BUG/MEDIUM: mworker: close unused transferred FDs on load failure

When the master process is reloaded on a new config, it will try to
connect to the previous process' socket to retrieve all known
listening FDs to be reused by the new listeners. If listeners were
removed, their unused FDs are simply closed.

However there's a catch. In case a socket fails to bind, the master
will cancel its startup and swithc to wait mode for a new operation
to happen. In this case it didn't close the possibly remaining FDs
that were left unused.

It is very hard to hit this case, but it can happen during a
troubleshooting session with fat fingers. For example, let's say
a config runs like this:

   frontend ftp
        bind 1.2.3.4:20000-29999

The admin wants to extend the port range down to 10000-29999 and
by mistake ends up with:

   frontend ftp
        bind 1.2.3.41:20000-29999

Upon restart the bind will fail if the address is not present, and the
master will then switch to wait mode without releasing the previous FDs
for 1.2.3.4:20000-29999 since they're now apparently unused. Then once
the admin fixes the config and does:

   frontend ftp
        bind 1.2.3.4:10000-29999

The service will start, but will bind new sockets, half of them
overlapping with the previous ones that were not properly closed. This
may result in a startup error (if SO_REUSEPORT is not enabled or not
available), in a FD number exhaustion (if the error is repeated many
times), or in connections being randomly accepted by the process if
they sometimes land on the old FD that nobody listens on.

This patch will need to be backported as far as 1.8, and depends on
previous patch:

   MINOR: sock: move the unused socket cleaning code into its own function

Note that before 2.3 most of the code was located inside haproxy.c, so
the patch above should probably relocate the function there instead of
sock.c.

src/haproxy.c

@@ -856,6 +856,9 @@ void reexec_on_failure()
 		child->failedreloads++;
 	}
 
+	/* do not keep unused FDs retrieved from the previous process */
+	sock_drop_unused_old_sockets();
+
 	usermsgs_clr(NULL);
 	ha_warning("Loading failure!\n");
 	mworker_reexec_waitmode();