XY wcfs: multiprocessing: Teach join to return what invoked function returned

For upcoming `wcfs status` we will need to be able to invoke functions
from stdlib and similar and retrieve their returned values.

XXX not sure the API is right. Maybe join should just join and return
value retrieved via .retvalue() method.

wcfs/internal/multiprocessing.py

@@ -79,14 +79,17 @@ class SubProcess(object):
         mod = importlib.import_module(modname)
         f = getattr(mod, funcname)
         procname = kw.pop('_procname', funcpath)
+        _ = _END()
         try:
-            f(*argv, **kw)
-            _ = 'END'
+            r = f(*argv, **kw)
+            _.ret = r
+            _.exc = None
         except BaseException as exc:
             # dump traceback so it appears in the log because Traceback objects are not picklable
             print("\nException in subprocess %s (pid%d):" % (procname, os.getpid()), file=sys.stderr)
             traceback.print_exc()
-            _ = exc
+            _.ret = None
+            _.exc = exc
         cout.send(_)
         cout.close()
 
@@ -100,11 +103,12 @@ class SubProcess(object):
     def exitcode(proc):
         return proc.popen.returncode
 
-    # join waits for the subprocess to end.
-    def join(proc, ctx):
+    # join waits for the subprocess to end and returns or raises its result.
+    def join(proc, ctx): # -> any | exception
         gotend = False
         goteof = False
         joined = False
+        end    = None
         while not (goteof and joined):
             if ctx.err() is not None:
                 raise ctx.err()
@@ -124,10 +128,16 @@ class SubProcess(object):
                 else:
                     if ok:
                         if not gotend:
-                            assert _ == 'END'
+                            assert isinstance(_, _END)
                             gotend = True
+                            end    = _
                         else:
                             raise AssertionError("got %r after END" % (_,))
+        if not gotend:
+            return None
+        if end.exc is not None:
+            raise end.exc
+        return end.ret
 
     # send sends object to subprocess input.
     def send(proc, obj):
@@ -152,3 +162,9 @@ class SubProcess(object):
         if isinstance(_, BaseException):
             raise _
         return _, True
+
+
+class _END:
+    # .ret  any       | None
+    # .exc  Exception | None
+    pass