BUG: disable garbage collector during memory allocation hook

When a cython object contains numpy arrays the pure python allocation
hook can trigger during cythons __dealloc__ method and trigger a second
deletion of the object currently being deleted.
To minimize the probabily that that happens disable the garbage
collector during the hook.
As this involves python calls it is still possible that a double delete
occurs but chances are lowered, a proper solution would be C only hook
like python 3.4 tracemalloc module.

Closes numpygh-4834

tools/allocation_tracking/track_allocations.py

@@ -1,6 +1,7 @@
 from __future__ import division, absolute_import, print_function
 
 import numpy as np
+import gc
 import inspect
 from alloc_hook import NumpyAllocHook
 
@@ -35,12 +36,21 @@ def __exit__(self, type, value, traceback):
         self.numpy_hook.__exit__()
 
     def hook(self, inptr, outptr, size):
+        # minimize the chances that the garbage collector kicks in during a
+        # cython __dealloc__ call and causes a double delete of the current
+        # object. To avoid this fully the hook would have to avoid all python
+        # api calls, e.g. by being implemented in C like python 3.4's
+        # tracemalloc module
+        gc_on = gc.isenabled()
+        gc.disable()
         if outptr == 0:  # it's a free
             self.free_cb(inptr)
         elif inptr != 0:  # realloc
             self.realloc_cb(inptr, outptr, size)
         else:  # malloc
             self.alloc_cb(outptr, size)
+        if gc_on:
+            gc.enable()
 
     def alloc_cb(self, ptr, size):
         if size >= self.threshold: