Workaround for libxml double-free in PHP 8.1.21

There is a quirky behaviour of the libxml integration in PHP 8.1.21
that results in "DOMException: Not Found Error" or in some cases a
SEGFAULT when nodes are appended to the DOM adjacent to nodes that
are being / have been removed from the DOM.

While we wait for our infrastructure (and especially mwcli
(T388411)) to move away from the version that has the bug, add a
workaroud so that we can make progress with MEX development.

Bug: T398821

Author: codders

src/Component.php

@@ -139,7 +139,31 @@ private function handleComponent( DOMElement $node, array $data ): bool {
 		}
 		$rendered = $this->app->renderComponentToDOM( $componentName, $componentData );
 		// TODO use adoptNode() instead of importNode() in PHP 8.3+ (see php-src commit ed6df1f0ad)
-		$node->replaceWith( $node->ownerDocument->importNode( $rendered, true ) );
+		$importNode = $node->ownerDocument->importNode( $rendered, true );
+		// TODO An issue in PHP 8.1.21's libxml integration causes a double-free if we replace a node
+		// directly with itself. T398821
+		if ( $node != $importNode ) {
+			$node->replaceWith( $importNode );
+		} else {
+			// TODO To work around the double-free, we detach all the children of the parent node and
+			// re-attach them in the correct sequence, replacing the target node with our newly-imported
+			// node. Once `mwcli` has moved off this outdated version of PHP (T388411) we should be able
+			// to remove this workaround. T398821
+			$parent = $node->parentNode;
+			$children = [];
+			foreach( iterator_to_array( $parent->childNodes ) as $child ) {
+				if ( $child !== $node ) {
+					$children[] = $child;
+				} else {
+					$children[] = $importNode;
+				}
+				$child->remove();
+			}
+
+			foreach ( $children as $child ) {
+				$parent->appendChild( $child );
+			}
+		}
 		return true;
 	}
 

tests/php/AppTest.php

@@ -61,6 +61,16 @@ public function testNestedComponentObjectProp(): void {
 		$this->assertSame( '<div><p>obj = { a: A, b: B }</p></div>', $result );
 	}
 
+	public function testComponentSubstitutionPreservesOrder(): void {
+		$app = new App( [] );
+		$app->registerComponentTemplate( 'root', '<div><x-a></x-a><div><p>Following Text</p></div>' );
+		$app->registerComponentTemplate( 'x-a', '<p>obj = { a: 1, b: 2 }</p>' );
+
+		$result = $app->renderComponent( 'root', [ ] );
+
+		$this->assertSame( '<div><p>obj = { a: 1, b: 2 }</p><div><p>Following Text</p></div></div>', $result );
+	}
+
 	public function testComponentPropKebabCase(): void {
 		$app = new App( [] );
 		$app->registerComponentTemplate(