X wcfs: Teach entrypoint frontend to handle subcomannds: serve, status, stop

This will be used to implement both wcfs service and wcfs promise.

wcfs/__init__.py

 # -*- coding: utf-8 -*-
-# Copyright (C) 2018-2020  Nexedi SA and Contributors.
+# Copyright (C) 2018-2021  Nexedi SA and Contributors.
 #                          Kirill Smelkov <kirr@nexedi.com>
 #
 # This program is free software: you can Use, Study, Modify and Redistribute
@@ -68,7 +68,7 @@ import logging as log
 from os.path import dirname
 from errno import ENOENT, EEXIST
 
-from golang import chan, select, default, func
+from golang import chan, select, default, func, defer
 from golang import sync, context
 from golang.gcompat import qq
 
@@ -377,21 +377,42 @@ def serve(zurl, optv, exec_=False):
 
 
 
-# if called as main just -> exec serve()
+# if called as main -> serve as frontend to wcfs service:
+#
+# wcfs serve  <zurl>
+# wcfs status <zurl>
+# wcfs stop   <zurl>
 def _usage(w):
     progname = os.path.basename(sys.argv[0])
     print("Wcfs serves WCFS filesystem for ZODB at zurl for wendelin.core .\n", file=w)
-    print("Usage: %s [-h | wcfs.go options] zurl" % progname, file=w)
+    print("Usage: %s (serve|stop|status) [-h | wcfs.go options] zurl" % progname, file=w)
     sys.exit(2)
 
+@func
 def main():
     argv = sys.argv[1:]
-    if len(argv) == 0:
+    if len(argv) < 2 or argv[0] == '-h':
         _usage(sys.stderr)
 
-    if argv[0] == '-h':
-        os.execv(_wcfs_exe(), [_wcfs_exe(), '-h'])
-
+    cmd  = argv[0]
+    argv = argv[1:]
     zurl = argv[-1]     # -a -b zurl    -> zurl
     optv = argv[:-1]    # -a -b zurl    -> -a -b
-    serve(zurl, optv, exec_=True)
+
+    if cmd == "serve":
+        if argv[0] == '-h':
+            os.execv(_wcfs_exe(), [_wcfs_exe(), '-h'])
+        serve(zurl, optv, exec_=True)
+
+    elif cmd == "status":
+        wc = join(zurl, autostart=False) # raises if wcfs is not started
+        defer(wc.close)
+        print("wcfs<%s>: serving ok" % zurl)
+
+    elif cmd == "stop":
+        mntpt = _mntpt_4zurl(zurl)
+        subprocess.check_call(["fusermount", "-u", mntpt])
+
+    else:
+        print("wcfs: unknown command %s" % qq(cmd), file=sys.stderr)
+        sys.exit(2)