Revert of rebaseline_server: multithreaded loading/diffing of images

gm/rebaseline_server/imagediffdb.py

Index: gm/rebaseline_server/imagediffdb.py
diff --git a/gm/rebaseline_server/imagediffdb.py b/gm/rebaseline_server/imagediffdb.py
index 10fcc98f3b13c35dd57a7aa9550dd813edcbaf04..3b1eb3ebc032a804b444c453dd28f9ed4cc06307 100644
--- a/gm/rebaseline_server/imagediffdb.py
+++ b/gm/rebaseline_server/imagediffdb.py
@@ -11,16 +11,12 @@
 
 import contextlib
 import csv
-import errno
 import logging
-import Queue
 import os
 import re
 import shutil
 import sys
 import tempfile
-import time
-import threading
 import urllib
 try:
   from PIL import Image, ImageChops
@@ -39,7 +35,6 @@
 
 DEFAULT_IMAGE_SUFFIX = '.png'
 DEFAULT_IMAGES_SUBDIR = 'images'
-DEFAULT_NUM_WORKERS = 8
 
 DISALLOWED_FILEPATH_CHAR_REGEX = re.compile('[^\w\-]')
 
@@ -55,14 +50,6 @@
 KEY__DIFFERENCE_DATA__PERCENT_DIFF_PIXELS = 'percentDifferingPixels'
 KEY__DIFFERENCE_DATA__PERCEPTUAL_DIFF = 'perceptualDifference'
 KEY__DIFFERENCE_DATA__WEIGHTED_DIFF = 'weightedDiffMeasure'
-
-# Special values within ImageDiffDB._diff_dict
-DIFFRECORD_FAILED = 'failed'
-DIFFRECORD_PENDING = 'pending'
-
-# TODO(epoger): Temporary(?) list to keep track of how many times we download
-# the same file in multiple threads.
-global_file_collisions = 0
 
 
 class DiffRecord(object):
@@ -76,6 +63,9 @@
                image_suffix=DEFAULT_IMAGE_SUFFIX):
     """Download this pair of images (unless we already have them on local disk),
     and prepare a DiffRecord for them.
+
+    TODO(epoger): Make this asynchronously download images, rather than blocking
+    until the images have been downloaded and processed.
 
     Args:
       storage_root: root directory on local disk within which we store all
@@ -229,50 +219,16 @@
   """ Calculates differences between image pairs, maintaining a database of
   them for download."""
 
-  def __init__(self, storage_root, num_workers=DEFAULT_NUM_WORKERS):
+  def __init__(self, storage_root):
     """
     Args:
       storage_root: string; root path within the DB will store all of its stuff
-      num_workers: integer; number of worker threads to spawn
     """
     self._storage_root = storage_root
 
     # Dictionary of DiffRecords, keyed by (expected_image_locator,
     # actual_image_locator) tuples.
-    # Values can also be DIFFRECORD_PENDING, DIFFRECORD_FAILED.
     self._diff_dict = {}
-
-    # Set up the queue for asynchronously loading DiffRecords, and start the
-    # worker threads reading from it.
-    self._tasks_queue = Queue.Queue(maxsize=2*num_workers)
-    self._workers = []
-    for i in range(num_workers):
-      worker = threading.Thread(target=self.worker, args=(i,))
-      worker.daemon = True
-      worker.start()
-      self._workers.append(worker)
-
-  def worker(self, worker_num):
-    """Launch a worker thread that pulls tasks off self._tasks_queue.
-
-    Args:
-      worker_num: (integer) which worker this is
-    """
-    while True:
-      params = self._tasks_queue.get()
-      key, expected_image_url, actual_image_url = params
-      try:
-        diff_record = DiffRecord(
-            self._storage_root,
-            expected_image_url=expected_image_url,
-            expected_image_locator=key[0],
-            actual_image_url=actual_image_url,
-            actual_image_locator=key[1])
-      except Exception:
-        logging.exception(
-            'exception while creating DiffRecord for key %s' % str(key))
-        diff_record = DIFFRECORD_FAILED
-      self._diff_dict[key] = diff_record
 
   def add_image_pair(self,
                      expected_image_url, expected_image_locator,
@@ -280,8 +236,13 @@
     """Download this pair of images (unless we already have them on local disk),
     and prepare a DiffRecord for them.
 
-    This method will block until the images are downloaded and DiffRecord is
-    available by calling get_diff_record().
+    TODO(epoger): Make this asynchronously download images, rather than blocking
+    until the images have been downloaded and processed.
+    When we do that, we should probably add a new method that will block
+    until all of the images have been downloaded and processed.  Otherwise,
+    we won't know when it's safe to start calling get_diff_record().
+    jcgregorio notes: maybe just make ImageDiffDB thread-safe and create a
+    thread-pool/worker queue at a higher level that just uses ImageDiffDB?
 
