERICSERV);
@@ -182,7 +183,19 @@
                 if (sockets_debug)
                     fprintf(stderr,"%s: bind(%s,%s,%d): OK\n", __FUNCTION__,
                         inet_strfamily(e->ai_family), uaddr, 
inet_getport(e));
-                goto listen;
+                if (listen(slisten,1) == 0) {
+                   goto listened;
+               }
+               else {
+                   int err = errno;
+
+                   perror("listen");
+                   closesocket(slisten);
+                   if (err == EADDRINUSE)
+                       goto rebind;
+                   freeaddrinfo(res);
+                   return -1;
+               }
             }
             try_next = to && (inet_getport(e) <= to + port_offset);
             if (!try_next || sockets_debug)
@@ -201,13 +214,7 @@
     freeaddrinfo(res);
     return -1;
 
-listen:
-    if (listen(slisten,1) != 0) {
-        perror("listen");
-        closesocket(slisten);
-        freeaddrinfo(res);
-        return -1;
-    }
+listened:
     snprintf(uport, sizeof(uport), "%d", inet_getport(e) - port_offset);
     qemu_opt_set(opts, "host", uaddr);
     qemu_opt_set(opts, "port", uport);