hush somewhat-real problems found by static analysis

These would have caused problems if a certain unlikely code path was taken:
mostly error-handling pathways.

misc/coding_tools/check-miscaptures.py

@@ -62,7 +62,7 @@ def check_loop(ast, results):
         # contains another 'for' loop.
         childnodes = funcnode.getChildNodes()[len(funcnode.defaults):]
         for child in childnodes:
-            check_ast(funcnode, results)
+            check_ast(child, results)
 
 def collect_assigned_and_nested(ast, assigned, nested):
     """

src/allmydata/frontends/magic_folder.py

@@ -243,7 +243,7 @@ class MagicFolder(service.MultiService):
         clock = clock or reactor
         db = magicfolderdb.get_magicfolderdb(dbfile, create_version=(magicfolderdb.SCHEMA_v1, 1))
         if db is None:
-            return Failure(Exception('ERROR: Unable to load magic folder db.'))
+            raise Exception('ERROR: Unable to load magic folder db.')
 
         # for tests
         self._client = client

src/allmydata/scripts/common_http.py

@@ -32,7 +32,7 @@ class BadResponse(object):
         self.status = -1
         self.reason = "Error trying to connect to %s: %s" % (url, err)
         self.error = err
-    def read(self):
+    def read(self, length=0):
         return ""
 
 

src/allmydata/util/log.py

@@ -36,10 +36,11 @@ class LogMixin(object):
     def log(self, msg, facility=None, parent=None, *args, **kwargs):
         if facility is None:
             facility = self._facility
+        pmsgid = None
         if parent is None:
             pmsgid = self._parentmsgid
-        if pmsgid is None:
-            pmsgid = self._grandparentmsgid
+            if pmsgid is None:
+                pmsgid = self._grandparentmsgid
         msgid = log.msg(msg, facility=facility, parent=pmsgid, *args, **kwargs)
         if self._parentmsgid is None:
             self._parentmsgid = msgid