From e7f9b94356c76eeb37608272c6320dde92a396c0 Mon Sep 17 00:00:00 2001
From: Patrik Oldsberg <poldsberg@gmail.com>
Date: Thu, 18 Mar 2021 10:48:26 +0100
Subject: [PATCH] core-api: remove duplicate element check during traversal

Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>
---
 .changeset/cold-birds-attack.md               |  5 +++++
 packages/core-api/src/extensions/traversal.ts |  9 ---------
 .../core-api/src/routing/collectors.test.tsx  | 20 -------------------
 3 files changed, 5 insertions(+), 29 deletions(-)
 create mode 100644 .changeset/cold-birds-attack.md

diff --git a/.changeset/cold-birds-attack.md b/.changeset/cold-birds-attack.md
new file mode 100644
index 0000000000000..95990c509794f
--- /dev/null
+++ b/.changeset/cold-birds-attack.md
@@ -0,0 +1,5 @@
+---
+'@backstage/core-api': patch
+---
+
+Allow elements to be used multiple times in the app element tree.
diff --git a/packages/core-api/src/extensions/traversal.ts b/packages/core-api/src/extensions/traversal.ts
index bdf02d16c8721..4431bbd24c113 100644
--- a/packages/core-api/src/extensions/traversal.ts
+++ b/packages/core-api/src/extensions/traversal.ts
@@ -37,7 +37,6 @@ export function traverseElementTree<Results>(options: {
   discoverers: Discoverer[];
   collectors: { [name in keyof Results]: Collector<Results[name], any> };
 }): Results {
-  const visited = new Set();
   const collectors: {
     [name in string]: ReturnType<Collector<any, any>>;
   } = {};
@@ -74,14 +73,6 @@ export function traverseElementTree<Results>(options: {
       if (!isValidElement(element)) {
         return;
       }
-      if (visited.has(element)) {
-        const anyType = element?.type as
-          | { displayName?: string; name?: string }
-          | undefined;
-        const name = anyType?.displayName || anyType?.name || String(anyType);
-        throw new Error(`Visited element ${name} twice`);
-      }
-      visited.add(element);
 
       const nextContexts: QueueItem['contexts'] = {};
 
diff --git a/packages/core-api/src/routing/collectors.test.tsx b/packages/core-api/src/routing/collectors.test.tsx
index 44d53856b4d3f..cd3e378fffafc 100644
--- a/packages/core-api/src/routing/collectors.test.tsx
+++ b/packages/core-api/src/routing/collectors.test.tsx
@@ -353,24 +353,4 @@ describe('discovery', () => {
       });
     }).toThrow('Mounted routable extension must have a path');
   });
-
-  it('should not visit the same element twice', () => {
-    const element = <Extension3 path="/baz" />;
-
-    expect(() =>
-      traverseElementTree({
-        root: (
-          <MemoryRouter>
-            <Extension1 path="/foo">{element}</Extension1>
-            <Extension2 path="/bar">{element}</Extension2>
-          </MemoryRouter>
-        ),
-        discoverers: [childDiscoverer, routeElementDiscoverer],
-        collectors: {
-          routes: routePathCollector,
-          routeParents: routeParentCollector,
-        },
-      }),
-    ).toThrow(`Visited element Extension(Component) twice`);
-  });
 });