Self-closing <mutation> tag in XML will break the resulting 'if' block

Describe the bug

If you provide toolbox XML with a self-closing mutation tag, the resulting ‘if’ block will be malformed.

To Reproduce

The easiest way to reproduce this is to apply this patch to the repo:

diff --git a/tests/playground.html b/tests/playground.html
index 3523caa6..6a07c912 100644
--- a/tests/playground.html
+++ b/tests/playground.html
@@ -692,6 +692,16 @@ var spaghettiXml = [
       <block type="logic_null" disabled="true"></block>
       <block type="logic_ternary"></block>
     </category>
+    <category name="Test" categorystyle="logic_category">
+      <block type="controls_if">
+        <mutation else="1"/>
+        <value name="IF0">
+          <block type="logic_boolean">
+            <field name="BOOL">TRUE</field>
+          </block>
+        </value>
+      </block>
+    </category>
     <category name="Loops" categorystyle="loop_category">
       <block type="controls_repeat_ext">
         <value name="TIMES">

Then, open blockly/tests/playground.html in the browser and click “Test” in the toolbox.

Expected: this screenshot that properly has the contents of the ‘if’ block:

Actual outcome: this screenshot which shows a blank ‘if’ block:

Desktop:

• OS: Windows 10
• Browser: Chrome 79.0.3945.117, Firefox 72.0.1, and Safari 13.0.4 (15608.4.9.1.3) (i.e. I don’t think this is browser- or OS-dependent)

Stack Traces

There is no error message in the console for this.

Additional context

I investigated for quite a while, but I can’t definitively say what the problem is. What I did notice is that domToBlockHeadless_ won’t form any connections between blocks when the XML includes the self-closing tag.

Note that Blockly generates XML with self-closing mutation tags, which is the only way I even discovered this bug since I tried using my workspace’s XML as a toolbox:

1. Configure an ‘if’ block with an ‘else’ mutation:

2. Click “Export to XML”:

[moniika]

The issue with the block in the toolbox is that modern HTML does not permit self-closing tags and so the block xml:

<block type="controls_if">
  <mutation else="1"/>
    <value name="IF0">
    <block type="logic_boolean">
      <field name="BOOL">TRUE</field>
    </block>
  </value>
</block>

Is treated as though it was:

<block type="controls_if">
  <mutation else="1">
    <value name="IF0">
    <block type="logic_boolean">
      <field name="BOOL">TRUE</field>
    </block>
    </value>
  </mutation>
</block>

and so when html element is parsed, we lose the boolean value because it is nested inside the mutation element instead of being a direct child of the block element. This can be observed by inspecting the html. Technically, self-closing mutation tag does work if it is after the value, but otherwise there is an issue.

[Adam13531]

Great job, everyone! Thanks for the quick turnaround time. 😄