util: add socket_set_fast_reuse function which will replace setting SO_REUSEADDR

If a socket is closed it remains in TIME_WAIT state for some time. On operating
systems using BSD sockets the endpoint of the socket may not be reused while in
this state unless SO_REUSEADDR was set on the socket. On windows on the other
hand the default behaviour is to allow reuse (i.e. identical to SO_REUSEADDR on
other operating systems) and setting SO_REUSEADDR on a socket allows it to be
bound to a endpoint even if the endpoint is already used by another socket
independently of the other sockets state. This can even result in undefined
behaviour.

Many sockets used by QEMU should not block the use of their endpoint after being
closed while they are still in TIME_WAIT state. Currently QEMU sets SO_REUSEADDR
for such sockets, which can lead to problems on Windows. This patch introduces
the function socket_set_fast_reuse that should be used instead of setting
SO_REUSEADDR when fast socket reuse is desired and behaves correctly on all
operating systems.

As a failure of this function can only be caused by bad QEMU internal errors, an
assertion handles these situations. The return value is still passed on, to
minimize changes in client code and prevent unused variable warnings if NDEBUG
is defined.

Signed-off-by: Sebastian Ottlik <ottlik@fzi.de>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Stefan Weil <sw@weilnetz.de>

include/qemu/sockets.h

@@ -39,6 +39,7 @@ int socket_set_cork(int fd, int v);
 int socket_set_nodelay(int fd);
 void qemu_set_block(int fd);
 void qemu_set_nonblock(int fd);
+int socket_set_fast_reuse(int fd);
 int send_all(int fd, const void *buf, int len1);
 int recv_all(int fd, void *buf, int len1, bool single_read);
 

util/oslib-posix.c

@@ -157,6 +157,18 @@ void qemu_set_nonblock(int fd)
     fcntl(fd, F_SETFL, f | O_NONBLOCK);
 }
 
+int socket_set_fast_reuse(int fd)
+{
+    int val = 1, ret;
+
+    ret = setsockopt(fd, SOL_SOCKET, SO_REUSEADDR,
+                     (const char *)&val, sizeof(val));
+
+    assert(ret == 0);
+
+    return ret;
+}
+
 void qemu_set_cloexec(int fd)
 {
     int f;

util/oslib-win32.c

@@ -124,6 +124,16 @@ void qemu_set_nonblock(int fd)
     qemu_fd_register(fd);
 }
 
+int socket_set_fast_reuse(int fd)
+{
+    /* Enabling the reuse of an endpoint that was used by a socket still in
+     * TIME_WAIT state is usually performed by setting SO_REUSEADDR. On Windows
+     * fast reuse is the default and SO_REUSEADDR does strange things. So we
+     * don't have to do anything here. More info can be found at:
+     * http://msdn.microsoft.com/en-us/library/windows/desktop/ms740621.aspx */
+    return 0;
+}
+
 int inet_aton(const char *cp, struct in_addr *ia)
 {
     uint32_t addr = inet_addr(cp);