Simplify the ExtensionAttachTo type

Simplified the `ExtensionAttachTo` type to only support a single
attachment target, removing the array form for attaching to multiple
extension points. Also removed the deprecated `ExtensionAttachToSpec`
type alias.

Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>
Made-with: Cursor

packages/frontend-app-api/src/tree/resolveAppTree.test.ts

@@ -137,60 +137,6 @@ describe('buildAppTree', () => {
     `);
   });
 
-  it('should create a tree with clones', () => {
-    const tree = resolveAppTree(
-      'a',
-      [
-        { ...baseSpec, id: 'a' },
-        { ...baseSpec, id: 'b', attachTo: { id: 'a', input: 'x' } },
-        {
-          ...baseSpec,
-          id: 'c',
-          attachTo: [
-            { id: 'a', input: 'x' },
-            { id: 'b', input: 'x' },
-          ],
-        },
-        {
-          ...baseSpec,
-          id: 'd',
-          attachTo: [
-            { id: 'b', input: 'x' },
-            { id: 'c', input: 'x' },
-          ],
-        },
-      ],
-      collector,
-    );
-
-    expect(Array.from(tree.nodes.keys())).toEqual(['a', 'b', 'c', 'd']);
-
-    expect(String(tree.root)).toMatchInlineSnapshot(`
-      "<a>
-        x [
-          <b>
-            x [
-              <c>
-                x [
-                  <d />
-                ]
-              </c>
-              <d />
-            ]
-          </b>
-          <c>
-            x [
-              <d />
-            ]
-          </c>
-        ]
-      </a>"
-    `);
-
-    const orphans = Array.from(tree.orphans).map(String);
-    expect(orphans).toMatchInlineSnapshot(`[]`);
-  });
-
   it('should create a tree out of order', () => {
     const tree = resolveAppTree(
       'b',

packages/frontend-app-api/src/tree/resolveAppTree.ts

@@ -153,7 +153,6 @@ export function resolveAppTree(
   }
 
   const orphans = new Array<SerializableAppNode>();
-  const clones = new Map<string, Array<SerializableAppNode>>();
 
   // A node with the provided rootNodeId must be found in the tree, and it must not be attached to anything
   let rootNode: AppNode | undefined = undefined;
@@ -164,46 +163,6 @@ export function resolveAppTree(
     // TODO: For now we simply ignore the attachTo spec of the root node, but it'd be cleaner if we could avoid defining it
     if (spec.id === rootNodeId) {
       rootNode = node;
-    } else if (Array.isArray(spec.attachTo)) {
-      // eslint-disable-next-line no-console
-      console.warn(
-        `Extension '${spec.id}' is using multiple attachment points which is deprecated and will be removed in a future release. ` +
-          `Use a Utility API instead to share functionality across multiple locations. ` +
-          `See https://backstage.io/docs/frontend-system/architecture/27-sharing-extensions for migration guidance.`,
-      );
-      let foundFirstParent = false;
-      for (const origAttachTo of spec.attachTo) {
-        let attachTo = origAttachTo;
-
-        if (!isValidAttachmentPoint(attachTo, nodes)) {
-          attachTo =
-            redirectTargetsByKey.get(makeRedirectKey(attachTo)) ?? attachTo;
-        }
-
-        const parent = nodes.get(attachTo.id);
-        if (parent) {
-          const cloneParents = clones.get(attachTo.id) ?? [];
-
-          if (!foundFirstParent) {
-            foundFirstParent = true;
-            node.setParent(parent, attachTo.input);
-          } else {
-            cloneParents.unshift(parent);
-          }
-
-          for (const extraParent of cloneParents) {
-            const clonedNode = new SerializableAppNode(spec);
-            clonedNode.setParent(extraParent, attachTo.input);
-            clones.set(
-              spec.id,
-              clones.get(spec.id)?.concat(clonedNode) ?? [clonedNode],
-            );
-          }
-        }
-      }
-      if (!foundFirstParent) {
-        orphans.push(node);
-      }
     } else {
       let attachTo = spec.attachTo;
       if (!isValidAttachmentPoint(attachTo, nodes)) {