Revert of rebaseline_server: multithreaded loading/diffing of images (https://codereview.chromium.org/235923002/)

Reason for revert:
Caused https://code.google.com/p/skia/issues/detail?id=2423 ('"Deadline exceeded" error when connecting to live rebaseline_server').  Reverting until I can figure it out.

Original issue's description:
> rebaseline_server: multithreaded loading/diffing of images
>
> BUG=skia:2414
> NOTRY=True
>
> Committed: http://code.google.com/p/skia/source/detail?r=14184

R=rmistry@google.com
TBR=rmistry@google.com
NOTREECHECKS=true
NOTRY=true
BUG=skia:2414

Author: epoger@google.com

Review URL: https://codereview.chromium.org/239623002

git-svn-id: http://skia.googlecode.com/svn/trunk@14207 2bbb7eff-a529-9590-31e7-b0007b416f81

4 files changed, 40 insertions, 190 deletions

diff --git a/gm/rebaseline_server/compare_to_expectations.py b/gm/rebaseline_server/compare_to_expectations.py
index ab16b36290..2389b61dad 100755
--- a/gm/rebaseline_server/compare_to_expectations.py
+++ b/gm/rebaseline_server/compare_to_expectations.py
@@ -89,8 +89,6 @@ class ExpectationComparisons(results.BaseComparisons):
     self._expected_root = expected_root
     self._load_actual_and_expected()
     self._timestamp = int(time.time())
-    logging.info('Number of download file collisions: %s' %
-                 imagediffdb.global_file_collisions)
     logging.info('Results complete; took %d seconds.' %
                  (self._timestamp - time_start))
 
diff --git a/gm/rebaseline_server/imagediffdb.py b/gm/rebaseline_server/imagediffdb.py
index 10fcc98f3b..3b1eb3ebc0 100644
--- a/gm/rebaseline_server/imagediffdb.py
+++ b/gm/rebaseline_server/imagediffdb.py
@@ -11,16 +11,12 @@ Calulate differences between image pairs, and store them in a database.
 
 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 @@ SKPDIFF_BINARY = find_run_binary.find_path_to_program('skpdiff')
 
 DEFAULT_IMAGE_SUFFIX = '.png'
 DEFAULT_IMAGES_SUBDIR = 'images'
-DEFAULT_NUM_WORKERS = 8
 
 DISALLOWED_FILEPATH_CHAR_REGEX = re.compile('[^\w\-]')
 
@@ -56,14 +51,6 @@ 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):
   """ Record of differences between two images. """
@@ -77,6 +64,9 @@ class DiffRecord(object):
     """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
           images
@@ -229,59 +219,30 @@ class ImageDiffDB(object):
   """ 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,
                      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 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 @@ class ImageDiffDB(object):
       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 @@ class ImageDiffDB(object):
         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.
+    Raises a KeyError if we don't have a DiffRecord for this image pair.
     """
-    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
+    key = (_sanitize_locator(expected_image_locator),
+           _sanitize_locator(actual_image_locator))
+    return self._diff_dict[key]
 