     Args:
       expected_image_url: file or HTTP url from which we will download the
@@ -294,11 +255,10 @@
       actual_image_locator: a unique ID string under which we will store the
           actual image within storage_root (probably including a checksum to
           guarantee uniqueness)
-
-    Raises:
-      Exception if we are unable to create a DiffRecord for this image pair.
-    """
-    key = _generate_key(expected_image_locator, actual_image_locator)
+    """
+    expected_image_locator = _sanitize_locator(expected_image_locator)
+    actual_image_locator = _sanitize_locator(actual_image_locator)
+    key = (expected_image_locator, actual_image_locator)
     if not key in self._diff_dict:
       try:
         new_diff_record = DiffRecord(
@@ -318,70 +278,14 @@
         new_diff_record = None
       self._diff_dict[key] = new_diff_record
 
-  def add_image_pair_async(self,
-                           expected_image_url, expected_image_locator,
-                           actual_image_url, actual_image_locator):
-    """Download this pair of images (unless we already have them on local disk),
-    and prepare a DiffRecord for them.
-
-    This method will return quickly; calls to get_diff_record() will block
-    until the DiffRecord is available (or we have given up on creating it).
-
-    Args:
-      expected_image_url: file or HTTP url from which we will download the
-          expected image
-      expected_image_locator: a unique ID string under which we will store the
-          expected image within storage_root (probably including a checksum to
-          guarantee uniqueness)
-      actual_image_url: file or HTTP url from which we will download the
-          actual image
-      actual_image_locator: a unique ID string under which we will store the
-          actual image within storage_root (probably including a checksum to
-          guarantee uniqueness)
-    """
-    key = _generate_key(expected_image_locator, actual_image_locator)
-    if not key in self._diff_dict:
-      # If we have already requested a diff between these two images,
-      # we don't need to request it again.
-      #
-      # Threading note: If multiple threads called into this method with the
-      # same key at the same time, there will be multiple tasks on the queue
-      # with the same key.  But that's OK; they will both complete successfully,
-      # and just waste a little time in the process. Nothing will break.
-      self._diff_dict[key] = DIFFRECORD_PENDING
-      self._tasks_queue.put((key, expected_image_url, actual_image_url))
-
   def get_diff_record(self, expected_image_locator, actual_image_locator):
     """Returns the DiffRecord for this image pair.
 
-    Args:
-      expected_image_locator: a unique ID string under which we will store the
-          expected image within storage_root (probably including a checksum to
-          guarantee uniqueness)
-      actual_image_locator: a unique ID string under which we will store the
-          actual image within storage_root (probably including a checksum to
-          guarantee uniqueness)
-
-    Returns the DiffRecord for this image pair, or None if we were unable to
-    generate one.
-    """
-    key = _generate_key(expected_image_locator, actual_image_locator)
-    diff_record = self._diff_dict[key]
-
-    # If we have no results yet, block until we do.
-    while diff_record == DIFFRECORD_PENDING:
-      time.sleep(1)
-      diff_record = self._diff_dict[key]
-
-    # Once we have the result...
-    if diff_record == DIFFRECORD_FAILED:
-      logging.error(
-          'failed to create a DiffRecord for expected_image_locator=%s , '
-          'actual_image_locator=%s' % (
-              expected_image_locator, actual_image_locator))
-      return None
-    else:
-      return diff_record
+    Raises a KeyError if we don't have a DiffRecord for this image pair.
+    """
+    key = (_sanitize_locator(expected_image_locator),
+           _sanitize_locator(actual_image_locator))
+    return self._diff_dict[key]
 
 
 # Utility functions
@@ -470,28 +374,11 @@
 
   Returns: a PIL image object
   """
-  global global_file_collisions
   if not os.path.exists(local_filepath):
     _mkdir_unless_exists(os.path.dirname(local_filepath))
     with contextlib.closing(urllib.urlopen(url)) as url_handle:
-
-      # First download the file contents into a unique filename, and
-      # then rename that file.  That way, if multiple threads are downloading
-      # the same filename at the same time, they won't interfere with each
-      # other (they will both download the file, and one will "win" in the end)
-      temp_filename = '%s-%d' % (local_filepath,
-                                 threading.current_thread().ident)
-      with open(temp_filename, 'wb') as file_handle:
+      with open(local_filepath, 'wb') as file_handle:
         shutil.copyfileobj(fsrc=url_handle, fdst=file_handle)
-
-      # Keep count of how many colliding downloads we encounter;
-      # if it's a large number, we may want to change our download strategy
-      # to minimize repeated downloads.
-      if os.path.exists(local_filepath):
-        global_file_collisions += 1
-      else:
-        os.rename(temp_filename, local_filepath)
-
   return _open_image(local_filepath)
 
 
@@ -532,11 +419,8 @@
   Args:
     path: path on local disk
   """
-  try:
+  if not os.path.isdir(path):
     os.makedirs(path)
-  except OSError as e:
-    if e.errno == errno.EEXIST:
-      pass
 
 
 def _sanitize_locator(locator):
@@ -549,21 +433,6 @@
   return DISALLOWED_FILEPATH_CHAR_REGEX.sub('_', str(locator))
 
 
-def _generate_key(expected_image_locator, actual_image_locator):
-  """Returns a key suitable for looking up this image pair.
-
-  Args:
-    expected_image_locator: a unique ID string under which we will store the
-        expected image within storage_root (probably including a checksum to
-        guarantee uniqueness)
-    actual_image_locator: a unique ID string under which we will store the
-        actual image within storage_root (probably including a checksum to
-        guarantee uniqueness)
-  """
-  return (_sanitize_locator(expected_image_locator),
-          _sanitize_locator(actual_image_locator))
-
-
 def _get_difference_locator(expected_image_locator, actual_image_locator):
   """Returns the locator string used to look up the diffs between expected_image
   and actual_image.