scripts/runqemu: allocate unfsd ports in a way that doesn't race or clash with unrelated processes

There is already a neat check_free_port() function for finding an available port
atomically, so use that and make two additional tweaks:

- no need to allocate two separate ports; per unfsd documentation they can be the same

- move lockfile release until after unfsd has been shut down and the port(s) used has been freed

[YOCTO #15077]

(From OE-Core rev: dee96e82fb04ea99ecd6c25513c7bd368df3bd37)

Signed-off-by: Alexander Kanavin <alex@linutronix.de>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

scripts/runqemu

@@ -1015,17 +1015,14 @@ to your build configuration.
             else:
                 self.nfs_server = '192.168.7.@GATEWAY@'
 
-        # Figure out a new nfs_instance to allow multiple qemus running.
-        ps = subprocess.check_output(("ps", "auxww")).decode('utf-8')
-        pattern = '/bin/unfsd .* -i .*\.pid -e .*/exports([0-9]+) '
-        all_instances = re.findall(pattern, ps, re.M)
-        if all_instances:
-            all_instances.sort(key=int)
-            self.nfs_instance = int(all_instances.pop()) + 1
-
-        nfsd_port = 3049 + 2 * self.nfs_instance
-        mountd_port = 3048 + 2 * self.nfs_instance
+        nfsd_port = 3048 + self.nfs_instance
+        lockdir = "/tmp/qemu-port-locks"
+        self.make_lock_dir(lockdir)
+        while not self.check_free_port('localhost', nfsd_port, lockdir):
+            self.nfs_instance += 1
+            nfsd_port += 1
 
+        mountd_port = nfsd_port
         # Export vars for runqemu-export-rootfs
         export_dict = {
             'NFS_INSTANCE': self.nfs_instance,
@@ -1600,13 +1597,13 @@ to your build configuration.
             logger.debug('Running %s' % str(cmd))
             subprocess.check_call(cmd)
         self.release_taplock()
-        self.release_portlock()
 
         if self.nfs_running:
             logger.info("Shutting down the userspace NFS server...")
             cmd = ("runqemu-export-rootfs", "stop", self.rootfs)
             logger.debug('Running %s' % str(cmd))
             subprocess.check_call(cmd)
+        self.release_portlock()
 
         if self.saved_stty:
             subprocess.check_call(("stty", self.saved_stty))