[PATCH qemu-traditional] qemu: Fix race condition when binding ports

[Andrew Cooper <andrew.cooper3@citrix.com>]

Two Qemus can race to bind the same VNC port.  It is valid for multiple
listen()s on the same socket to succeed, but only the first bind() will
succeed.

In the case that two Qemus are racing, the second one will fail with an
EADDRINUSE, and bail with a fatal error which renders the domain functionally
useless.

In the case that bind() fails with EADDRINUSE, rebind the socket again and try
for the next port.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
CC: Ian Campbell <Ian.Campbell@citrix.com>
CC: Ian Jackson <Ian.Jackson@eu.citrix.com>
CC: Stefano Stabellini <stefano.stabellini@citrix.com>
CC: George Dunlap <george.dunlap@eu.citrix.com>

---

This fix has been in XenServer for two and a half years
---
 qemu-sockets.c |   23 ++++++++++++++++-------
 1 file changed, 16 insertions(+), 7 deletions(-)

diff --git a/qemu-sockets.c b/qemu-sockets.c
index 9b9fa77..e54f6ad 100644
--- a/qemu-sockets.c
+++ b/qemu-sockets.c
@@ -163,6 +163,7 @@ int inet_listen(const char *str, char *ostr, int olen,
 
     /* 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);
@@ -186,7 +187,20 @@ int inet_listen(const char *str, char *ostr, int olen,
                 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)
@@ -205,12 +219,7 @@ int inet_listen(const char *str, char *ostr, int olen,
     freeaddrinfo(res);
     return -1;
 
-listen:
-    if (listen(slisten,1) != 0) {
-        perror("listen");
-        closesocket(slisten);
-        return -1;
-    }
+listened:
     if (ostr) {
         if (e->ai_family == PF_INET6) {
             snprintf(ostr, olen, "[%s]:%d%s", uaddr,
-- 
1.7.10.4