base: Fix race condition in the socket listen function

gem5 makes the incorrect assumption that by binding a socket, it
effectively has allocated a port. Linux only allocates ports once you call
listen on the given socket, not when you call bind.  So even if the port was
free when bind was called, another process (gem5 instance) could race in
between the bind & listen calls and steal the port. In the current code, if
the call to bind fails due to the port being in use (EADDRINUSE), gem5 retries
for a different port.  However if listen fails, gem5 just panics. The fix is
testing the return value of listen and re-trying if it was due to EADDRINUSE.

Committed by: Nilay Vaish <nilay@cs.wisc.edu>

src/base/socket.cc

@@ -103,11 +103,14 @@ ListenSocket::listen(int port, bool reuse)
         return false;
     }
 
-    if (::listen(fd, 1) == -1)
-        panic("ListenSocket(listen): listen() failed!");
+    if (::listen(fd, 1) == -1) {
+        if (errno != EADDRINUSE)
+            panic("ListenSocket(listen): listen() failed!");
+
+        return false;
+    }
 
     listening = true;
-
     anyListening = true;
     return true;
 }