 
 # Utility functions
@@ -470,28 +374,11 @@ def _download_and_open_image(local_filepath, url):
 
   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 @@ def _mkdir_unless_exists(path):
   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 @@ def _sanitize_locator(locator):
   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.
diff --git a/gm/rebaseline_server/imagepair.py b/gm/rebaseline_server/imagepair.py
index a89066fca3..33385ab522 100644
--- a/gm/rebaseline_server/imagepair.py
+++ b/gm/rebaseline_server/imagepair.py
@@ -48,43 +48,27 @@ class ImagePair(object):
     self.extra_columns_dict = extra_columns
     if not imageA_relative_url or not imageB_relative_url:
       self._is_different = True
-      self._diff_record = None
-      self._diff_record_set = True
+      self.diff_record = None
     elif imageA_relative_url == imageB_relative_url:
       self._is_different = False
-      self._diff_record = None
-      self._diff_record_set = True
+      self.diff_record = None
     else:
-      # Tell image_diff_db to add this ImagePair.
-      # It will do so in a separate thread so as not to block this one;
-      # when you call self.get_diff_record(), it will block until the results
-      # are ready.
-      image_diff_db.add_image_pair_async(
+      # TODO(epoger): Rather than blocking until image_diff_db can read in
+      # the image pair and generate diffs, it would be better to do it
+      # asynchronously: tell image_diff_db to download a bunch of file pairs,
+      # and only block later if we're still waiting for diff_records to come
+      # back.
+      self._is_different = True
+      image_diff_db.add_image_pair(
           expected_image_locator=imageA_relative_url,
           expected_image_url=posixpath.join(base_url, imageA_relative_url),
           actual_image_locator=imageB_relative_url,
           actual_image_url=posixpath.join(base_url, imageB_relative_url))
-      self._image_diff_db = image_diff_db
-      self._diff_record_set = False
-
-  def get_diff_record(self):
-    """Returns the DiffRecord associated with this ImagePair.
-
-    Returns None if the images are identical, or one is missing.
-    This method will block until the DiffRecord is available.
-    """
-    if not self._diff_record_set:
-      self._diff_record = self._image_diff_db.get_diff_record(
-          expected_image_locator=self.imageA_relative_url,
-          actual_image_locator=self.imageB_relative_url)
-      self._image_diff_db = None  # release reference, no longer needed
-      if (self._diff_record and
-          self._diff_record.get_num_pixels_differing() == 0):
+      self.diff_record = image_diff_db.get_diff_record(
+          expected_image_locator=imageA_relative_url,
+          actual_image_locator=imageB_relative_url)
+      if self.diff_record and self.diff_record.get_num_pixels_differing() == 0:
         self._is_different = False
-      else:
-        self._is_different = True
-      self._diff_record_set = True
-    return self._diff_record
 
   def as_dict(self):
     """Returns a dictionary describing this ImagePair.
@@ -95,12 +79,11 @@ class ImagePair(object):
         KEY__IMAGE_A_URL: self.imageA_relative_url,
         KEY__IMAGE_B_URL: self.imageB_relative_url,
     }
+    asdict[KEY__IS_DIFFERENT] = self._is_different
     if self.expectations_dict:
       asdict[KEY__EXPECTATIONS_DATA] = self.expectations_dict
     if self.extra_columns_dict:
       asdict[KEY__EXTRA_COLUMN_VALUES] = self.extra_columns_dict
-    diff_record = self.get_diff_record()
-    if diff_record and (diff_record.get_num_pixels_differing() > 0):
-      asdict[KEY__DIFFERENCE_DATA] = diff_record.as_dict()
-    asdict[KEY__IS_DIFFERENT] = self._is_different
+    if self.diff_record and (self.diff_record.get_num_pixels_differing() > 0):
+      asdict[KEY__DIFFERENCE_DATA] = self.diff_record.as_dict()
     return asdict
diff --git a/gm/rebaseline_server/imagepairset.py b/gm/rebaseline_server/imagepairset.py
index ae02d8a811..04aea90342 100644
--- a/gm/rebaseline_server/imagepairset.py
+++ b/gm/rebaseline_server/imagepairset.py
@@ -50,20 +50,20 @@ class ImagePairSet(object):
     self._descriptions = descriptions or DEFAULT_DESCRIPTIONS
     self._extra_column_tallies = {}  # maps column_id -> values
                                      #                -> instances_per_value
-    self._image_pairs = []
+    self._image_pair_dicts = []
     self._image_base_url = None
     self._diff_base_url = diff_base_url
 
   def add_image_pair(self, image_pair):
     """Adds an ImagePair; this may be repeated any number of times."""
     # Special handling when we add the first ImagePair...
-    if not self._image_pairs:
+    if not self._image_pair_dicts:
       self._image_base_url = image_pair.base_url
 
     if image_pair.base_url != self._image_base_url:
       raise Exception('added ImagePair with base_url "%s" instead of "%s"' % (
           image_pair.base_url, self._image_base_url))
-    self._image_pairs.append(image_pair)
+    self._image_pair_dicts.append(image_pair.as_dict())
     extra_columns_dict = image_pair.extra_columns_dict
     if extra_columns_dict:
       for column_id, value in extra_columns_dict.iteritems():
@@ -142,7 +142,7 @@ class ImagePairSet(object):
     key_base_url = KEY__IMAGESETS__FIELD__BASE_URL
     return {
         KEY__EXTRACOLUMNHEADERS: self._column_headers_as_dict(),
-        KEY__IMAGEPAIRS: [pair.as_dict() for pair in self._image_pairs],
+        KEY__IMAGEPAIRS: self._image_pair_dicts,
         KEY__IMAGESETS: {
             KEY__IMAGESETS__SET__IMAGE_A: {
                 key_description: self._descriptions[0],