Make the Debugger a singleton and do not reset the stopframe to None …

…during postmortem.

This fixes #607. Making the Debugger a singleton should be enough to fix #607.
HOWEVER, pytest def postmortem() calls .reset() which resets the stopframe.
Somehow, this causes the debugger to stop somewhere inside internal debugger source code,
instead of in the code the postmortem should be done.

So, this also includes an override of the .reset() method and detects when the debugger
is already running and does not reset the stopframe. See also #67.

pudb/debugger.py

@@ -183,15 +183,21 @@
 
 # {{{ debugger interface
 
-class Debugger(bdb.Bdb):
+class Singleton(type):
+    _instance = None
+
+    def __call__(cls, *args, **kwargs):
+        if not cls._instance:
+            cls._instance = super(Singleton, cls).__call__(*args, **kwargs)
+        return cls._instance
+
+
+class Debugger(bdb.Bdb, metaclass=Singleton):
     _current_debugger = []
 
     def __init__(self, stdin=None, stdout=None, term_size=None, steal_output=False,
                  _continue_at_start=False, tty_file=None, **kwargs):
 
-        if Debugger._current_debugger:
-            raise ValueError("a Debugger instance already exists")
-
         # Pass remaining kwargs to python debugger framework
         bdb.Bdb.__init__(self, **kwargs)
         self.ui = DebuggerUI(self, stdin=stdin, stdout=stdout, term_size=term_size)
@@ -214,6 +220,30 @@ def __init__(self, stdin=None, stdout=None, term_size=None, steal_output=False,
         # Okay, now we have a debugger
         self._current_debugger.append(self)
 
+    def reset(self):
+        """Set values of attributes as ready to start debugging.
+
+        Override from Bdb.reset()
+
+        When pytest starts a postmortem analysis, but the debugger is already active,
+        calling .reset() in src/_pytest/debugging.py::post_mortem
+        causes the self.stopframe to be set to None.
+        This pauses the debugger somewhere in the source code of the debugger. See #67
+
+        We detect using _current_debugger that this is the case and do not set the
+        stopframe to None then.
+
+        Related #607, #52
+        """
+        import linecache
+        linecache.checkcache()
+        self.botframe = None
+        if not self._current_debugger:
+            self.stopframe = None
+        self.returnframe = None
+        self.quitting = False
+        self.stoplineno = 0
+
     def __del__(self):
         # according to https://stackoverflow.com/a/1481512/1054322, the garbage
         # collector cannot be relied on to  call this, so we call it explicitly