Fix segfaults accessing _OpenSlideMap after OpenSlide released

Rework destructor and close() handling to ensure that:

- Operations on an _OpenSlideMap object don't crash if its parent
OpenSlide object has been freed.

- Operations on an _OpenSlideMap object raise an exception, rather than
crashing, if the parent OpenSlide object has been explicitly closed.

- Freeing an OpenSlide object during interpreter shutdown correctly
releases the OpenSlide handle.

Closes #6.

Author: bgilbert

openslide/__init__.py

@@ -144,10 +144,6 @@ def __init__(self, filename):
         AbstractSlide.__init__(self)
         self._osr = lowlevel.open(filename)
 
-    def __del__(self):
-        if getattr(self, '_osr', None) is not None:
-            self.close()
-
     @classmethod
     def can_open(cls, filename):
         """Return True if OpenSlide can read the specified file."""
@@ -156,7 +152,6 @@ def can_open(cls, filename):
     def close(self):
         """Close the OpenSlide object."""
         lowlevel.close(self._osr)
-        self._osr = None
 
     @property
     def layer_count(self):

openslide/lowlevel.py

@@ -51,13 +51,26 @@ class _OpenSlide(object):
 
     def __init__(self, ptr):
         self._as_parameter_ = ptr
+        self._valid = True
+        # Retain a reference to close() to avoid GC problems during
+        # interpreter shutdown
+        self._close = close
+
+    def __del__(self):
+        if self._valid:
+            self._close(self)
+
+    def invalidate(self):
+        self._valid = False
 
     @classmethod
     def from_param(cls, obj):
         if obj.__class__ != cls:
             raise ValueError("Not an OpenSlide reference")
         if not obj._as_parameter_:
             raise ValueError("Passing undefined slide object")
+        if not obj._valid:
+            raise ValueError("Passing closed slide object")
         return obj
 
 # check for errors opening an image file and wrap the resulting handle
@@ -66,6 +79,10 @@ def _check_open(result, _func, _args):
         raise OpenSlideError("Could not open image file")
     return _OpenSlide(result)
 
+# prevent further operations on slide handle after it is closed
+def _check_close(_result, _func, args):
+    args[0].invalidate()
+
 # check if the library got into an error state after each library call
 def _check_error(result, _func, args):
     err = get_error(args[0])
@@ -108,7 +125,7 @@ def _load_image(buf, size):
 
 open = _func('openslide_open', c_void_p, [c_char_p], _check_open)
 
-close = _func('openslide_close', None, [_OpenSlide], None)
+close = _func('openslide_close', None, [_OpenSlide], _check_close)
 
 get_layer_count = _func('openslide_get_layer_count', c_int32, [_OpenSlide])