Re: [Qemu-devel] [PATCH v5 1/3] contrib: add ivshmem client and server

[Michael S. Tsirkin]

On Fri, Sep 05, 2014 at 02:07:54PM +0200, David Marchand wrote:
> Hello Michael,
> 
> On 09/04/2014 05:56 PM, Michael S. Tsirkin wrote:
> >>+    /* create the unix listening socket */
> >>+    sock_fd = socket(AF_UNIX, SOCK_STREAM, 0);
> >>+    if (sock_fd < 0) {
> >>+        debug_log(server, "cannot create socket: %s\n", strerror(errno));
> >>+        goto err_close_shm;
> >>+    }
> >>+
> >>+    sun.sun_family = AF_UNIX;
> >>+    snprintf(sun.sun_path, sizeof(sun.sun_path), "%s", 
> >>server->unix_sock_path);
> >>+    unlink(sun.sun_path);
> >
> >why unlink it?
> 
> Yes, this is wrong, because this means that when starting multiple
> servers on the same socket, the last server is the one who wins ...
> while I think it should be the opposite (first server wins, as it may
> have some connected clients).
> 
> I have been scratching my head about this: when should I unlink ?
> 
> My current fix unlinks from ivshmem_server_close() (which should be the
> right place).
> I need to call this when exiting, but I can only do this when the server
> exits gracefully (when an error occurs on the server socket or when
> receiving a SIGTERM).
> 
> If something unexpected happens (like a bug/crash or a SIGKILL), the
> socket won't be unlinked and the next server process will refuse to start.
> Is this something acceptable ?
> 
> Do you have a better idea ?
> 
> 
> Thanks.

Is the point to prevent multiple servers from opening
the same socket? Either leave it up to the user,
or try to use some lock - unlink will never be
a safe way to do this.  I advise for the former.


> -- 
> David Marchand