Consolidate collected XBlock static assets.

Before this commit, XBlock static assets were extracted during the
collectstatic process by iterating through all installed XBlock classes
and creating a separate directory for each. However, some packages
install many XBlocks that actually share the same static assets. The
most notable example is problem_builder, though we also see this with
schoolyourself and google_drive.

This commit uses the parent module name to do package asset lookup,
collapsing those cases down and eliminating duplicates. For a default
install of edx-platform XBlocks, this reduces assets generated from 31M
to 14M.

openedx/core/lib/xblock_pipeline/finder.py

@@ -11,6 +11,8 @@ from django.core.files.storage import Storage
 from pkg_resources import resource_exists, resource_filename, resource_isdir, resource_listdir
 from xblock.core import XBlock
 
+from openedx.core.lib.xblock_utils import xblock_resource_pkg
+
 
 class XBlockPackageStorage(Storage):
     """
@@ -109,13 +111,25 @@ class XBlockPipelineFinder(BaseFinder):
     A static files finder that gets static assets from xblocks.
     """
     def __init__(self, *args, **kwargs):
+        """
+        The XBlockPipelineFinder creates a separate XBlockPackageStorage for
+        every installed XBlock package when its initialized. After that
+        initialization happens, we just proxy all list()/find() requests by
+        iterating through the XBlockPackageStorage objects.
+        """
         super(XBlockPipelineFinder, self).__init__(*args, **kwargs)
-        xblock_classes = set()
-        for __, xblock_class in XBlock.load_classes():
-            xblock_classes.add(xblock_class)
+
+        # xblock_resource_info holds (package_name, resources_dir) tuples. While
+        # it never happens in practice, the XBlock API does allow different
+        # XBlocks installed with the same setup.py to refer to their shared
+        # static assets using different prefixes.
+        xblock_resource_info = {
+            (xblock_resource_pkg(xblock_class), xblock_class.get_resources_dir())
+            for __, xblock_class in XBlock.load_classes()
+        }
         self.package_storages = [
-            XBlockPackageStorage(xblock_class.__module__, xblock_class.get_resources_dir())
-            for xblock_class in xblock_classes
+            XBlockPackageStorage(pkg_name, resources_dir)
+            for pkg_name, resources_dir in xblock_resource_info
         ]
 
     def list(self, ignore_patterns):

openedx/core/lib/xblock_utils/__init__.py

@@ -464,7 +464,7 @@ def xblock_local_resource_url(block, uri):
     xblock_class = getattr(block.__class__, 'unmixed_class', block.__class__)
     if settings.PIPELINE_ENABLED or not settings.REQUIRE_DEBUG:
         return staticfiles_storage.url('xblock/resources/{package_name}/{path}'.format(
-            package_name=xblock_class.__module__,
+            package_name=xblock_resource_pkg(xblock_class),
             path=uri
         ))
     else:
@@ -472,3 +472,30 @@ def xblock_local_resource_url(block, uri):
             'block_type': block.scope_ids.block_type,
             'uri': uri,
         })
+
+
+def xblock_resource_pkg(block):
+    """
+    Return the module name needed to find an XBlock's shared static assets.
+
+    This method will return the full module name that is one level higher than
+    the one the block is in. For instance, problem_builder.answer.AnswerBlock
+    has a __module__ value of 'problem_builder.answer'. This method will return
+    'problem_builder' instead. However, for edx-ora2's
+    openassessment.xblock.openassessmentblock.OpenAssessmentBlock, the value
+    returned is 'openassessment.xblock'.
+
+    XModules are special cased because they're local to this repo and they
+    actually don't share their resource files when compiled out as part of the
+    XBlock asset pipeline. This only covers XBlocks and XModules using the
+    XBlock-style of asset specification. If they use the XModule bundling part
+    of the asset pipeline (xmodule_assets), their assets are compiled through an
+    entirely separate mechanism and put into lms-modules.js/css.
+    """
+    # XModules are a special case because they map to different dirs for
+    # sub-modules.
+    module_name = block.__module__
+    if module_name.startswith('xmodule.'):
+        return module_name
+
+    return module_name.rsplit('.', 1)[0]