Have the squashing system write the data (via a double queue system and multiprocessing) to both the client and the stack's storage. On subsequent calls, if the synthetic image exists, it will be returned directly instead of being recomputed

endpoints/verbs.py

@@ -11,6 +11,8 @@ from auth.permissions import ReadRepositoryPermission
 from data import model
 from endpoints.registry import set_cache_headers
 
+from util.queuefile import QueueFile
+from util.queueprocess import QueueProcess
 from util.gzipwrap import GzipWrap
 from util.streamlayerformat import StreamLayerMerger
 
@@ -19,6 +21,24 @@ from werkzeug.wsgi import wrap_file
 verbs = Blueprint('verbs', __name__)
 logger = logging.getLogger(__name__)
 
+def _open_stream(namespace, repository, image_list):
+  def get_next_layer():
+    for current_image_id in image_list:
+      current_image_entry = model.get_repo_image(namespace, repository, current_image_id)
+      current_image_path = store.image_layer_path(current_image_entry.storage.uuid)
+      current_image_stream = store.stream_read_file(current_image_entry.storage.locations,
+                                                    current_image_path)
+        
+      logger.debug('Returning image layer %s: %s' % (current_image_id, current_image_path))
+      yield current_image_stream
+
+  stream = GzipWrap(StreamLayerMerger(get_next_layer).get_generator())
+  return stream.read
+
+def _write_image_to_storage(namespace, repository, locations, image_id, queue_file):
+  image_path = store.image_layer_path(image_id)
+  store.stream_write(locations, image_path, queue_file)
+  queue_file.close()
 
 @verbs.route('/<namespace>/<repository>/<tag>/squash', methods=['GET'])
 @process_auth
@@ -42,22 +62,43 @@ def get_squashed_tag(namespace, repository, tag, headers):
     unhashed = str(repo_image.storage.id) + ':' + app.config['SECRET_KEY']
     synthetic_image_id = hashlib.sha256(unhashed).hexdigest()
 
+    # Check to see if the synthetic image ID exists in storage. If so, we just return a 302.
+    logger.debug('Looking up synthetic image %s', synthetic_image_id)
+
+    locations = repo_image.storage.locations
+    saved_image_path = store.image_layer_path(synthetic_image_id)
+    if store.exists(locations, saved_image_path):
+      logger.debug('Synthetic image %s exists in storage', synthetic_image_id)
+      download_url = store.get_direct_download_url(locations, saved_image_path)
+      if download_url:
+        logger.debug('Redirecting to download URL for synthetic image %s', synthetic_image_id)
+        return redirect(download_url, code=302)
+
+      logger.debug('Sending cached synthetic image %s', synthetic_image_id)
+      return send_file(store.stream_read_file(locations, saved_image_path))
+
     # Load the ancestry for the image.
+    logger.debug('Building and returning synthetic image %s', synthetic_image_id)
     uuid = repo_image.storage.uuid
     ancestry_data = store.get_content(repo_image.storage.locations, store.image_ancestry_path(uuid))
     full_image_list = json.loads(ancestry_data)
-
-    def get_next_layer():
-      for current_image_id in full_image_list:
-        current_image_entry = model.get_repo_image(namespace, repository, current_image_id)
-        current_image_path = store.image_layer_path(current_image_entry.storage.uuid)
-        current_image_stream = store.stream_read_file(current_image_entry.storage.locations,
-                                                      current_image_path)
         
-        logger.debug('Returning image layer %s: %s' % (current_image_id, current_image_path))
-        yield current_image_stream
+    # Create a queue process to generate the data. The queue files will read from the process
+    # and send the results to the client and storage.
+    args = (namespace, repository, full_image_list)
+    queue_process = QueueProcess(_open_stream, 8 * 1024, 10 * 1024 * 1024, args) # 8K/10M chunk/max
 
-    stream = GzipWrap(StreamLayerMerger(get_next_layer).get_generator())
-    return send_file(stream)
+    client_queue_file = QueueFile(queue_process.create_queue(), 'client')
+    storage_queue_file = QueueFile(queue_process.create_queue(), 'storage')
+    
+    # Start building.
+    queue_process.run()
+
+    # Start the storage saving.
+    storage_args = (namespace, repository, locations, synthetic_image_id, storage_queue_file)
+    QueueProcess.run_process(_write_image_to_storage, storage_args)
+
+    # Return the client's data.
+    return send_file(client_queue_file)
 
   abort(403)

util/queuefile.py

@@ -0,0 +1,37 @@
+from multiprocessing import Queue
+import os
+
+class QueueFile(object):
+  """ Class which implements a file-like interface and reads from a blocking
+      multiprocessing queue.
+  """
+  def __init__(self, queue, name=None):
+    self._queue = queue
+    self._closed = False
+    self._done = False
+    self._buffer = ''
+    self._total_size = 0
+    self._name = name
+
+  def read(self, size=8192):
+    if self._closed or self._done:
+      return None
+
+    while len(self._buffer) < size:
+      result = self._queue.get(block=True)
+      if result is None:
+        self._done = True
+        break
+      
+      self._buffer += result
+      self._total_size += len(result)
+    
+    buf = self._buffer[0:size]
+    self._buffer = self._buffer[size:]
+    return buf
+
+  def flush(self):
+    pass
+
+  def close(self):
+    self._closed = True

util/queueprocess.py

@@ -0,0 +1,57 @@
+from multiprocessing import Process, Queue
+import logging
+import multiprocessing
+import os
+import time
+import gipc
+
+logger = multiprocessing.log_to_stderr()
+logger.setLevel(logging.INFO)
+
+class QueueProcess(object):
+  """ Helper class which invokes a worker in a process to produce
+      data for one (or more) queues.
+  """
+  def __init__(self, get_producer, chunk_size, max_size, args):
+    self._get_producer = get_producer
+    self._queues = []
+    self._chunk_size = chunk_size
+    self._max_size = max_size
+    self._args = args or []
+
+  def create_queue(self):
+    """ Adds a multiprocessing queue to the list of queues. Any queues added
+        will have the data produced appended.
+    """
+    queue = Queue(self._max_size / self._chunk_size)
+    self._queues.append(queue)
+    return queue
+
+  @staticmethod
+  def run_process(target, args):
+    gipc.start_process(target=target, args=args)
+
+  def run(self):
+    # Important! gipc is used here because normal multiprocessing does not work
+    # correctly with gevent when we sleep.
+    args = (self._get_producer, self._queues, self._chunk_size, self._args)
+    QueueProcess.run_process(_run, args)
+
+def _run(get_producer, queues, chunk_size, args):
+  producer = get_producer(*args)
+  while True:
+    data = producer(chunk_size) or None
+    for queue in queues:
+      try:
+        queue.put(data, block=True, timeout=10)
+      except Exception as ex:
+        # One of the listeners stopped listening.
+        return
+            
+    if data is None:
+      break
+
+    # Important! This allows the thread that writes the queue data to the pipe
+    # to do so. Otherwise, this hangs.
+    time.sleep(0)
+