When and why did pointer syntax break for external blocks?

[kdmccormick]

Original behavior

We believe that, in the past, all XBlocks would currently import via either "inline" or "pointer" syntax. We believe that this worked regardless of whether they were built-in to edx-platform or installed from an external repo.

Furthermore, we believe that CMS would export using pointer syntax rather than inline syntx.

1. Inline syntax (one file)

<!-- vertical/foo.xml -->
<vertical url_name="foo">
    <problem url_name="bar" display_name="The Bar">Bar content goes here</problem>
    <drag-and-drop-v2 url_name="baz" display_name="The Baz">Baz content goes here</drag-and-drop-v2>
</vertical>

2. Pointer syntax (three files)

<!-- vertical/foo.xml -->
<vertical url_name="foo">
    <problem url_name="bar" />
    <drag-and-drop-v2 url_name="baz" />
</vertical>

<!-- problem/bar.xml -->
<problem url_name="bar" display_name="The Bar">Bar content goes here</problem>

<!-- drag-and-drop-v2/baz.xml -->
<drag-and-drop-v2 url_name="baz" display_name="The Baz">Baz content goes here</drag-and-drop-v2>

Current behavior

Pointer syntax still imports fine for blocks which are built-in to edx-platform and have XmlMixin. CMS still exports using pointer syntax for these blocks as well.

BUG: Pointer syntax will not import for blocks which are external to edx-platform and do not have XmlMixin. CMS exports these using inline syntax.

Open questions

• Why did this behavior change? Perhaps, we used to mix XmlMixin into every block, and at some point that accidentally changed?
• What is the latest release that pointer syntax worked on?
  • Kyle tried drag-and-drop-v2 in Redwood with pointer syntax, and it did not work.
  • Kyle hears that a major Open edX running Quince uses exports with pointer syntax. Need to investigate. If true, this would imply that the breakage happened between Quince and Redwood.
• When we fix this, should we backport the fix to any of those old releases?

Related

Completing his issue will restore pointer syntax importing on master and in Teak:

• Add logic to parse XBlock's XML with the pointer tag correctly. XBlock#823

[kdmccormick]

FYI @farhan , @feanil , @ormsbee

[kdmccormick]

Forum post: https://discuss.openedx.org/t/olx-users-does-this-bug-affect-your-site/15277