Make __getitem__ for raw imageseries threadsafe

This ensures that for multithreaded situations (such as writing out
a frame cache), `__getitem__` will be thread-safe. The change ensures
that the frame will be obtained immediately after seeking, and no other
thread can seek until the frame is obtained.

When I test the example Darren gave us (#608) on the master branch with
no multithreading, it ran in 5m 36.826s. With this change, and using
multithreading, it ran in 5m 12.811s. So the multithreading produced
a minor speed increase.

Fixes: #608

Signed-off-by: Patrick Avery <[email protected]>

hexrd/imageseries/load/rawimage.py

@@ -1,5 +1,6 @@
 """ Adapter class for raw image reader"""
 import os
+import threading
 
 import numpy as np
 import yaml
@@ -30,6 +31,7 @@ def __init__(self, fname, **kwargs):
         self._shape = tuple((int(si) for si in y['shape'].split()))
         self._frame_size = self._shape[0] * self._shape[1]
         self._frame_bytes = self._frame_size * self.dtype.itemsize
+        self._frame_read_lock = threading.Lock()
         self.skipbytes = y['skip']
         self._len = self._get_length()
         self._meta = dict()
@@ -105,8 +107,12 @@ def __iter__(self):
 
     def __getitem__(self, key):
         count = key * self._frame_bytes + self.skipbytes
-        self.f.seek(count, 0)
-        frame = np.fromfile(self.f, self.dtype, count=self._frame_size)
+
+        # Ensure reading a frame the file is thread-safe
+        with self._frame_read_lock:
+            self.f.seek(count, 0)
+            frame = np.fromfile(self.f, self.dtype, count=self._frame_size)
+
         return frame.reshape(self.shape)
 
     @property